CBC · California Building Code

How must foundations be protected from groundwater and moisture?

If your lowest floor sits below exterior grade CRC requires a geotechnical check for groundwater within 5 ft. If groundwater or hydrostatic pressure is present you must waterproof walls and floors; where pressure won’t occur (or the water table is engineered to stay ≥6 in below the floor) dampproofing plus perimeter drains and a gravel base are permitted—follow the material thicknesses, drain elevations and membrane extents given in § 1803.5.4 and § 1805.

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Last reviewed: July 5, 2026

What the code requires — plain English (controlling §)

The CBC requires a geotechnical investigation to determine groundwater depth where lowest floors are below exterior grade (§ 1803.5.4). If groundwater or hydrostatic conditions can affect the building, walls and floors must be either dampproofed or waterproofed and appropriate subsoil drainage and base courses provided, with specific material/thickness and placement requirements in Chapter 18 (§ 1805 and its subsections). These thresholds and the permitted methods are set out in § 1803.5.4 and § 1805 (dampproofing, waterproofing, subsoil drainage).

Requirements in detail

1) Trigger: geotechnical investigation (groundwater)

A geotechnical investigation must determine whether groundwater is above or within 5 feet (1,524 mm) below the elevation of the lowest floor when that floor is located below the finished ground adjacent to the foundation — i.e., check if groundwater will affect design or construction (§ 1803.5.4). If the investigation shows groundwater is close enough to affect the structure, the designer must address it in the foundation design and in waterproofing/dampproofing decisions.

Decision-relevant dimensions/values (quick reference)

Decision / item	Value or requirement	Code Reference
Geotechnical trigger: depth to groundwater	Groundwater above or within 5 ft (1,524 mm) below lowest floor (below exterior grade)	§ 1803.5.4
Allowable lowered groundwater to use dampproofing	Groundwater may be lowered & maintained ≥ 6 in (152 mm) below bottom of lowest floor to use dampproofing	§ 1805 A.1.3
Dampproofing under slab membrane	≥ 6‑mil (0.006 in; 0.152 mm) polyethylene with 6‑in lap (or other approved) when under slab	§ 1805 A.2.1
Dampproofing on top of slab	Mopped bitumen, or ≥ 4‑mil (0.004 in; 0.102 mm) polyethylene, or approved material	§ 1805 A.2.1
Wall dampproofing	Exterior-applied bituminous, 3 lb/yd² acrylic-modified cement, 1/8 in surface-bonding mortar (ASTM C887) or materials permitted for waterproofing	§ 1805 A.2.2
Waterproofing (walls) vertical extent	From bottom of wall to ≥ 12 in (305 mm) above maximum groundwater elevation	§ 1805 A.3.2
Waterproofing (walls) acceptable membranes	Two‑ply hot‑mopped felts, ≥6‑mil PVC, 40‑mil polymer‑modified asphalt, 6‑mil polyethylene, or other approved membranes capable of bridging nonstructural cracks	§ 1805 A.3.2
Waterproofing (floors under hydrostatic)	Membrane options: rubberized asphalt, butyl rubber, fully adhered HDPE/polyolefin composite, or ≥6‑mil PVC with 6‑in lap (under slab)	§ 1805 A.3.1
Floor base course thickness (under basement slab)	Minimum 4 in (102 mm) clean gravel/crushed stone (≤10% passing No.4) unless well‑drained site exception applies	§ 1805 A.4.1
Foundation drain placement	Drain extends ≥ 12 in (305 mm) beyond outside edge of footing; bottom of drain not higher than bottom of floor base; top of drain ≥ 6 in (152 mm) above top of footing; cover with approved filter membrane	§ 1805 A.4.2

2) Dampproofing vs. Waterproofing — when to use each

Dampproofing (lighter moisture control) is required where hydrostatic pressure will not occur. The code expressly authorizes dampproofing for floors and walls where hydrostatic pressure is not expected, and also allows dampproofing when an engineered groundwater‑control system lowers the table to at least 6 inches below the bottom of the lowest floor (§ 1805 A.2 and § 1805 A.1.3).
Waterproofing (heavier, hydrostatic‑resistant systems) is required where the geotechnical investigation shows hydrostatic pressure exists or the design does not include a groundwater control system. Waterproofing membranes and structural detailing must resist hydrostatic loads; walls must be designed to withstand the lateral loads from water and be waterproofed from wall bottom to at least 12 inches above maximum water table (§ 1805 A.3 and subsections).

3) Drainage, base course, joints and penetrations

Where hydrostatic pressure does not exist, the code requires a subsoil drainage system: a base under the floor and a perimeter foundation drain constructed of gravel/crushed stone and/or perforated pipe, with invert not higher than floor elevation, filter fabric cover, and specified extents relative to footing and base course (§ 1805 A.4 and § 1805 A.4.2).
Joints and penetrations of walls and floors must be made watertight with approved methods and materials (waterproofing section, § 1805 A.3.3). Surface preparation for dampproofing/waterproofing (sealing form‑tie holes, parging masonry) is required (§ 1805 A.2.2.1 and § 1805 A.3.2.1).

4) Under‑floor spaces and basements

Under‑floor (crawlspace) finished ground level should not be below the bottom of footings; if groundwater rises to within 6 in (152 mm) of outside finished ground level or surface water doesn’t drain, the under‑floor ground must match outside finished grade unless a drainage system is provided — and certain § 1805 provisions may not apply in that case (§ 1805 A.1.2). Basements that qualify as a story above grade plane and have >25% of perimeter with exterior ground higher have specific dampproofing and drain requirements (§ 1805 A.1.1).

Exceptions & special cases

Story‑above‑grade plane basements: If a basement is treated as a story above grade plane and the outside finished ground adjacent to ≥25% of the perimeter is above the basement floor, the code requires dampproofing and a foundation drain around the below‑grade portion; some waterproofing/subsoil base requirements do not apply in that configuration (§ 1805 A.1.1).
Well‑drained sites: The 4‑inch floor base course under basement slabs can be omitted where the site is well‑drained gravel or sand/gravel mix soils (§ 1805 A.4.1 exception).
Parging exception: Unit masonry walls need not be parged if an approved material is permitted for direct application (§ 1805 A.2.2.1 exception).
Groundwater control tradeoff: Where an engineered groundwater‑lowering system is provided and maintains the table ≥ 6 in below the bottom of the lowest floor, dampproofing (not full waterproofing) is permitted; design of the control system must follow accepted engineering practice and consider soil permeability, inflow rates, pump capacity/head, disposal capacity (§ 1805 A.1.3).

Common mistakes

Treating dampproofing as equivalent to waterproofing: Dampproofing is acceptable only where no hydrostatic pressure is expected; if hydrostatic conditions exist per the geotechnical report, waterproofing is required (§ 1805 A.2 vs § 1805 A.3).
Skipping the geotechnical check: Failing to have a geotechnical investigation (or ignoring its groundwater findings) when the lowest floor is below adjacent grade violates § 1803.5.4 and can lead to under‑designed waterproofing/drainage.
Incorrect drain placement / elevation: Installing foundation drain invert above the floor elevation or failing to extend the drain the required 12 in beyond footing, or not covering with approved filter membrane, will reduce drain effectiveness (§ 1805 A.4.2).
Not extending waterproofing high enough on walls: Waterproofing must extend to at least 12 in above the maximum groundwater elevation; stopping too low leaves walls exposed to seepage when water rises (§ 1805 A.3.2).
Poor surface preparation: Not sealing form‑tie holes, not parging masonry where required, or not sealing joint penetrations undermines membrane performance (§ 1805 A.2.2.1 and § 1805 A.3.2.1).

Worked example — applying the rule with numbers

Scenario: New house with a basement floor 4.0 ft (1,219 mm) below exterior finished grade at the nearest exterior wall. Geotechnical borings show seasonal high groundwater at 1.5 ft (457 mm) below exterior grade.

Does § 1803.5.4 require a geotechnical investigation? Yes — lowest floor is below finished ground and groundwater is within 5 ft of that floor elevation, so groundwater investigation is required and has been performed (trigger = within 5 ft). § 1803.5.4
Is hydrostatic pressure likely? Groundwater is 1.5 ft below exterior grade, while basement floor is 4.0 ft below grade — groundwater is 2.5 ft above the basement floor elevation (i.e., the water table is above the floor elevation) → hydrostatic condition exists. The geotechnical report would therefore indicate hydrostatic pressure and the code requires waterproofing (not mere dampproofing) unless an engineered groundwater‑control system is provided and demonstrated. § 1805 A.3 and § 1805 A.1.3
Code‑compliant measures (minimum):
- Design basement walls as concrete/masonry capable of resisting lateral hydrostatic loads and apply waterproofing membrane from wall bottom to at least 12 in (305 mm) above the maximum groundwater elevation (i.e., 12 in above the 1.5 ft below grade mark) — per § 1805 A.3.2.
- Provide an under‑slab waterproof membrane (e.g., fully adhered HDPE or rubberized asphalt, or ≥6‑mil PVC with 6‑in laps) per § 1805 A.3.1.
- Install a 4‑inch clean gravel base under the slab (unless the site is well‑drained gravel, which it is not in this scenario) and a perimeter foundation drain with invert at or below floor elevation, drain extending at least 12 in beyond footing and top of drain ≥ 6 in above top of footing, covered by filter fabric (§ 1805 A.4.1 & A.4.2).

Alternatively, if an engineered groundwater control system is designed and shown to lower and maintain the water table to at least 6 in below the bottom of the lowest floor (i.e., to ≥ 6 in below basement floor elevation), then dampproofing per § 1805 A.2 could be used instead — but the groundwater control design must address permeability, inflow rate, pump capacity/head, disposal capacity, etc. (§ 1805 A.1.3).

Related provisions (quick list)

§ 1803.5.4 — Geotechnical investigation: groundwater depth and effect on design.
§ 1805 (and subsections A.1 through A.4) — Dampproofing and Waterproofing general requirements, dampproofing details, waterproofing details, and subsoil drainage.
§ 1805 A.1.3 — Ground‑water control system conditions that permit dampproofing (6‑in threshold).
§ 1805 A.2 / A.2.1 / A.2.2 — Dampproofing materials and installation (floors and walls).
§ 1805 A.3 / A.3.1 / A.3.2 — Waterproofing materials, walls and floors; membranes and performance expectations.
§ 1805 A.4 / A.4.1 / A.4.2 — Subsoil drainage systems, floor base course and foundation drains.

Code references

Grounded in the retrieved California Building Code — click a citation to read the verbatim passage:

CBC § 1.3 High relevance — show source text
1805 A .1.3 Ground-water control. Where the ground-water table is lowered and maintained at an elevation not less than 6 inches (152 mm) below the bottom of the lowest floor, the floor and walls shall be dampproofed in accordance with Section 1805 A .2. The design of the system to lower the ground-water table shall be based on accepted principles of engineering that shall consider, but not necessarily be limited to, permeability of the soil, rate at which water enters the drainage system, rated capacity of pumps, head against which pumps are to operate and the rated capacity of the disposal area of the system.

1805 A .2 Dampproofing. Where hydrostatic pressure will not occur as determined by Section 1803 A .5.4, floors and walls shall be dampproofed in accordance with this section.

1805 A .2.1 Floors. Dampproofing materials for floors shall be installed between the floor and the base course required by Section 1805 A .4.1, except where a separate floor is provided above a concrete slab.

Where installed beneath the slab, dampproofing shall consist of not less than 6-mil (0.006 inch; 0.152 mm) polyethylene with joints lapped not less than 6 inches (152 mm), or other approved methods or materials. Where permitted to be installed on top of the slab, dampproofing shall consist of mopped-on bitumen, not less than 4-mil (0.004 inch; 0.102 mm) polyethylene, or other approved methods or materials. Joints in the membrane shall be lapped and sealed in accordance with the manufacturer’s installation instructions.

1805 A .2.2 Walls. Dampproofing materials for walls shall be installed on the exterior surface of the wall, and shall extend from the top of the footing to above ground level. Dampproofing shall consist of a bituminous material, 3 pounds per square yard (16 N/m [2] ) of acrylic modified cement, [1] / 8 inch (3.2 mm) coat of surface-bonding mortar complying with ASTM C887, any of the materials permitted for waterproofing by Section 1805.3.2 or other approved methods or materials.

1805 A .2.2.1 Surface preparation of walls. Prior to application of dampproofing materials on concrete walls, holes and recesses resulting from the removal of form ties shall be sealed with a bituminous material or other approved methods or materials. Unit masonry walls shall be parged on the exterior surface below ground level with not less than [3] / 8 inch (9.5 mm) of Portland cement mortar. The parging shall be coved at the footing.

Exception: Parging of unit masonry walls is not required where a material is approved for direct application to the masonry.

1805 A .3 Waterproofing. Where the ground-water investigation required by Section 1803 A .5.4 indicates that a hydrostatic pressure condition exists, and the design does not include a ground-water control system as described in Section 1805 A .1.3, walls and floors shall be waterproofed in accordance with this section.

1805 A .3.1 Floors. Floors required to be waterproofed shall be of concrete and designed and constructed to withstand the hydrostatic pressures to which the floors will be subjected.
CBC § 1.2 High relevance — show source text
1805 A .1.2 Under-floor space. The finished ground level of an under-floor space such as a crawl space shall not be located below the bottom of the footings. Where there is evidence that the ground-water table rises to within 6 inches (152 mm) of the ground

2025 CALIFORNIA BUILDING CODE 18A-7

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SOILS AND FOUNDATIONS

level at the outside building perimeter, or that the surface water does not readily drain from the building site, the ground level of the under-floor space shall be as high as the outside finished ground level, unless an approved drainage system is provided. The provisions of Sections 1803 A .5.4, 1805 A .2, 1805 A .3 and 1805 A .4 shall not apply in this case.

1805 A .1.2.1 Flood hazard areas. For buildings and structures in flood hazard areas as established in Section 1612 A .3, the finished ground level of an under-floor space such as a crawl space shall be equal to or higher than the outside finished ground level on one side or more.

Exception: Under-floor spaces of Group R-3 buildings that meet the requirements of FEMA TB 11.

1805 A .1.3 Ground-water control. Where the ground-water table is lowered and maintained at an elevation not less than 6 inches (152 mm) below the bottom of the lowest floor, the floor and walls shall be dampproofed in accordance with Section 1805 A .2. The design of the system to lower the ground-water table shall be based on accepted principles of engineering that shall consider, but not necessarily be limited to, permeability of the soil, rate at which water enters the drainage system, rated capacity of pumps, head against which pumps are to operate and the rated capacity of the disposal area of the system.

1805 A .2 Dampproofing. Where hydrostatic pressure will not occur as determined by Section 1803 A .5.4, floors and walls shall be dampproofed in accordance with this section.

1805 A .2.1 Floors. Dampproofing materials for floors shall be installed between the floor and the base course required by Section 1805 A .4.1, except where a separate floor is provided above a concrete slab.

Where installed beneath the slab, dampproofing shall consist of not less than 6-mil (0.006 inch; 0.152 mm) polyethylene with joints lapped not less than 6 inches (152 mm), or other approved methods or materials. Where permitted to be installed on top of the slab, dampproofing shall consist of mopped-on bitumen, not less than 4-mil (0.004 inch; 0.102 mm) polyethylene, or other approved methods or materials. Joints in the membrane shall be lapped and sealed in accordance with the manufacturer’s installation instructions.
CBC § 4.505.2 High relevance — show source text
4.505.2 Concrete slab foundations. Concrete slab foundations required to have a vapor retarder by the California Building Code, Chapter 19 or concrete slab-on-ground floors required to have a vapor retarder by the California Residential Code, Chapter 5, shall also comply with this section.

4.505.2.1 Capillary break. A capillary break shall be installed in compliance with at least one of the following:
1. A 4-inch-thick (101.6 mm) base of [1] / 2 inch (12.7 mm) or larger clean aggregate shall be provided with a vapor retarder in direct contact with concrete and a concrete mix design, which will address bleeding, shrinkage and curling, shall be used. For additional information, see American Concrete Institute, ACI 302.2R-06.
2. Other equivalent methods approved by the enforcing agency.
3. A slab design specified by a licensed design professional.
4.505.3 Moisture content of building materials. Building materials with visible signs of water damage shall not be installed. Wall and floor framing shall not be enclosed when the framing members exceed 19 percent moisture content. Moisture content shall be verified in compliance with the following:
1. Moisture content shall be determined with either a probe-type or contact-type moisture meter. Equivalent moisture verification methods may be approved by the enforcing agency and shall satisfy requirements found in Section 101.8 of this code.
2. Moisture readings shall be taken at a point 2 feet (610 mm) to 4 feet (1219 mm) from the grade stamped end of each piece to be verified.
3. At least three random moisture readings shall be performed on wall and floor framing with documentation acceptable to the enforcing agency provided at the time of approval to enclose the wall and floor framing.
Insulation products which are visibly wet or have a high moisture content shall be replaced or allowed to dry prior to enclosure in wall or floor cavities. Wet-applied insulation products shall follow the manufacturers’ drying recommendations prior to enclosure.

SECTION 4.506—INDOOR AIR QUALITY AND EXHAUST

4.506.1 Bathroom exhaust fans. Each bathroom shall be mechanically ventilated and shall comply with the following:
1. Fans shall be ENERGY STAR compliant and be ducted to terminate outside the building.
2. Unless functioning as a component of a whole house ventilation system, fans must be controlled by a humidity control. a. Humidity controls shall be capable of adjustment between a relative humidity range of ≤ 50 percent to a maximum of 80 percent. A humidity control may utilize manual or automatic means of adjustment. b. A humidity control may be a separate component to the exhaust fan and is not required to be integral (i.e., built-in).
Notes:
1. For the purposes of this section, a bathroom is a room which contains a bathtub, shower or tub/shower combination.
2. Lighting integral to bathroom exhaust fans shall comply with the California Energy Code.
SECTION 4.507—ENVIRONMENTAL COMFORT

4.507.1 Reserved.

2025 CALIFORNIA GREEN BUILDING STANDARDS CODE 4-17

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RESIDENTIAL MANDATORY MEASURES
CBC § 3.1 High relevance — show source text
R406.3 Dampproofing for wood foundations. Wood foundations enclosing habitable or usable spaces located below grade shall be dampproofed in accordance with Sections R406.3.1 through R406.3.4.

R406.3.1 Panel joint sealed. Plywood panel joints in the foundation walls shall be sealed full length with a caulking compound capable of producing a moistureproof seal under the conditions of temperature and moisture content at which it will be applied and used.

R406.3.2 Below-grade moisture barrier. A 6-mil-thick (0.15 mm) polyethylene film shall be applied over the below-grade portion of exterior foundation walls prior to backfilling. Joints in the polyethylene film shall be lapped 6 inches (152 mm) and sealed with adhesive. The top edge of the polyethylene film shall be bonded to the sheathing to form a seal. Film areas at grade level shall be protected from mechanical damage and exposure by a pressure-preservative treated lumber or plywood strip attached to the wall several inches above finished grade level and extending approximately 9 inches (229 mm) below grade. The joint between the strip and the wall shall be caulked full length prior to fastening the strip to the wall. Where approved, other coverings appropriate to the architectural treatment shall be permitted to be used. The polyethylene film shall extend down to the bottom of the wood footing plate but shall not overlap or extend into the gravel or crushed stone footing.

R406.3.3 Porous fill. The space between the excavation and the foundation wall shall be backfilled with the same material used for footings, up to a height of 1 foot (305 mm) above the footing for well-drained sites, or one-half the total backfill height for poorly drained sites. The porous fill shall be covered with strips of 30-pound (13.6 kg) asphalt paper or 6-mil (0.15 mm) polyethylene to permit water seepage while avoiding infiltration of fine soils.

R406.3.4 Backfill. The remainder of the excavated area shall be backfilled with the same type of soil as was removed during the excavation.

R406.4 Precast concrete foundation system dampproofing. Except where required by Section R406.2 to be waterproofed, precast concrete foundation walls enclosing habitable or useable spaces located below grade shall be dampproofed in accordance with Section R406.1.

R406.4.1 Panel joints sealed. Precast concrete foundation panel joints shall be sealed full height with a sealant meeting ASTM C920, Type S or M, Grade NS, Class 25, Use NT, M or A. Joint sealant shall be installed in accordance with the manufacturer’s instructions.

2025 CALIFORNIA RESIDENTIAL CODE 4-43

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FOUNDATIONS

SECTION R407—COLUMNS

R407.1 Wood column protection. Wood columns shall be protected against decay as set forth in Section R304.

R407.2 Steel column protection. All surfaces (inside and outside) of steel columns shall be given a shop coat of rust-inhibitive paint, except for corrosion-resistant steel and steel treated with coatings to provide corrosion resistance.
CBC § 3.2 High relevance — show source text
1805 A .3.2 Walls. Walls required to be waterproofed shall be of concrete or masonry and shall be designed and constructed to withstand the hydrostatic pressures and other lateral loads to which the walls will be subjected.

Waterproofing shall be applied from the bottom of the wall to not less than 12 inches (305 mm) above the maximum elevation of the ground-water table. The remainder of the wall shall be dampproofed in accordance with Section 1805 A .2.2. Waterproofing shall consist of two-ply hot-mopped felts, not less than 6-mil (0.006 inch; 0.152 mm) polyvinyl chloride, 40-mil (0.040 inch; 1.02 mm) polymer-modified asphalt, 6-mil (0.006 inch; 0.152 mm) polyethylene or other approved methods or materials capable of bridging nonstructural cracks. Joints in the membrane shall be lapped and sealed in accordance with the manufacturer’s installation instructions.

1805 A .3.2.1 Surface preparation of walls. Prior to the application of waterproofing materials on concrete or masonry walls, the walls shall be prepared in accordance with Section 1805 A .2.2.1.

1805 A .3.3 Joints and penetrations. Joints in walls and floors, joints between the wall and floor and penetrations of the wall and floor shall be made watertight utilizing approved methods and materials.

1805 A .4 Subsoil drainage system. Where a hydrostatic pressure condition does not exist, dampproofing shall be provided and a base shall be installed under the floor and a drain installed around the foundation perimeter. A subsoil drainage system designed and constructed in accordance with Section 1805 A .1.3 shall be deemed adequate for lowering the ground-water table.

1805 A .4.1 Floor base course. Floors of basements, except as provided for in Section 1805 A .1.1, shall be placed over a floor base course not less than 4 inches (102 mm) in thickness that consists of gravel or crushed stone containing not more than 10 percent of material that passes through a No. 4 (4.75 mm) sieve.

Exception: Where a site is located in well-drained gravel or sand/gravel mixture soils, a floor base course is not required.

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1805 A .4.2 Foundation drain. A drain shall be placed around the perimeter of a foundation that consists of gravel or crushed stone containing not more than 10-percent material that passes through a No. 4 (4.75 mm) sieve. The drain shall extend not less than 12 inches (305 mm) beyond the outside edge of the footing. The thickness shall be such that the bottom of the drain is not higher than the bottom of the base under the floor, and that the top of the drain is not less than 6 inches (152 mm) above the top of the footing. The top of the drain shall be covered with an approved filter membrane material. Where a drain tile or perforated pipe is used, the invert of the pipe or tile shall not be higher than the floor elevation.
CBC § 1803.5.3 High relevance — show source text
1803.5.3 Expansive soil. In areas likely to have expansive soil, the building official shall require soil tests to determine where such soils do exist.

Soils meeting all four of the following provisions shall be considered to be expansive, except that tests to show compliance with Items 1, 2 and 3 shall not be required if the test prescribed in Item 4 is conducted:
1. Plasticity index (PI) of 15 or greater, determined in accordance with ASTM D4318.
2. More than 10 percent of the soil particles pass a No.200 sieve (75 µm), determined in accordance with ASTM D6913.
3. More than 10 percent of the soil particles are less than 5 micrometers in size, determined in accordance with ASTM D6913.
4. Expansion index greater than 20, determined in accordance with ASTM D4829.
1803.5.4 Groundwater. A geotechnical investigation shall be performed to determine if:
1. Groundwater is above or within 5 feet (1524 mm) below the elevation of the lowest floor level where such floor is located below the finished ground level adjacent to the foundation.
2. The groundwater depth will affect the design and construction of buildings and structures.
1803.5.5 Deep foundations. Where deep foundations will be used, a geotechnical investigation shall be conducted and shall include all of the following, unless sufficient data on which to base the design and installation is otherwise available:
1. Recommended deep foundation types and installed capacities.
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1. Recommended center-to-center spacing of deep foundation elements.
2. Driving criteria.
3. Installation procedures.
4. Field inspection and reporting procedures (to include procedures for verification of the installed bearing capacity where required).
5. Load test requirements.
6. Suitability of deep foundation materials for the intended environment.
7. Designation of bearing stratum or strata.
8. Reductions for group action, where necessary.
1803.5.6 Rock strata. Where foundations are to be constructed on or in rock, the geotechnical investigation shall assess variations in rock strata depth, competency and load-bearing capacity.

1803.5.7 Excavation near foundations. Where excavation will reduce support from any foundation, a registered design professional shall prepare an assessment of the structure as determined from examination of the structure, available design documents, available subsurface data, and, if necessary, excavation of test pits. The registered design professional shall determine the requirements for support and protection of any existing foundation and prepare site-specific plans, details and sequence of work for submission. Such support shall be provided by under-pinning, bracing, excavation retention systems or by other means acceptable to the building official.

1803.5.8 Compacted fill material. Where shallow foundations will bear on compacted fill material more than 12 inches (305 mm) in depth, a geotechnical investigation shall be conducted and shall include all of the following:
1. Specifications for the preparation of the site prior to placement of compacted fill material.
2. Specifications for material to be used as compacted fill.
3. Test methods to be used to determine the maximum dry density and optimum moisture content of the material to be used as compacted fill.
4. Maximum allowable thickness of each lift of compacted fill material.
5. Field test method for determining the in-place dry density of the compacted fill.
CBC § 1804.6 High relevance — show source text
1. In coastal high hazard areas, unless such fill is conducted or placed to avoid diversion of water and waves toward any building or structure.
2. Where design flood elevations are specified but floodways have not been designated, unless it has been demonstrated that the cumulative effect of the proposed flood hazard area encroachment, when combined with all other existing and anticipated flood hazard area encroachment, will not increase the design flood elevation more than 1 foot (305 mm) at any point.
1804.6 Compacted fill material. Where shallow foundations will bear on compacted fill material, the compacted fill shall comply with the provisions of an approved geotechnical report, as set forth in Section 1803.

Exception: Compacted fill material 12 inches (305 mm) in depth or less need not comply with an approved report, provided that the in-place dry density is not less than 90 percent of the maximum dry density at optimum moisture content determined in accordance with ASTM D1557. The compaction shall be verified by special inspection in accordance with Section 1705.6.

1804.7 Controlled low-strength material (CLSM). Where shallow foundations will bear on controlled low-strength material (CLSM), the CLSM shall comply with the provisions of an approved geotechnical report, as set forth in Section 1803.

SECTION 1805—DAMPPROOFING AND WATERPROOFING

1805.1 General. Walls or portions thereof that retain earth and enclose interior spaces and floors below grade shall be waterproofed and dampproofed in accordance with this section, with the exception of those spaces containing groups other than residential and institutional where such omission is not detrimental to the building or occupancy.

Ventilation for crawl spaces shall comply with Section 1202.4.

1805.1.1 Story above grade plane. Where a basement is considered a story above grade plane and the finished ground level adjacent to the basement wall is below the basement floor elevation for 25 percent or more of the perimeter, the floor and walls shall be dampproofed in accordance with Section 1805.2 and a foundation drain shall be installed in accordance with Section 1805.4.2. The foundation drain shall be installed around the portion of the perimeter where the basement floor is below ground level. The provisions of Sections 1803.5.4, 1805.3 and 1805.4.1 shall not apply in this case.

2025 CALIFORNIA BUILDING CODE 18-7

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SOILS AND FOUNDATIONS

1805.1.2 Under-floor space. The finished ground level of an under-floor space such as a crawl space shall not be located below the bottom of the footings. Where there is evidence that the ground-water table rises to within 6 inches (152 mm) of the ground level at the outside building perimeter, or that the surface water does not readily drain from the building site, the ground level of the under-floor space shall be as high as the outside finished ground level, unless an approved drainage system is provided. The provisions of Sections 1803.5.4, 1805.2, 1805.3 and 1805.4 shall not apply in this case.
CBC § 5.4 High relevance — show source text
1. Expansion index greater than 20, determined in accordance with ASTM D4829.
1803 A .5.4 Groundwater. A geotechnical investigation shall be performed to determine if:
1. Groundwater is above or within 5 feet (1524 mm) below the elevation of the lowest floor level where such floor is located below the finished ground level adjacent to the foundation.
2. The groundwater depth will affect the design and construction of buildings and structures.
1803 A .5.5 Deep foundations. Where deep foundations will be used, a geotechnical investigation shall be conducted and shall include all of the following, unless sufficient data on which to base the design and installation is otherwise available:
1. Recommended deep foundation types and installed capacities.
2. Recommended center-to-center spacing of deep foundation elements.
3. Driving criteria.
4. Installation procedures.
5. Field inspection and reporting procedures (to include procedures for verification of the installed bearing capacity where required).
6. Load test requirements.
7. Suitability of deep foundation materials for the intended environment.
8. Designation of bearing stratum or strata.
9. Reductions for group action, where necessary.
1803 A .5.6 Rock strata. Where foundations are to be constructed on or in rock, the geotechnical investigation shall assess variations in rock strata depth, competency and load-bearing capacity.

1803 A .5.7 Excavation near foundations. Where excavation will reduce support from any foundation, a registered design professional shall prepare an assessment of the structure as determined from examination of the structure, available design documents, available subsurface data, and, if necessary, excavation of test pits. The registered design professional shall determine the requirements for support and protection of any existing foundation and prepare site-specific plans, details and sequence of work

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for submission. Such support shall be provided by under-pinning, bracing, excavation retention systems or by other means acceptable to the building official.

1803 A .5.8 Compacted fill material. Where shallow foundations will bear on compacted fill material more than 12 inches (305 mm) in depth, a geotechnical investigation shall be conducted and shall include all of the following:
1. Specifications for the preparation of the site prior to placement of compacted fill material.
2. Specifications for material to be used as compacted fill.
3. Test methods to be used to determine the maximum dry density and optimum moisture content of the material to be used as compacted fill.
4. Maximum allowable thickness of each lift of compacted fill material.
5. Field test method for determining the in-place dry density of the compacted fill.
6. Minimum acceptable in-place dry density expressed as a percentage of the maximum dry density determined in accordance with Item 3.
7. Number and frequency of field tests required to determine compliance with Item 6.
CBC § 1202.4. High relevance — show source text
1804 A .7 Controlled low-strength material (CLSM). Where shallow foundations will bear on controlled low-strength material (CLSM), the CLSM shall comply with the provisions of an approved geotechnical report, as set forth in Section 1803 A .

SECTION 1805 A —DAMPPROOFING AND WATERPROOFING

1805 A .1 General. Walls or portions thereof that retain earth and enclose interior spaces and floors below grade shall be waterproofed and dampproofed in accordance with this section, with the exception of those spaces containing groups other than residential and institutional where such omission is not detrimental to the building or occupancy.

Ventilation for crawl spaces shall comply with Section 1202.4.

1805 A .1.1 Story above grade plane. Where a basement is considered a story above grade plane and the finished ground level adjacent to the basement wall is below the basement floor elevation for 25 percent or more of the perimeter, the floor and walls shall be dampproofed in accordance with Section 1805 A .2 and a foundation drain shall be installed in accordance with Section 1805 A .4.2. The foundation drain shall be installed around the portion of the perimeter where the basement floor is below ground level. The provisions of Sections 1803 A .5.4, 1805 A .3 and 1805 A .4.1 shall not apply in this case.

1805 A .1.2 Under-floor space. The finished ground level of an under-floor space such as a crawl space shall not be located below the bottom of the footings. Where there is evidence that the ground-water table rises to within 6 inches (152 mm) of the ground

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level at the outside building perimeter, or that the surface water does not readily drain from the building site, the ground level of the under-floor space shall be as high as the outside finished ground level, unless an approved drainage system is provided. The provisions of Sections 1803 A .5.4, 1805 A .2, 1805 A .3 and 1805 A .4 shall not apply in this case.

1805 A .1.2.1 Flood hazard areas. For buildings and structures in flood hazard areas as established in Section 1612 A .3, the finished ground level of an under-floor space such as a crawl space shall be equal to or higher than the outside finished ground level on one side or more.

Exception: Under-floor spaces of Group R-3 buildings that meet the requirements of FEMA TB 11.

CBC § 4.504.5 High relevance — show source text

Exterior grade products marked as meeting the PS-1 or PS-2 standards of the Engineered Wood Association, the Australian AS/NZS 2269, European 636 3S, and Canadian CSA O121, CSA O151, CSA O153 and CSA O325 standards.
Other methods acceptable to the enforcing agency.

TABLE 4.504.5—FORMALDEHYDE LIMITS1 Maximum Formaldehyde Emissions in Parts per Million	Col2
PRODUCT	CURRENT LIMIT
Hardwood plywood veneer core	0.05
Hardwood plywood composite core	0.05
Particleboard	0.09
Medium density fiberboard	0.11
Thin medium density fiberboard2	0.13
1. Values in this table are derived from those specified by the California Air Resources Board, Air Toxics Control Measure for Composite Wood as tested in accordance with ASTM E1333. For additional information, see_California Code of Regulations_, Title 17, Sections 93120 through 93120.12. 2. Thin medium density fiberboard has a maximum thickness of 5/16 inch (8 mm).	1. Values in this table are derived from those specified by the California Air Resources Board, Air Toxics Control Measure for Composite Wood as tested in accordance with ASTM E1333. For additional information, see_California Code of Regulations_, Title 17, Sections 93120 through 93120.12. 2. Thin medium density fiberboard has a maximum thickness of 5/16 inch (8 mm).

SECTION 4.505—INTERIOR MOISTURE CONTROL

4.505.1 General. Buildings shall meet or exceed the provisions of the California Building Standards Code.

4.505.2 Concrete slab foundations. Concrete slab foundations required to have a vapor retarder by the California Building Code, Chapter 19 or concrete slab-on-ground floors required to have a vapor retarder by the California Residential Code, Chapter 5, shall also comply with this section.

4.505.2.1 Capillary break. A capillary break shall be installed in compliance with at least one of the following:

A 4-inch-thick (101.6 mm) base of [1] / 2 inch (12.7 mm) or larger clean aggregate shall be provided with a vapor retarder in direct contact with concrete and a concrete mix design, which will address bleeding, shrinkage and curling, shall be used. For additional information, see American Concrete Institute, ACI 302.2R-06.
Other equivalent methods approved by the enforcing agency.
A slab design specified by a licensed design professional.

4.505.3 Moisture content of building materials. Building materials with visible signs of water damage shall not be installed. Wall and floor framing shall not be enclosed when the framing members exceed 19 percent moisture content. Moisture content shall be verified in compliance with the following:

Moisture content shall be determined with either a probe-type or contact-type moisture meter. Equivalent moisture verification methods may be approved by the enforcing agency and shall satisfy requirements found in Section 101.8 of this code.
Moisture readings shall be taken at a point 2 feet (610 mm) to 4 feet (1219 mm) from the grade stamped end of each piece to be verified.
At least three random moisture readings shall be performed on wall and floor framing with documentation acceptable to the enforcing agency provided at the time of approval to enclose the wall and floor framing.

CBC § 2.2.1 High relevance — show source text
1805 A .2.2.1 Surface preparation of walls. Prior to application of dampproofing materials on concrete walls, holes and recesses resulting from the removal of form ties shall be sealed with a bituminous material or other approved methods or materials. Unit masonry walls shall be parged on the exterior surface below ground level with not less than [3] / 8 inch (9.5 mm) of Portland cement mortar. The parging shall be coved at the footing.

Exception: Parging of unit masonry walls is not required where a material is approved for direct application to the masonry.

1805 A .3 Waterproofing. Where the ground-water investigation required by Section 1803 A .5.4 indicates that a hydrostatic pressure condition exists, and the design does not include a ground-water control system as described in Section 1805 A .1.3, walls and floors shall be waterproofed in accordance with this section.

1805 A .3.1 Floors. Floors required to be waterproofed shall be of concrete and designed and constructed to withstand the hydrostatic pressures to which the floors will be subjected.

Waterproofing shall be accomplished by placing a membrane of rubberized asphalt, butyl rubber, fully adhered/fully bonded HDPE or polyolefin composite membrane or not less than 6-mil [0.006 inch (0.152 mm)] polyvinyl chloride with joints lapped not less than 6 inches (152 mm) or other approved materials under the slab. Joints in the membrane shall be lapped and sealed in accordance with the manufacturer’s installation instructions.

1805 A .3.2 Walls. Walls required to be waterproofed shall be of concrete or masonry and shall be designed and constructed to withstand the hydrostatic pressures and other lateral loads to which the walls will be subjected.

Waterproofing shall be applied from the bottom of the wall to not less than 12 inches (305 mm) above the maximum elevation of the ground-water table. The remainder of the wall shall be dampproofed in accordance with Section 1805 A .2.2. Waterproofing shall consist of two-ply hot-mopped felts, not less than 6-mil (0.006 inch; 0.152 mm) polyvinyl chloride, 40-mil (0.040 inch; 1.02 mm) polymer-modified asphalt, 6-mil (0.006 inch; 0.152 mm) polyethylene or other approved methods or materials capable of bridging nonstructural cracks. Joints in the membrane shall be lapped and sealed in accordance with the manufacturer’s installation instructions.

1805 A .3.2.1 Surface preparation of walls. Prior to the application of waterproofing materials on concrete or masonry walls, the walls shall be prepared in accordance with Section 1805 A .2.2.1.

1805 A .3.3 Joints and penetrations. Joints in walls and floors, joints between the wall and floor and penetrations of the wall and floor shall be made watertight utilizing approved methods and materials.
CBC § 1202.4. Medium relevance — show source text
The procedure used to establish the final ground level adjacent to the foundation shall account for additional settlement of the backfill.

1804 A .5 Grading and fill in flood hazard areas. In flood hazard areas established in Section 1612 A .3, grading, fill, or both, shall not be approved:
1. Unless such fill is placed, compacted and sloped to minimize shifting, slumping and erosion during the rise and fall of flood water and, as applicable, wave action.
2. In floodways, unless it has been demonstrated through hydrologic and hydraulic analyses performed by a registered design professional in accordance with standard engineering practice that the proposed grading or fill, or both, will not result in any increase in flood levels during the occurrence of the design flood.
3. In coastal high hazard areas, unless such fill is conducted or placed to avoid diversion of water and waves toward any building or structure.
4. Where design flood elevations are specified but floodways have not been designated, unless it has been demonstrated that the cumulative effect of the proposed flood hazard area encroachment, when combined with all other existing and anticipated flood hazard area encroachment, will not increase the design flood elevation more than 1 foot (305 mm) at any point.
1804 A .6 Compacted fill material. Where shallow foundations will bear on compacted fill material, the compacted fill shall comply with the provisions of an approved geotechnical report, as set forth in Section 1803 A .

Exception: Compacted fill material 12 inches (305 mm) in depth or less need not comply with an approved report, provided that the in-place dry density is not less than 90 percent of the maximum dry density at optimum moisture content determined in accordance with ASTM D1557. The compaction shall be verified by special inspection in accordance with Section 1705 A .6.

1804 A .7 Controlled low-strength material (CLSM). Where shallow foundations will bear on controlled low-strength material (CLSM), the CLSM shall comply with the provisions of an approved geotechnical report, as set forth in Section 1803 A .

SECTION 1805 A —DAMPPROOFING AND WATERPROOFING

1805 A .1 General. Walls or portions thereof that retain earth and enclose interior spaces and floors below grade shall be waterproofed and dampproofed in accordance with this section, with the exception of those spaces containing groups other than residential and institutional where such omission is not detrimental to the building or occupancy.

Ventilation for crawl spaces shall comply with Section 1202.4.

1805 A .1.1 Story above grade plane. Where a basement is considered a story above grade plane and the finished ground level adjacent to the basement wall is below the basement floor elevation for 25 percent or more of the perimeter, the floor and walls shall be dampproofed in accordance with Section 1805 A .2 and a foundation drain shall be installed in accordance with Section 1805 A .4.2. The foundation drain shall be installed around the portion of the perimeter where the basement floor is below ground level. The provisions of Sections 1803 A .5.4, 1805 A .3 and 1805 A .4.1 shall not apply in this case.

Frequently asked questions

When do I need a geotechnical report for groundwater?

If the lowest floor is below the finished exterior grade, the code requires a geotechnical investigation to determine if groundwater is above or within 5 ft (1,524 mm) of that floor elevation and whether groundwater will affect design (§ 1803.5.4).

How do I know whether to dampproof or waterproof?

Use dampproofing where hydrostatic pressure will not occur or when an engineered groundwater‑lowering system keeps the table at least 6 in (152 mm) below the bottom of the lowest floor (§ 1805 A.2 and § 1805 A.1.3). If hydrostatic pressure exists according to the geotechnical report (and no control system is used), waterproofing is required (§ 1805 A.3).

How high must waterproofing go on a foundation wall?

Waterproofing must extend from the bottom of the wall to at least 12 in (305 mm) above the maximum elevation of the ground‑water table (§ 1805 A.3.2).

What are the minimum drain/base dimensions I must follow?

Under basement slabs use a 4 in (102 mm) base of gravel/crushed stone (unless the site is well‑drained gravel), and perimeter drains must extend ≥ 12 in (305 mm) beyond footing, with the invert not higher than the floor elevation and top of drain ≥ 6 in (152 mm) above footing top (§ 1805 A.4.1 & A.4.2).

Can I rely on a polyethylene sheet alone for protection?

Under‑slab polyethylene (6‑mil) is an allowed dampproofing layer when hydrostatic pressure will not occur; however, where hydrostatic pressure exists the code requires a waterproofing membrane designed to resist the hydrostatic load (§ 1805 A.2.1 and § 1805 A.3.1).

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