CRC · California Residential Code
Wood floors on ground and preservative treatment
Homeowners: The California Residential Code (§R504) requires on‑grade wood floors to be built with labeled pressure‑treated, dried lumber set on a compacted site with a 4‑inch gravel base and a 6‑mil polyethylene moisture barrier lapped 6 inches. Joists, blocking and plywood must be treated, installed to transfer lateral soil loads, and meet joist deflection rules; if exterior fills differ by more than 2 feet, an engineered solution is required.
Last reviewed: July 6, 2026
What the code requires — plain English
The California Residential Code requires that wood floors built directly on grade or in basements be constructed with pressure preservative‑treated wood, be designed to resist soil lateral loads and normal floor loads, and be installed over a prepared granular base and a 6‑mil polyethylene moisture barrier. The controlling requirements appear in § R504 (see § R504.1, § R504.2, § R504.2.1, § R504.2.2, § R504.3) .
The single most important rule: use pressure‑preservative‑treated, dried‑after‑treatment, labeled wood for sleepers/joists and follow the site preparation and moisture‑barrier requirements in § R504 .
Requirements in detail
General design and loads
- Wood basement floors and on‑grade wood floors must be engineered to resist axial forces and bending moments from lateral soil pressures, as well as the floor live and dead loads; joist deflection must meet Section § R301 requirements (design rule in § R504.1) .
- Where unbalanced lateral soil loads exist, wood basement floors are limited to installations where the differential depth of fill on opposite exterior foundation walls is 2 feet (610 mm) or less, unless special provisions are made to resist sliding (§ R504.1.1) .
- Joist bearing, plywood continuity and blocking requirements to transfer lateral forces are specified in § R504.1.2 and must be followed (joists bear against studs or band joist; plywood continuous over overlapped joists; blocking between joists) .
- Where uplift or buckling restraint is needed, provide interior bearing walls or properly designed stub walls anchored into supporting soil below (§ R504.1.3) .
Site preparation and base
- Remove all vegetation, topsoil and foreign material inside foundation walls; added fill must be free of vegetation/foreign material and compacted to provide uniform support for sleepers (§ R504.2) .
- Place a minimum 4‑inch (102 mm) granular base of gravel (maximum 3/4‑inch (19.1 mm)) or crushed stone (maximum 1/2‑inch (12.7 mm)) over the compacted earth (§ R504.2.1) .
- Install polyethylene sheeting, minimum 6‑mil (0.15 mm), over the granular base; joints must be lapped 6 inches (152 mm) and left unsealed, and the membrane shall be placed over the pressure‑preservative‑treated wood sleepers but shall not extend beneath the footing plates of the exterior walls (§ R504.2.2) .
Materials and preservative treatment
- Framing materials — sleepers, joists, blocking and plywood subflooring — used for on‑ground wood floors must be pressure‑preservative treated and dried after treatment in accordance with AWPA U1, and must bear the label of an accredited agency (§ R504.3) .
- Field‑cut ends, notches and drilled holes of preservative‑treated wood must be retreated in the field in accordance with AWPA M4; fasteners and connectors in contact with treated wood must meet corrosion‑resistant/coating requirements (§ R304.1.1, § R304.3) .
Quick decision table (key values / thresholds)
| Decision item | Required value / action | Code reference |
|---|---|---|
| Use of treated wood | Pressure preservative-treated; dried after treatment; labeled | § R504.3 |
| Joist design | Meet joist deflection requirements per Section § R301 | § R504.1 |
| Differential fill limit (no special sliding provisions) | ≤ 2 ft (610 mm) | § R504.1.1 |
| Granular base thickness | Minimum 4 in (102 mm) | § R504.2.1 |
| Granular aggregate size | Gravel ≤ 3/4 in (19.1 mm) or crushed stone ≤ 1/2 in (12.7 mm) | § R504.2.1 |
| Vapor/moisture barrier | Polyethylene, minimum 6‑mil (0.15 mm); joints lapped 6 in (152 mm); left unsealed; placed over sleepers; not under footing plates | § R504.2.2 |
| Plywood subfloor | Continuous over overlapped joists / butt joints; blocking to transfer lateral forces | § R504.1.2 |
| Field treatment of cuts | Retreat in field per AWPA M4 | § R304.1.1 |
| Fasteners/connectors | Use corrosion‑resistant or specified galvanized/stainless fasteners per § R304.3 | § R304.3 |
Exceptions & special cases
- Differential fill > 2 ft (610 mm): the prescriptive limitation in § R504.1.1 allows on‑ground wood basement floors only when differential depth is 2 ft or less unless special measures (engineered sliding/retention) are provided; the Code does not list those special measures in § R504 — an engineer must design them (§ R504.1.1) .
- Untreated wood entirely below the groundwater level or continuously submerged in fresh water is excepted from preservative treatment requirements (this is covered under ground‑contact rules in § R304.1.2) .
- Polyethylene placement: the membrane is required to be placed as described in § R504.2.2; note the Code explicitly says the membrane “shall not extend beneath the footing plates of the exterior walls” (i.e., do not wrap under footing plates) .
- Materials must bear the quality mark/label showing treating plant, preservative type, retention, end use, standard, inspection agency and “Dry” when applicable (see § R304.2 and § R304.2.1) .
If you need prescriptive details for sliding resistance, soil‑retaining stub walls, or engineered uplift/buckling anchors the Code points to design by calculation (see § R504.1, § R504.1.3) but those engineered measures are not prescriptively specified in § R504 .
Common mistakes
- Using non‑treated or improperly labeled lumber for sleepers/joists — the Code requires pressure‑preservative treatment, drying after treatment (AWPA U1) and a visible quality mark (§ R504.3, § R304.2) .
- Forgetting to retreat field cuts, notches or drilled holes — must be treated in the field per AWPA M4 (§ R304.1.1) .
- Installing the polyethylene membrane incorrectly: sealing lap joints (laps must be left unsealed), or extending the membrane beneath footing plates (the membrane shall not extend beneath footing plates) — see § R504.2.2 .
- Using ordinary (non‑corrosion resistant) fasteners in contact with treated wood — fasteners must meet § R304.3 corrosion coating/material requirements; otherwise premature corrosion and structural failure can occur .
- Ignoring lateral soil loads or differential fill limits — failing to design for lateral soil thrust or exceed a 2‑ft (610 mm) differential without engineered resistance can lead to sliding or structural distress (§ R504.1, § R504.1.1) .
- Omitting blocking to transfer lateral forces at end walls — Code requires sufficient blocking between joists to transmit lateral forces into the floor system (§ R504.1.2) .
Worked example
Scenario: 20 ft × 30 ft living room built as an on‑grade wood floor inside foundation walls. Opposite exterior foundation wall fills differ by 18 in (less than 2 ft).
Steps to comply:
- Clear vegetation and topsoil inside foundation perimeter and compact added fill to uniform support for sleepers (§ R504.2) .
- Place a 4 in (102 mm) layer of gravel (max 3/4 in (19.1 mm)) over the compacted earth across the 20×30 area (§ R504.2.1) .
- Lay 6‑mil polyethylene over the granular base, lap joints 6 in (152 mm) and leave laps unsealed; position the membrane as required (do not run it under footing plates) (§ R504.2.2) .
- Set pressure‑preservative‑treated sleepers (labeled and dried after treatment per AWPA U1) on the granular base/poly over the area, spaced per the floor framing design that meets joist deflection per § R301; use blocking to transfer lateral forces and ensure joists bear tightly against studs or the band joist per § R504.1.2 (§ R504.3, § R504.1.2, § R504.1) .
- Install plywood subfloor continuous over overlapped joists (or butt joints over in‑line joists) and secure with corrosion‑resistant fasteners conforming to § R304.3; retreat any field cuts per AWPA M4 (§ R504.1.2, § R304.3, § R304.1.1) .
- Because the fill differential is 18 in (< 2 ft), the prescriptive limit in § R504.1.1 is satisfied without additional engineered sliding resistance (§ R504.1.1) .
This scenario meets the numeric thresholds in the Code: 4‑inch base, 6‑mil poly, 6‑in lap, and differential fill ≤ 2 ft — all cited to § R504 provisions .
Related provisions
- § R304.1.1 — Field treatment (AWPA M4) for field‑cut ends, notches, holes of preservative‑treated wood .
- § R304.1.2 — Ground contact: requirement for pressure‑preservative treatment where wood is in contact with ground (exceptions for continuously submerged wood) .
- § R304.2 / R304.2.1 — Quality mark and required information on treated lumber/plywood labels (treatment plant, preservative, retention, end use, standard, agency, “Dry”) .
- § R304.3 — Fasteners and connectors in contact with preservative‑treated wood (corrosion resistance and coating requirements) .
- § R301 — Structural design criteria including joist deflection requirements referenced by § R504.1 (design obligation) .
- § R305 — Termite protection methods and related provisions referenced elsewhere in Chapter 3 (see R305 and Figure R305.4 for termite probability) .
Code references
Grounded in the retrieved California Residential Code — click a citation to read the verbatim passage:
CRC § 3.1 High relevance — show source text
R503.3 Particleboard.
R503.3.1 Identification and grade. Particleboard shall conform to ANSI A208.1 and shall be so identified by a grade mark or certificate of inspection issued by an approved agency. R503.3.2 Floor underlayment. Particleboard floor underlayment shall conform to Type PBU and shall be not less than [1] / 4 inch (6.4 mm) in thickness.
R503.3.3 Installation. Particleboard underlayment shall be installed in accordance with the recommendations of the manufacturer and attached to framing in accordance with Table R602.3(1).
SECTION R504—PRESSURE PRESERVATIVE-TREATED WOOD FLOORS (ON GROUND)
R504.1 General. Pressure preservative-treated wood basement floors and floors on ground shall be designed to withstand axial forces and bending moments resulting from lateral soil pressures at the base of the exterior walls and floor live and dead loads. Floor framing shall be designed to meet joist deflection requirements in accordance with Section R301.
R504.1.1 Unbalanced soil loads. Unless special provision is made to resist sliding caused by unbalanced lateral soil loads, wood basement floors shall be limited to applications where the differential depth of fill on opposite exterior foundation walls is 2 feet (610 mm) or less.
R504.1.2 Construction. Joists in wood basement floors shall bear tightly against the narrow face of studs in the foundation wall or directly against a band joist that bears on the studs. Plywood subfloor shall be continuous over lapped joists or over butt joints between in-line joists. Sufficient blocking shall be provided between joists to transfer lateral forces at the base of the end walls into the floor system.
R504.1.3 Uplift and buckling. Where required, resistance to uplift or restraint against buckling shall be provided by interior bearing walls or properly designed stub walls anchored in the supporting soil below.
R504.2 Site preparation. The area within the foundation walls shall have all vegetation, topsoil and foreign material removed, and any fill material that is added shall be free of vegetation and foreign material. The fill shall be compacted to ensure uniform support of the pressure preservative-treated wood floor sleepers. R504.2.1 Base. A minimum 4-inch-thick (102 mm) granular base of gravel having a maximum size of [3] / 4 inch (19.1 mm) or crushed stone having a maximum size of [1] / 2 inch (12.7 mm) shall be placed over the compacted earth.
R504.2.2 Moisture barrier. Polyethylene sheeting of minimum 6-mil (0.15 mm) thickness shall be placed over the granular base. Joints shall be lapped 6 inches (152 mm) and left unsealed. The polyethylene membrane shall be placed over the pressure preservative-treated wood sleepers and shall not extend beneath the footing plates of the exterior walls.
R504.3 Materials. Framing materials, including sleepers, joists, blocking and plywood subflooring, shall be pressure-preservative treated and dried after treatment in accordance with AWPA U1 (Commodity Specification A, Special Requirement 4.2), and shall bear the label of an accredited agency.
SECTION R505—COLD-FORMED STEEL FLOOR FRAMING
CRC § 2304.12.2 High relevance — show source text
2304.12.2 Other locations. Wood used in the locations specified in Sections 2304.12.2.1 through 2304.12.2.8 shall be naturally durable wood or preservative-treated wood in accordance with AWPA U1. Preservative-treated wood used in interior locations shall be protected with two coats of urethane, shellac, latex epoxy or varnish unless waterborne preservatives are used. Prior to application of the protective finish, the wood shall be dried in accordance with the manufacturer’s recommendations. 2304.12.2.1 Girder ends. The ends of wood girders entering exterior masonry or concrete walls shall be provided with a [1] / 2 inch (12.7 mm) airspace on top, sides and end, unless naturally durable or preservative-treated wood is used.
2304.12.2.2 Posts or columns. Posts or columns supporting permanent structures and supported by a concrete or masonry slab or footing that is in direct contact with the earth shall be of naturally durable or preservative-treated wood.
Exception: Posts or columns that meet all of the following:
Are not exposed to the weather, or are protected by a roof, eave, overhang, or other covering if exposed to the weather.
Are supported by concrete piers or metal pedestals projected not less than 1 inch (25 mm) above the slab or deck and are separated from the concrete pier by an impervious moisture barrier.
Are located not less than 8 inches (203 mm) above exposed earth.
2304.12.2.3 Supporting member for permanent appurtenances. Naturally durable or preservative-treated wood shall be utilized for those portions of wood members that form the structural supports of buildings, balconies, porches or similar permanent building appurtenances where such members are exposed to the weather without adequate protection from a roof, eave, overhang or other covering to prevent moisture or water accumulation on the surface or at joints between members.
Exception: Sawn lumber in buildings located in a geographical region where experience has demonstrated that climatic conditions preclude the need to use durable materials where the structure is exposed to the weather.
2304.12.2.4 Supporting members for permeable floors and roofs. Wood structural members that support moisture-permeable floors or roofs that are exposed to the weather, such as concrete or masonry slabs, shall be of naturally durable or preservative-treated wood unless separated from such floors or roofs by an impervious moisture barrier. The impervious moisture barrier system protecting the structure supporting floors shall provide positive drainage of water that infiltrates the moisture-permeable floor topping.
2304.12.2.5 Ventilation beneath balcony or elevated walking surfaces. Enclosed framing in exterior balconies and elevated walking surfaces that have weather-exposed surfaces shall be provided with openings that provide a net free cross-ventilation area not less than [1] / 150 of the area of each separate space.
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WOOD
2304.12.2.6 Wood in contact with the ground or fresh water. Wood used in contact with exposed earth shall be naturally durable for both decay and termite resistance or preservative treated in accordance with AWPA U1 for soil or freshwater use.
Exception: Untreated wood is permitted where such wood is continuously and entirely below the ground-water level or submerged in fresh water.
CRC § 25.4 High relevance — show source text
Applicable to all grades of sanded exterior-type plywood.|For SI: 1 inch = 25.4 mm, 1 pound per square foot = 0.0479 kPa.
a. Plywood continuous over two or more spans and face grain perpendicular to supports. Unsupported edges shall be tongue-and-groove or blocked except where nominal1/4-
inch-thick wood panel-type underlayment, fiber-cement underlayment or3/4-inch wood finish floor is used. Fiber-cement underlayment shall comply with ASTM C1288 or ISO
8336 Category C. Allowable uniform live load at maximum span based on deflection of1/360 of span is 100 psf.
b. Applicable to all grades of sanded exterior-type plywood.|2025 CALIFORNIA RESIDENTIAL CODE 5-13
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FLOORS
R503.2.2 Allowable spans. The maximum allowable span for wood structural panels used as subfloor or combination subfloor underlayment shall be as set forth in Table R503.2.1.1(1), or APA E30. The maximum span for sanded plywood combination subfloor underlayment shall be as set forth in Table R503.2.1.1(2).
R503.2.3 Installation. Wood structural panels used as subfloor or combination subfloor underlayment shall be attached to wood framing in accordance with Table R602.3(1) and shall be attached to cold-formed steel framing in accordance with Table R505.3.1(2).
R503.3 Particleboard.
R503.3.1 Identification and grade. Particleboard shall conform to ANSI A208.1 and shall be so identified by a grade mark or certificate of inspection issued by an approved agency. R503.3.2 Floor underlayment. Particleboard floor underlayment shall conform to Type PBU and shall be not less than [1] / 4 inch (6.4 mm) in thickness.
R503.3.3 Installation. Particleboard underlayment shall be installed in accordance with the recommendations of the manufacturer and attached to framing in accordance with Table R602.3(1).
SECTION R504—PRESSURE PRESERVATIVE-TREATED WOOD FLOORS (ON GROUND)
R504.1 General. Pressure preservative-treated wood basement floors and floors on ground shall be designed to withstand axial forces and bending moments resulting from lateral soil pressures at the base of the exterior walls and floor live and dead loads. Floor framing shall be designed to meet joist deflection requirements in accordance with Section R301.
R504.1.1 Unbalanced soil loads. Unless special provision is made to resist sliding caused by unbalanced lateral soil loads, wood basement floors shall be limited to applications where the differential depth of fill on opposite exterior foundation walls is 2 feet (610 mm) or less.
R504.1.2 Construction. Joists in wood basement floors shall bear tightly against the narrow face of studs in the foundation wall or directly against a band joist that bears on the studs. Plywood subfloor shall be continuous over lapped joists or over butt joints between in-line joists. Sufficient blocking shall be provided between joists to transfer lateral forces at the base of the end walls into the floor system.
CRC § 4-6 High relevance — show source text
R403 Footings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-6
R404 Foundation and Retaining Walls . . . . . . . . . . . . . . . . 4-24
R405 Foundation Drainage . . . . . . . . . . . . . . . . . . . . . . . . . . 4-42
R406 Foundation Waterproofing and Dampproofing . . . 4-43
R407 Columns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-44
R408 Under-Floor Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-44
CHAPTER 5 FLOORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3
R501 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3
R502 Wood Floor Framing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3
R503 Floor Sheathing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-12
R504 Pressure Preservative-Treated Wood
Floors (On Ground) . . . . . . . . . . . . . . . . . . . . . . . . . . 5-14
R505 Cold-Formed Steel Floor Framing . . . . . . . . . . . . . . . 5-14
R506 Concrete Floors (On Ground) . . . . . . . . . . . . . . . . . . . 5-26
R507 Exterior Decks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-26
CHAPTER 6 WALL CONSTRUCTION. . . . . . . . . . . . . . . . . . . 6-3
R601 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3
R602 Wood Wall Framing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3
R603 Cold-Formed Steel Wall Framing. . . . . . . . . . . . . . . . 6-53
R604 Wood Structural Panels . . . . . . . . . . . . . . . . . . . . . . . . 6-93
R605 Particleboard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-93
R606 General Masonry Construction. . . . . . . . . . . . . . . . . . 6-93
CRC § 1.1 High relevance — show source text
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BUILDING PLANNING
Wood siding, sheathing and wall framing on the exterior of a building having a clearance of less than 6 inches (152 mm) from the ground or less than 2 inches (51 mm) measured vertically from concrete steps, porch slabs, patio slabs and similar horizontal surfaces exposed to the weather.
Wood structural members supporting moisture-permeable floors or roofs that are exposed to the weather, such as concrete or masonry slabs, unless separated from such floors or roofs by an impervious moisture barrier. The impervious moisture barrier system protecting the structure supporting floors shall provide positive drainage of water that infiltrates the moisture- permeable floor topping.
Wood furring strips or other wood framing members attached directly to the interior of exterior masonry walls or concrete walls below grade except where an approved vapor retarder is applied between the wall and the furring strips or framing members.
Portions of wood structural members that form the structural supports of buildings, decks, balconies, porches or similar permanent building appurtenances where those members are exposed to the weather without adequate protection from a roof, eave, overhang or other covering that prevents moisture or water accumulation on the surface or at joints between members. Exception: Sawn lumber used in structures located in a geographical region where experience has demonstrated that climatic conditions preclude the need to use naturally durable or preservative-treated wood where the structure is exposed to the weather.
Wood columns in contact with basement floor slabs unless supported by concrete piers or metal pedestals projecting not less than 1 inch (25 mm) above the concrete floor and separated from the concrete pier by an impervious moisture barrier.
R304.1.1 Field treatment. Field-cut ends, notches and drilled holes of preservative-treated wood shall be treated in the field in accordance with AWPA M4.
R304.1.2 Ground contact. All wood in contact with the ground, embedded in concrete in direct contact with the ground or embedded in concrete exposed to the weather that supports permanent structures intended for human occupancy shall be approved pressure-preservative-treated wood suitable for ground contact use, except that untreated wood used entirely below groundwater level or continuously submerged in fresh water shall not be required to be pressure-preservative treated.
R304.1.3 Ventilation required beneath balcony or elevated walking surfaces. Enclosed framing in exterior balconies and elevated walking surfaces that are exposed to rain, snow or drainage from irrigation shall be provided with openings that provide a net-free cross-ventilation area not less than [1] / 150 of the area of each separate space.
R304.2 Quality mark. Lumber and plywood required to be pressure-preservative treated in accordance with Section R304.1 shall bear the quality mark of an approved inspection agency that maintains continuing supervision, testing and inspection over the quality of the product and that has been approved by an accreditation body that complies with the requirements of the American Lumber Standard Committee treated wood program.
R304.2.1 Required information. The required quality mark on each piece of pressure-preservative-treated lumber or plywood shall contain the following information:
- Identification of the treating plant.
- Type of preservative.
- The minimum preservative retention.
- End use for which the product was treated.
- Standard to which the product was treated.
- Identity of the approved inspection agency.
- The designation “Dry,” if applicable.
CRC § 1.1 High relevance — show source text
R304.1.1 Field treatment. Field-cut ends, notches and drilled holes of preservative-treated wood shall be treated in the field in accordance with AWPA M4.
R304.1.2 Ground contact. All wood in contact with the ground, embedded in concrete in direct contact with the ground or embedded in concrete exposed to the weather that supports permanent structures intended for human occupancy shall be approved pressure-preservative-treated wood suitable for ground contact use, except that untreated wood used entirely below groundwater level or continuously submerged in fresh water shall not be required to be pressure-preservative treated.
R304.1.3 Ventilation required beneath balcony or elevated walking surfaces. Enclosed framing in exterior balconies and elevated walking surfaces that are exposed to rain, snow or drainage from irrigation shall be provided with openings that provide a net-free cross-ventilation area not less than [1] / 150 of the area of each separate space.
R304.2 Quality mark. Lumber and plywood required to be pressure-preservative treated in accordance with Section R304.1 shall bear the quality mark of an approved inspection agency that maintains continuing supervision, testing and inspection over the quality of the product and that has been approved by an accreditation body that complies with the requirements of the American Lumber Standard Committee treated wood program.
R304.2.1 Required information. The required quality mark on each piece of pressure-preservative-treated lumber or plywood shall contain the following information:
- Identification of the treating plant.
- Type of preservative.
- The minimum preservative retention.
- End use for which the product was treated.
- Standard to which the product was treated.
- Identity of the approved inspection agency.
- The designation “Dry,” if applicable.
Exception: Quality marks on lumber less than 1 inch (25 mm) nominal thickness, or lumber less than nominal 1 inch by 5 inches (25 mm by 127 mm) or 2 inches by 4 inches (51 mm by 102 mm) or lumber 36 inches (914 mm) or less in length shall be applied by stamping the faces of exterior pieces or by end labeling not less than 25 percent of the pieces of a bundled unit.
R304.3 Fasteners and connectors in contact with preservative-treated and fire-retardant-treated wood. Fasteners, including nuts and washers, and connectors in contact with preservative-treated wood and fire-retardant-treated wood shall be in accordance with this section. The coating weights for zinc-coated fasteners shall be in accordance with ASTM A153. The coating weight for zinccoated nails shall be in accordance with ASTM A153 Class D or ASTM A641 Class 3S. Stainless steel driven fasteners shall be in accordance with the material requirements of ASTM F1667.
R304.3.1 Fasteners for preservative-treated wood. Fasteners, including nuts and washers, for preservative-treated wood shall be of hot-dipped, zinc-coated galvanized steel, stainless steel, silicon bronze or copper. Staples shall be of stainless steel. Coating types and weights for connectors in contact with preservative-treated wood shall be in accordance with the connector manufacturer’s recommendations. In the absence of manufacturer’s recommendations, not less than ASTM A653 type G185 zinc-coated galvanized steel, or equivalent, shall be used.
CRC § 2304.12.2.4 High relevance — show source text
Exception: Sawn lumber in buildings located in a geographical region where experience has demonstrated that climatic conditions preclude the need to use durable materials where the structure is exposed to the weather.
2304.12.2.4 Supporting members for permeable floors and roofs. Wood structural members that support moisture-permeable floors or roofs that are exposed to the weather, such as concrete or masonry slabs, shall be of naturally durable or preservative-treated wood unless separated from such floors or roofs by an impervious moisture barrier. The impervious moisture barrier system protecting the structure supporting floors shall provide positive drainage of water that infiltrates the moisture-permeable floor topping.
2304.12.2.5 Ventilation beneath balcony or elevated walking surfaces. Enclosed framing in exterior balconies and elevated walking surfaces that have weather-exposed surfaces shall be provided with openings that provide a net free cross-ventilation area not less than [1] / 150 of the area of each separate space.
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WOOD
2304.12.2.6 Wood in contact with the ground or fresh water. Wood used in contact with exposed earth shall be naturally durable for both decay and termite resistance or preservative treated in accordance with AWPA U1 for soil or freshwater use.
Exception: Untreated wood is permitted where such wood is continuously and entirely below the ground-water level or submerged in fresh water.
2304.12.2.6.1 Posts or columns. Posts and columns that are supporting permanent structures and embedded in concrete that is exposed to the weather or in direct contact with the earth shall be of preservative-treated wood.
2304.12.2.7 Termite protection. In geographical areas where hazard of termite damage is known to be very heavy, wood floor framing in the locations specified in Section 2304.12.1.1 and exposed framing of exterior decks or balconies shall be of naturally durable species (termite resistant) or preservative treated in accordance with AWPA U1 for the species, product preservative and end use or provided with approved methods of termite protection.
2304.12.2.8 Wood used in retaining walls and cribs. Wood installed in retaining or crib walls shall be preservative treated in accordance with AWPA U1 for soil and freshwater use.
2304.12.3 Attic ventilation. For attic ventilation, see Section 1202.2.2.
2304.12.4 Under-floor ventilation (crawl space). For under-floor ventilation (crawl space), see Section 1202.4.
2304.12.8 Separate wood framing. [SPCB] Correct the conditions in frame and stucco walls and similar appurtenant construction so that the wood framing is separate from the main structure by a complete concrete or masonry plug with no voids that will allow infestations to enter the structure from the wall. If there is no plug, the foundation shall be 2 inches (51 mm) or more above the grade levels and at least as high as the adjoining slabs or 4-inch (102 mm) concrete barrier seat off installed.
CRC § 2304.12.1.1.1 High relevance — show source text
2304.12.1.1.1 [SPCB] There shall be a clearance of at least 18 inches (457 mm) between the underside of wood floor joists and the finished surface of the ground, and at least 12 inches (305 mm) between the underside of any other wood horizontal framing member and the finished surface of the ground. The ground underneath floor joists shall be leveled or smoothed off so as to maintain a reasonably even surface.
Exception: For purposes of structural pest control inspection, a minimum of 12 inches (305 mm) of clearance under-floor joists shall be considered adequate except that such clearance shall not be necessary where the subarea soil is of such a nature as to prevent excavation or where excavation would create a hazard from shifting soil or other causes.
2304.12.1.2 Wood supported by exterior foundation walls. Wood framing members, including wood sheathing, that are in contact with exterior foundation walls and are less than 8 inches (203 mm) from exposed earth shall be of naturally durable or preservative-treated wood.
Exception: [DSA-SS and OSHPD 1, 1R, 2, 4 & 5] At exterior walls where the earth is paved with an asphalt or concrete slab at least 18 inches (457 mm) wide and draining away from the building, the bottom of sills are permitted to be 6 inches (152 mm) above the top of such slab. Other equivalent means of termite and decay protection may be accepted by the enforcement agency.
2304.12.1.3 Exterior walls below grade. Wood framing members and furring strips in direct contact with the interior of exterior masonry or concrete walls below grade shall be of naturally durable or preservative-treated wood.
2304.12.1.4 Sleepers and sills. Sleepers and sills on a concrete or masonry slab that is in direct contact with earth shall be of naturally durable or preservative-treated wood.
2304.12.1.4.1 Additional requirements. [DSA-SS and OSHPD 1, 1R, 2, 4 & 5] Stud walls or partitions at shower or toilet rooms with more than two plumbing fixtures, excluding floor drains, and stud walls adjacent to unroofed paved areas shall rest on a concrete curb extending at least 6 inches (152 mm) above finished floor and pavement level.
2304.12.1.5 Wood siding. Clearance between wood siding and earth on the exterior of a building shall be not less than 6 inches (152 mm) or less than 2 inches (51 mm) vertical from concrete steps, porch slabs, patio slabs and similar horizontal surfaces exposed to the weather except where siding, sheathing and wall framing are of naturally durable or preservativetreated wood.
CRC § 2.1 Medium relevance — show source text
Chemical termiticide treatment in accordance with Section R305.2.
Termite-baiting system installed and maintained in accordance with the label.
Pressure-preservative-treated wood in accordance with the provisions of Section R304.1.
Naturally durable termite-resistant wood.
Physical barriers in accordance with Section R305.3 and used in locations as specified in Section R304.1.
Cold-formed steel framing in accordance with Sections R505.2.1 and R603.2.1.
R305.1.1 Quality mark. Lumber and plywood required to be pressure-preservative treated in accordance with Section R305.1 shall bear the quality mark of an approved inspection agency that maintains continuing supervision, testing and inspection over the quality of the product and that has been approved by an accreditation body that complies with the requirements of the American Lumber Standard Committee treated wood program.
R305.1.2 Field treatment. Field-cut ends, notches and drilled holes of pressure-preservative-treated wood shall be retreated in the field in accordance with AWPA M4.
R305.2 Chemical termiticide treatment. Chemical termiticide treatment shall include soil treatment or field-applied wood treatment. The concentration, rate of application and method of treatment of the chemical termiticide shall be in strict accordance with the termiticide label.
R305.3 Barriers. Approved physical barriers, such as metal or plastic sheeting or collars specifically designed for termite prevention, shall be installed in a manner to prevent termites from entering the structure. Shields placed on top of an exterior foundation wall shall be used only if in combination with another method of protection.
R305.4 Foam plastic protection. In areas where the probability of termite infestation is “very heavy” as indicated in Figure R305.4, extruded and expanded polystyrene, polyisocyanurate and other foam plastics shall not be installed on the exterior face or under interior or exterior foundation walls or slab foundations located below grade. The clearance between foam plastics installed above grade and exposed earth shall be not less than 6 inches (152 mm).
Exceptions:
Buildings where the structural members of walls, floors, ceilings and roofs are entirely of noncombustible materials or pressure-preservative-treated wood.
Where in addition to the requirements of Section R305.1, an approved method of protecting the foam plastic and structure from subterranean termite damage is used.
On the interior side of basement walls.
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FIGURE R305.4—TERMITE INFESTATION PROBABILITY MAP
Note: Lines defining areas are approximate only. Local conditions may be more or less severe than indicated by the region classification.
SECTION R306—FLOOD-RESISTANT CONSTRUCTION
R306.1 General. Buildings and structures constructed in whole or in part in flood hazard areas established in Table R301.2, and substantial improvement and repair of substantial damage of buildings and structures located in whole or in part in flood hazard areas, shall be designed and constructed in accordance with the provisions contained in this section. Buildings and structures that are located in more than one flood hazard area, including A Zones, Coastal A Zones and V Zones, shall comply with the provisions associated with the most restrictive flood hazard area. Buildings and structures located in whole or in part in identified floodways shall be designed and constructed in accordance with ASCE 24.
CRC § 5-1 Medium relevance — show source text
The state agency does not adopt sections identified with the following symbol: †
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5-2 2025 CALIFORNIA RESIDENTIAL CODE
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5 FLOORS
User notes:
About this chapter: Chapter 5 provides the requirements for the design and construction of floor systems that will be capable of supporting minimum required design loads. This chapter covers wood floor framing, wood floors on the ground, cold-formed steel floor framing and concrete slabs on the ground. Allowable span tables are provided that greatly simplify the determination of joist, girder and sheathing sizes for raised floor systems of wood framing and cold-formed steel framing. This chapter also contains prescriptive requirements for wood-framed exterior decks and their attachment to the main building.
SECTION R501—GENERAL
R501.1 Application. The provisions of this chapter shall control the design and construction of the floors for buildings, including the floors of attic spaces used to house mechanical or plumbing fixtures and equipment.
R501.2 Requirements. Floor construction shall be capable of accommodating all loads in accordance with Section R301 and of transmitting the resulting loads to the supporting structural elements.
SECTION R502—WOOD FLOOR FRAMING
R502.1 General. Wood and wood-based products used for load-supporting purposes shall conform to the applicable provisions of this section.
R502.1.1 Sawn lumber. Sawn lumber shall be identified by a grade mark of an accredited lumber grading or inspection agency and have design values certified by an accreditation body that complies with DOC PS 20. In lieu of a grade mark, a certificate of inspection issued by a lumber grading or inspection agency meeting the requirements of this section shall be accepted.
Note: See Section R301.1.1.1 for limited-density owner- built rural dwellings.
R502.1.1.1 Preservative-treated lumber. Preservative treated dimension lumber shall be identified as required by Section R304.2.
R502.1.1.2 End-jointed lumber. Approved end-jointed lumber identified by a grade mark conforming to Section R502.1.1 shall be permitted to be used interchangeably with solid-sawn members of the same species and grade. End-jointed lumber used in an assembly required elsewhere in this code to have a fire-resistance rating shall have the designation “Heat-Resistant Adhesive” or “HRA” included in its grade mark.
R502.1.2 Prefabricated wood I-joists. Structural capacities and design provisions for prefabricated wood I-joists shall be established and monitored in accordance with ASTM D5055.
R502.1.3 Structural glued-laminated timbers. Glued-laminated timbers shall be manufactured and identified as required in ANSI A190.1, ANSI 117 and ASTM D3737.
R502.1.4 Structural log members. Structural log members shall comply with the provisions of ICC 400.
R502.1.5 Structural composite lumber. Structural capacities for structural composite lumber shall be established and monitored in accordance with ASTM D5456.
R502.1.6 Cross-laminated timber. Cross-laminated timber shall be manufactured and identified as required by ANSI/APA PRG 320.
CRC § 1.6 Medium relevance — show source text
Foam plastics R302.8 Insulation R302.10, R302.14 Penetrations R302.4
Townhouse separation R302.2 Two-family dwellings R302.3 Under-stair protection R302.7 Wall and ceiling finishes R302.9 Fire-Separation Distance Exterior walls and wall projections R302.1 Ground-mounted photovoltaic systems R329.7, R329.8 Fixtures
Clearance R327.1
Flood protection R306.1.6 Plumbing fixture, general R326, R327 Flame Spread Index R302.10, R302.9 Flashing R703.4, R703.8.5, R903.2, R905 Flood Protection Plan Areas protected by the facilities of Central Valley Appendix CK Flood-Resistant Construction
General and structural R301.2.4, R306 Floors
Bathtub and shower R327.2 Concrete (on ground) R506 Loads R301, R501.2 Steel framing R505 Treated wood (on ground) R504
Wood framing R502 Flue
Area R1003.14, R1003.15 Lining R1003.11, R1003.12 Multiple R1003.13 Foam Plastics R303 Footings R403 Foundations Chapter 4 Cripple walls R602.10.9, R602.11.2, R602.9
Frost protection R403.1.4.1 Inspection R109.1.1 Walls R404
Frame
Inspection R109.1.4 Frost Protection R507.3, R507.3.3, R1001.2 Fuel Cell Power System, Stationary R322 Fuel-Burning Appliances (see Appliances)
Garbage Disposal (see Food-Waste Disposers) Garages R317, R330.8.1 Electric vehicle charging infrastructure R317.6 Protection of Energy Storage Systems R330.8
Generators R331
Girders R502.5 Glazing Aggregate area R325.1 Greenhouses R324.6.6 Protection of openings R301.2.1.2 Safety glazing at hazardous locations R324
Site-built R324.5 Skylights and sloped glazing R324.6 Grade
Of lumber R502.1.1, R602.1.1, R802.1.1 Grade Plane R101.2, R301.2.2.7, R316.3 Ground Floors (on ground) R504, R506 Guards or Guardrail R321, R324.4.4 Exterior R507.10 Guestroom (see Lodging House) Gypsum Board and Gypsum Panel Products R702.3
Hallways R318.6 Hand Shower (see Shower) Handrails R318.7.8, R318.8.3, R320, R324.4.4 Alternating tread devices R318.7.12.2 Ship’s ladders R318.7.13.2 Hazardous Location R324
Headers
SIP R610.8
Steel R603.6
Wood R602.7
CRC § 12.7 Medium relevance — show source text
R303.8 Wind resistance. Foam plastic insulation complying with ASTM C578 and ASTM C1289 and used as exterior wall sheathing on framed wall assemblies shall comply with ANSI/ABTG FS 100 for wind pressure resistance unless installed directly over or under a sheathing material that is separately capable of resisting the wind load or otherwise exempted from the scope of ANSI/ABTG FS 100.
SECTION R304—PROTECTION OF WOOD AND WOOD-BASED PRODUCTS AGAINST DECAY
R304.1 Location required. Protection of wood and wood-based products from decay shall be provided in the following locations by the use of naturally durable wood or wood that is preservative-treated in accordance with AWPA U1.
- In crawl spaces or unexcavated areas located within the periphery of the building foundation, wood joists or the bottom of a wood structural floor where closer than 18 inches (457 mm) to exposed ground, wood girders where closer than 12 inches (305 mm) to exposed ground, and wood columns where closer than 8 inches (204 mm) to exposed ground.
- Wood framing members, including columns, that rest directly on concrete or masonry exterior foundation walls and are less than 8 inches (203 mm) from the exposed ground.
- Sills and sleepers on a concrete or masonry slab that is in direct contact with the ground unless separated from such slab by an impervious moisture barrier.
- The ends of wood girders entering exterior masonry or concrete walls having clearances of less than [1] / 2 inch (12.7 mm) on tops, sides and ends.
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Wood siding, sheathing and wall framing on the exterior of a building having a clearance of less than 6 inches (152 mm) from the ground or less than 2 inches (51 mm) measured vertically from concrete steps, porch slabs, patio slabs and similar horizontal surfaces exposed to the weather.
Wood structural members supporting moisture-permeable floors or roofs that are exposed to the weather, such as concrete or masonry slabs, unless separated from such floors or roofs by an impervious moisture barrier. The impervious moisture barrier system protecting the structure supporting floors shall provide positive drainage of water that infiltrates the moisture- permeable floor topping.
Wood furring strips or other wood framing members attached directly to the interior of exterior masonry walls or concrete walls below grade except where an approved vapor retarder is applied between the wall and the furring strips or framing members.
Portions of wood structural members that form the structural supports of buildings, decks, balconies, porches or similar permanent building appurtenances where those members are exposed to the weather without adequate protection from a roof, eave, overhang or other covering that prevents moisture or water accumulation on the surface or at joints between members. Exception: Sawn lumber used in structures located in a geographical region where experience has demonstrated that climatic conditions preclude the need to use naturally durable or preservative-treated wood where the structure is exposed to the weather.
Wood columns in contact with basement floor slabs unless supported by concrete piers or metal pedestals projecting not less than 1 inch (25 mm) above the concrete floor and separated from the concrete pier by an impervious moisture barrier.
Frequently asked questions
What thickness of gravel and polyethylene does the CRC require under on‑grade wood floors?
The CRC requires a minimum 4‑inch (102 mm) granular base (gravel ≤ 3/4 in or crushed stone ≤ 1/2 in) and 6‑mil (0.15 mm) polyethylene sheeting with 6‑inch (152 mm) lap joints (§ R504.2.1, § R504.2.2) .
Must all sleepers and joists be preservative treated?
Yes. Sleepers, joists, blocking and plywood subflooring for floors on ground must be pressure‑preservative treated and dried after treatment in accordance with AWPA U1, and bear the treating quality mark (§ R504.3, § R304.2) .
Can I seal the laps of the polyethylene vapor barrier?
No. The Code specifies that vapor barrier joints shall be lapped 6 inches and left unsealed (§ R504.2.2) .
What if the exterior fills differ by more than 2 feet?
If the differential depth of fill on opposite exterior foundation walls exceeds 2 ft (610 mm), special engineered provisions are required to resist sliding — the prescriptive allowance of § R504.1.1 no longer applies and an engineered solution is necessary (§ R504.1.1) .
Are ordinary nails acceptable for fastening treated sleepers and subfloor?
No. Fasteners and connectors in contact with preservative‑treated wood must be corrosion‑resistant per § R304.3 (hot‑dipped galvanized, stainless, silicon bronze or copper per the listed requirements) to avoid premature corrosion and failure .
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