CRC · California Residential Code
Underlayment and finished flooring fastening requirements
For homeowners: the CRC requires underlayment panels (plywood, particleboard, fiber‑cement, etc.) to be attached to the framing using the exact fastener type, length, corrosion resistance and spacing shown in Table R602.3(2). That means using the specified nail or staple size and placing fasteners at the table’s **edge** and **field** on‑center spacing (and different prescriptions apply when the finish is tile). Always follow the table entries in **§ R602.3(2)** and the table footnotes.
Last reviewed: July 6, 2026
What the code requires — plain English
The California Residential Code requires that underlayment panels (plywood, particleboard, hardboard, fiber‑cement, etc.) be fastened with the specific fastener type, length, corrosion resistance, and spacing listed in Table R602.3(2). Follow the table for the panel nominal thickness, the prescribed fastener (nail, screw or staple) and the edges vs field/body spacing; where the table includes alternative fasteners (nails vs. staples) use the spacing shown for that fastener. See § R602.3(2) for the table entries and related notes.
The single most important rule: use the exact fastener type, length and on‑center spacing specified in § R602.3(2) for the underlayment material and thickness you are installing.
Requirements in detail
Scope and controlling text
- The detailed fastening prescriptions for common floor underlayment materials appear in Table R602.3(2) (§ R602.3(2)) — this is the controlling reference for underlayment fastening.
- Installation of wood structural panels used as subfloor or combination subfloor/underlayment is required to follow Table R602.3(1) and R602.3(2) as applicable; see § R503.2.3.
Key defined terms (first mention)
- Underlayment — the panel sheeting placed over the subfloor to receive finished flooring (plywood, particleboard, hardboard, fiber‑cement, etc.) as regulated in § R602.3(2).
- Finished flooring — the final floor covering (hardwood, tile, resilient, etc.) placed over underlayment; Table R602.3(2) differentiates fasteners for underlayment when the finished flooring is tile vs. other finishes.
- Edges (of panel) — the perimeter rows/lines of fasteners at panel edges; the table specifies a tighter spacing at edges than in the body (field) of the panel.
- Body (field) — the interior area of the panel away from edges; the table gives the field (body) spacing.
Decision‑relevant values (summary table)
The table below extracts the most commonly used lines from § R602.3(2). Always refer to the complete table in the code for additional thicknesses/materials and any footnotes.
| Material | Nominal thickness | Permitted fastener (type & length) | Edge spacing (in.) | Field/body spacing (in.) | Code Reference |
|---|---|---|---|---|---|
| Fiber‑cement underlayment (finished flooring other than tile) | 1/4" | 1-1/4" long × 0.099" corrosion‑resistant ring‑shank nails | 3 | 6 | § R602.3(2) |
| Fiber‑cement (for tile finish) | 1/4" | 1-1/4" × .121 shank × .375 head diameter corrosion‑resistant roofing nails (or No. 8 × .375 ribbed wafer‑head screws) | 8 | 8 | § R602.3(2) |
| Plywood underlayment | 1/4" & 5/16" | 1-1/4" ring or screw‑shank nail, min 12½ ga. (0.099" shank dia.) | 3 | 6 | § R602.3(2) |
| Plywood underlayment (alt.) | 1/4" & 5/16" | Staple 18 ga., 7/8" long, 3/16" crown | 2 | 5 | § R602.3(2) |
| Hardboard underlayment | 0.200" | 1-1/2" × 0.080" ring‑grooved shank underlayment nail (or 1-3/8" polymer cement‑coated sinker) | 6 | 6 | § R602.3(2) |
| Particleboard underlayment | 1/4" | 1-1/2" × 0.099" ring‑grooved shank underlayment nail | 3 | 6 | § R602.3(2) |
| Particleboard | 3/8" or 1/2" | 2" × 0.120" ring‑grooved shank underlayment nail | 6 | 10 (for some thicknesses) | § R602.3(2) |
Notes on the table entries:
- Many entries specify corrosion‑resistant fasteners (galvanized or stainless); where the table word "corrosion‑resistant" is used, the fastener must meet that requirement.
- Staples are an allowed alternate where the table shows them; the table also specifies minimum crown widths for staples and minimum nail gauges — follow the listed gauge and crown requirements.
- Where tile is the finish, Table R602.3(2) prescribes different fasteners (roofing nails or specific screws) and wider spacing in many cases; do not assume the same fasteners as for non‑tile finishes.
Footnotes and spacing limits that affect fastening
- For panels spanning 48 inches or greater, the code requires nails or staples at not more than 6 inches on center at all supports; intermediate supports: not more than 12 inches o.c. for floors unless the table prescribes otherwise. This affects the chosen spacing when panels are installed over wide spans.
- The code permits staples with equivalent holding power and penetration as an alternate to nails (see table notes); check the crown width and gauge requirements in the table.
Exceptions & special cases
- Unsupported panel edges: panel edges must be tongue‑and‑groove or supported by blocking, unless a minimum 1/4" wood panel‑type underlayment or fiber‑cement underlayment (with joint offsets specified), 1-1/2" lightweight/cellular concrete over the subfloor, or 3/4" wood finish flooring installed at right angles to the supports is provided. See the table notes in R503/R602.
- Tile finishes: where finished flooring is tile, Table R602.3(2) often prescribes roofing nails or specific wafer‑head screws and different spacing (commonly 8" o.c.). Use the tile‑finish row in the table — do not use the non‑tile spacing when tile is specified.
- Wind or other structural provisions that reference Table R602.3(1): when panels are serving structural functions (sheathing resisting wind) other tables and sections apply — Table R602.3(2) is the “alternate attachments” table for underlayment; consult the other tables and sections where applicable.
Common mistakes
- Using the wrong fastener length or gauge. The table lists both minimum shank diameter (nail gauge) or head dimensions for roofing nails/screws — do not substitute shorter/ thinner fasteners.
- Missing the corrosion‑resistant requirement. Several underlayment entries explicitly require corrosion‑resistant fasteners; failing to use them (especially in high‑moisture areas) violates the table.
- Applying the same spacing for edges and field. The edge spacing is frequently tighter than the field spacing — installing only field spacing at the edges is non‑compliant.
- Confusing underlayment fastening with finished flooring fasteners. Table R602.3(2) governs how the underlayment panel is attached to framing; finished flooring attachment (nailing of hardwood, setting of tile) is a separate operation and may have manufacturer/installation requirements beyond the table.
- Ignoring span or support notes. Panels with long spans (e.g., 48" o.c.) may require closer fastener spacing at supports (see table footnotes) or different panel thickness.
Worked example — concrete scenario
Scenario: Installing 1/4" plywood underlayment under a finished hardwood floor (not tile) over joists at 16" o.c. You choose the nail option from the table.
- Table line for plywood 1/4" calls for 1-1/4" ring or screw‑shank nails, min 12½ ga. (0.099" shank dia.), with edges 3" o.c. and field 6" o.c. (see § R602.3(2)).
- Practical application for one 4' × 8' sheet:
- Perimeter/edge fasteners: the panel perimeter is 24 ft (288 in). At 3" o.c., perimeter fasteners ≈ 288 / 3 = 96 fasteners (distributed along edges).
- Field grid (6" o.c. both directions) = one fastener per 6"×6" = 4 fasteners/sqft. For a 32 sqft panel: 32 × 4 = 128 field fasteners. (The perimeter rows overlap the grid; the code expects the specified edge spacing in addition to the field pattern — follow the manufacturer/code practice when placing the final layout.)
- Nail type/finish: use corrosion‑resistant ring‑shank nails per the table (galvanized or stainless as appropriate).
This example shows why planning fastener counts and using the correct fastener type and spacing matters — the code‑required pattern is dense compared with some installer practices.
Related provisions
- § R503.2.3 — Installation: wood structural panels used as subfloor/underlayment shall be attached in accordance with Table R602.3(1) and related tables.
- § R503.2.2 — Allowable spans for wood structural panels used as subfloor/underlayment (see tables R503.2.1.1).
- § R602.3(2) — Table with alternate attachments (the controlling table for underlayment fastening).
- Table R503.2.1.1 / Table R503.2.1.1(2) — allowable spans and thickness selection for combination subfloor/underlayment.
- Table R602.3(1) — primary fastening table for wood structural panels (used in conjunction with the alternate Table R602.3(2)).
Code references
Grounded in the retrieved California Residential Code — click a citation to read the verbatim passage:
CRC § 1.5 High relevance — show source text
Maximum span 24 inches where3/4-inch wood finish flooring is installed at right angles to joists.
i. Maximum span 24 inches where 1.5 inches of lightweight concrete or approved cellular concrete is placed over the subfloor.
j. Unsupported edges shall have tongue-and-groove joints or shall be supported with blocking unless minimum nominal1/4-inch-thick wood panel-type underlayment, fiber-
cement underlayment with end and edge joints offset not less than 2 inches or 11/2 inches of lightweight concrete or approved cellular concrete is placed over the subfloor, or
3/4-inch wood finish flooring is installed at right angles to the supports. 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.
k. Unsupported edges shall have tongue-and-groove joints or shall be supported by blocking unless nominal1/4-inch-thick wood panel-type underlayment, fiber-cement under-
layment with end and edge joints offset not less than 2 inches or3/4-inch wood finish flooring is installed at right angles to the supports. 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, except panels with a span rating
of 48 on center are limited to 65 psf total uniform load at maximum span.
l. Allowable live load values at spans of 16 inches on center and 24 inches on center taken from referenced standard APA E30,APA Engineered Wood Construction Guide. Refer to
referenced standard for allowable spans not listed in the table.|For SI: 1 inch = 25.4 mm, 1 pound per square foot = 0.0479 kPa.
a. The allowable total loads were determined using a dead load of 10 psf. If the dead load exceeds 10 psf, then the live load shall be reduced accordingly.
b. Panels continuous over two or more spans with long dimension (strength axis) perpendicular to supports. Spans shall be limited to values shown because of possible effect of
concentrated loads.
c. Applies to panels 24 inches or wider.
d. Lumber blocking, panel edge clips (one midway between each support, except two equally spaced between supports where span is 48 inches), tongue-and-groove panel
edges, or other approved type of edge support.
e. Includes Structural I panels in these grades.
f. Uniform load deflection limitation:1/180 of span under live load plus dead load,1/240 of span under live load only.
g. Maximum span 24 inches for15/32- and1/2-inch panels.
h. Maximum span 24 inches where3/4-inch wood finish flooring is installed at right angles to joists.
i. Maximum span 24 inches where 1.5 inches of lightweight concrete or approved cellular concrete is placed over the subfloor.
j.CRC § 0.42 High relevance — show source text
Nails or staples shall be spaced at not more than 12
inches on center at intermediate supports for floors.
d. Fasteners shall be placed in a grid pattern throughout the body of the panel.
e. For 5-ply panels, intermediate nails shall be spaced not more than 12 inches on center each way.
f. Hardboard underlayment shall conform to CPA/ANSI A135.4.
g. Alternate fastening is only permitted for roof sheathing where the ultimate design wind speed is less than or equal to 110 mph, and where fasteners are installed 3 inches on
center at all supports, and where fastening is to wood framing of a species with specific gravity greater than or equal to 0.42 in accordance with AWC NDS.
h. Fiber-cement underlayment shall conform to ASTM C1288 or ISO 8336, Category C.|For SI: 1 inch = 25.4 mm.
a. Nail is a general description and shall be permitted to be T-head, modified round head or round head.
b. Staples shall have a minimum crown width of7/16-inch except as noted.
c. Nails or staples shall be spaced at not more than 6 inches on center at all supports where spans are 48 inches or greater. Nails or staples shall be spaced at not more than 12
inches on center at intermediate supports for floors.
d. Fasteners shall be placed in a grid pattern throughout the body of the panel.
e. For 5-ply panels, intermediate nails shall be spaced not more than 12 inches on center each way.
f. Hardboard underlayment shall conform to CPA/ANSI A135.4.
g. Alternate fastening is only permitted for roof sheathing where the ultimate design wind speed is less than or equal to 110 mph, and where fasteners are installed 3 inches on
center at all supports, and where fastening is to wood framing of a species with specific gravity greater than or equal to 0.42 in accordance with AWC NDS.
h. Fiber-cement underlayment shall conform to ASTM C1288 or ISO 8336, Category C.|For SI: 1 inch = 25.4 mm.
a. Nail is a general description and shall be permitted to be T-head, modified round head or round head.
b. Staples shall have a minimum crown width of7/16-inch except as noted.
c. Nails or staples shall be spaced at not more than 6 inches on center at all supports where spans are 48 inches or greater. Nails or staples shall be spaced at not more than 12
inches on center at intermediate supports for floors.
d. Fasteners shall be placed in a grid pattern throughout the body of the panel.
e. For 5-ply panels, intermediate nails shall be spaced not more than 12 inches on center each way.
f. Hardboard underlayment shall conform to CPA/ANSI A135.4.
g. Alternate fastening is only permitted for roof sheathing where the ultimate design wind speed is less than or equal to 110 mph, and where fasteners are installed 3 inches on
center at all supports, and where fastening is to wood framing of a species with specific gravity greater than or equal to 0.42 in accordance with AWC NDS.
h. Fiber-cement underlayment shall conform to ASTM C1288 or ISO 8336, Category C.||TABLE R602.
CRC § 0.083 High relevance — show source text
Minimum fastener size. Corrosion-resistant nails not less than No. 11 gage with5/16-inch head. Fasteners shall be long enough to penetrate into the sheathing3/4 inch or
through the thickness of the sheathing, whichever is less. Attaching wire for clay and concrete tile shall not be smaller than 0.083 inch.
b. Snow areas. Not fewer than two fasteners per tile are required or battens and one fastener.
c. Roof slopes greater than 24:12. The nose of all tiles shall be securely fastened.
d. Horizontal battens. Battens shall be not less than 1 inch by 2 inches nominal. Provisions shall be made for drainage by a riser of not less than1/8 inch at each nail or by 4-foot-
long battens with not less than a1/2-inch separation between battens. Horizontal battens are required for slopes over 7:12.
e. Perimeter fastening areas include three tile courses but not less than 36 inches from either side of hips or ridges and edges of eaves and_gable_ rakes.
f._ Vasd_ shall be determined in accordance with Section 1609.3.1.|15-12 2025 CALIFORNIA BUILDING CODE
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
ROOF ASSEMBLIES AND ROOFTOP STRUCTURES
1507.3.8 Application. Tile shall be applied according to the manufacturer’s installation instructions, based on the following:
Climatic conditions.
Roof slope.
Underlayment system.
Type of tile being installed.
1507.3.9 Flashing. At the juncture of the roof vertical surfaces, flashing and counterflashing shall be provided in accordance with the manufacturer’s installation instructions, and where of metal, shall be not less than 0.019-inch (0.48 mm) (No. 26 galvanized sheet gage) corrosion-resistant metal. The valley flashing shall extend not less than 11 inches (279 mm) from the centerline each way and have a splash diverter rib not less than 1 inch (25 mm) high at the flow line formed as part of the flashing. Sections of flashing shall have an end lap of not less than 4 inches (102 mm). For roof slopes of three units vertical in 12 units horizontal (25-percent slope) and over, the valley flashing shall have a 36-inch-wide (914 mm) underlayment of either one layer of Type I underlayment running the full length of the valley, or a self-adhering polymer-modified bitumen sheet bearing a label indicating compliance with ASTM D1970, in addition to other required underlayment. In areas where the average daily temperature in January is 25°F (-4°C) or less or where there is a possibility of ice forming along the eaves causing a backup of water, the metal valley flashing underlayment shall be solid cemented to the roofing underlayment for slopes under seven units vertical in 12 units horizontal (58-percent slope) or self-adhering polymer-modified bitumen sheet shall be installed.
CRC § 0.42 High relevance — show source text
Nail is a general description and shall be permitted to be T-head, modified round head or round head.
b. Staples shall have a minimum crown width of7/16-inch except as noted.
c. Nails or staples shall be spaced at not more than 6 inches on center at all supports where spans are 48 inches or greater. Nails or staples shall be spaced at not more than 12
inches on center at intermediate supports for floors.
d. Fasteners shall be placed in a grid pattern throughout the body of the panel.
e. For 5-ply panels, intermediate nails shall be spaced not more than 12 inches on center each way.
f. Hardboard underlayment shall conform to CPA/ANSI A135.4.
g. Alternate fastening is only permitted for roof sheathing where the ultimate design wind speed is less than or equal to 110 mph, and where fasteners are installed 3 inches on
center at all supports, and where fastening is to wood framing of a species with specific gravity greater than or equal to 0.42 in accordance with AWC NDS.
h. Fiber-cement underlayment shall conform to ASTM C1288 or ISO 8336, Category C.|For SI: 1 inch = 25.4 mm.
a. Nail is a general description and shall be permitted to be T-head, modified round head or round head.
b. Staples shall have a minimum crown width of7/16-inch except as noted.
c. Nails or staples shall be spaced at not more than 6 inches on center at all supports where spans are 48 inches or greater. Nails or staples shall be spaced at not more than 12
inches on center at intermediate supports for floors.
d. Fasteners shall be placed in a grid pattern throughout the body of the panel.
e. For 5-ply panels, intermediate nails shall be spaced not more than 12 inches on center each way.
f. Hardboard underlayment shall conform to CPA/ANSI A135.4.
g. Alternate fastening is only permitted for roof sheathing where the ultimate design wind speed is less than or equal to 110 mph, and where fasteners are installed 3 inches on
center at all supports, and where fastening is to wood framing of a species with specific gravity greater than or equal to 0.42 in accordance with AWC NDS.
h. Fiber-cement underlayment shall conform to ASTM C1288 or ISO 8336, Category C.|For SI: 1 inch = 25.4 mm.
a. Nail is a general description and shall be permitted to be T-head, modified round head or round head.
b. Staples shall have a minimum crown width of7/16-inch except as noted.
c. Nails or staples shall be spaced at not more than 6 inches on center at all supports where spans are 48 inches or greater. Nails or staples shall be spaced at not more than 12
inches on center at intermediate supports for floors.
d. Fasteners shall be placed in a grid pattern throughout the body of the panel.
e. For 5-ply panels, intermediate nails shall be spaced not more than 12 inches on center each way.
f. Hardboard underlayment shall conform to CPA/ANSI A135.4.
g.CRC § 1.1 High relevance — show source text
Distortions
in the underlayment shall not interfere with the ability of the
shingles to seal. End laps shall be 4 inches and shall be offset
by 6 feet.
2. For roof slopes of 4 units vertical in 12 units horizontal (4:12)
or greater, underlayment shall be one layer applied in the
following manner: underlayment shall be applied shingle
fashion, parallel to and starting from the eave and lapped 2
inches. Distortions in the underlayment shall not interfere
with the ability of the shingles to seal. End laps shall be 4
inches and shall be offset by 6 feet.
3. A single layer of self-adhering polymer modified bitumen
underlayment complying with ASTM D1970, installed in
accordance with the underlayment and roof covering manu-
facturer’s installation instructions for the deck material, roof
ventilation configuration and climate exposure of the roof
covering.|Underlayment shall be one of the following:
1. Two layers of mechanically fastened underlayment applied in
the following manner: Apply a strip of underlayment that is
half the width of a full sheet parallel to and starting at the
eaves, fastened sufficiently to hold in place. Starting at the
eave, apply full-width sheets of underlayment, overlapping
successive sheets half the width of a full sheet plus 2 inches.
Distortions in the underlayment shall not interfere with the
ability of the shingles to seal. End laps shall be 4 inches and
shall be offset by 6 feet.
2. A minimum 4-inch-wide strip of self-adhering polymer modi-
fied bitumen underlayment complying with ASTM D1970,
installed in accordance with the manufacturer’s installation
instructions for the deck material, shall be applied over all
joints in the roof decking. An approved underlayment comply-
ing with Table R905.1.1(1) for the applicable roof covering shall
be applied over the entire roof over the 4-inch- wide
membrane strips.
3. A single layer of self-adhering polymer modified bitumen
underlayment complying with ASTM D1970, installed in accor-
dance
with
the
underlayment
and
roof
covering
manufacturer’s installation instructions for the deck material,
roof ventilation configuration and climate exposure of the roof
covering.| |Clay and
concrete tile|R905.3|Underlayment shall be one of the following:
1. For roof slopes from 21/2 units vertical in 12 units horizontal
(21/2:12), up to 4 units vertical in 12 units horizontal (4:12),
underlayment shall be two layers applied in the following
manner: apply a strip of underlayment that is half the width
of a full sheet parallel to and starting at the eaves, fastened
sufficiently to hold in place. Starting at the eave, apply full-
width sheets of underlayment, overlapping successive
sheets half the width of a full sheet plus 2 inches. End laps
shall be 4 inches and shall be offset by 6 feet.
2.CRC § 25.4 High relevance — show source text
Fiber-cement underlayment shall conform to ASTM C1288 or ISO 8336, Category C.|For SI: 1 inch = 25.4 mm.
a. Nail is a general description and shall be permitted to be T-head, modified round head or round head.
b. Staples shall have a minimum crown width of7/16-inch except as noted.
c. Nails or staples shall be spaced at not more than 6 inches on center at all supports where spans are 48 inches or greater. Nails or staples shall be spaced at not more than 12
inches on center at intermediate supports for floors.
d. Fasteners shall be placed in a grid pattern throughout the body of the panel.
e. For 5-ply panels, intermediate nails shall be spaced not more than 12 inches on center each way.
f. Hardboard underlayment shall conform to CPA/ANSI A135.4.
g. Alternate fastening is only permitted for roof sheathing where the ultimate design wind speed is less than or equal to 110 mph, and where fasteners are installed 3 inches on
center at all supports, and where fastening is to wood framing of a species with specific gravity greater than or equal to 0.42 in accordance with AWC NDS.
h. Fiber-cement underlayment shall conform to ASTM C1288 or ISO 8336, Category C.|TABLE R602.3(3)—REQUIREMENTS FOR WOOD STRUCTURAL PANEL WALL SHEATHING USED TO RESIST WIND PRESSURESa, b, c Col2 Col3 Col4 Col5 Col6 Col7 Col8 Col9 Col10 ** MINIMUM NAIL** ** MINIMUM NAIL** MINIMUM
WOOD
STRUCTURAL
PANEL SPAN
RATING** MINIMUM**
NOMINAL
PANEL
THICKNESS
(inches)** MAXIMUM**
WALL
STUD
SPACING
(inches)** PANEL**
NAIL SPACING** PANEL**
NAIL SPACING** ULTIMATE DESIGN**
WIND SPEEDVult (mph)** ULTIMATE DESIGN**
WIND SPEEDVult (mph)** ULTIMATE DESIGN**
WIND SPEEDVult (mph)** Size** ** Penetration**
(inches)** Penetration**
(inches)** Penetration**
(inches)** Penetration**
(inches)** Edges**
(inches o.c.)** Field**
(inches o.c.)** Wind exposure category** ** Wind exposure category** ** Wind exposure category** ** Size** ** Penetration**
(inches)** Penetration**
(inches)** Penetration**
(inches)** Penetration**
(inches)** Edges**
(inches o.c.)** Field**
**(inches o.c.CRC § 0.097 High relevance — show source text
roofg and wall sheathing to framing and particleboard wall sheathing to framingf**| |Up to1/2|Staple 15 ga. 13/4|4|8| |Up to1/2|0.097–0.099 Nail 21/4|3|6| |Up to1/2|Staple 16 ga. 13/4|3|6| |19/32 and5/8|0.113 Nail 2|3|6| |19/32 and5/8|Staple 15 and 16 ga. 2|4|8| |19/32 and5/8|0.097–0.099 Nail 21/4|4|8| |23/32 and3/4|Staple 14 ga. 2|4|8| |23/32 and3/4|Staple 15 ga. 13/4|3|6| |23/32 and3/4|0.097–0.099 Nail 21/4|4|8| |23/32 and3/4|Staple 16 ga. 2|4|8| |1|Staple 14 ga. 21/4|4|8| |1|0.113 Nail 21/4|3|6| |1|Staple 15 ga. 21/4|4|8| |1|0.097–0.099 Nail 21/2|4|8|
2025 CALIFORNIA RESIDENTIAL CODE 6-7
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
WALL CONSTRUCTION
TABLE R602.3(2)—ALTERNATE ATTACHMENTS TO TABLE R602.3(1)—continued Col2 Col3 Col4 NOMINAL MATE-
RIAL THICKNESS
(inches)DESCRIPTIONa, b OF FASTENER AND LENGTH
(inches)SPACINGc OF FASTENERS SPACINGc OF FASTENERS NOMINAL MATE-
RIAL THICKNESS
(inches)DESCRIPTIONa, b OF FASTENER AND LENGTH
(inches)Edges
(inches)Body of paneld
(inches)Floor underlayment; plywood-hardboard-particleboardf-fiber-cementh Floor underlayment; plywood-hardboard-particleboardf-fiber-cementh Floor underlayment; plywood-hardboard-particleboardf-fiber-cementh Floor underlayment; plywood-hardboard-particleboardf-fiber-cementh Fiber-cement Fiber-cement Fiber-cement Fiber-cement 1/4 11/4 long × 0.099″ corrosion-resistant, ring shank nails (finished flooring other than tile) 3 6 1/4 Staple 18 ga.7/8 long,1/4 crown (finished flooring other than tile) 3 6 1/4 11/4 long × .121 shank × .375 head diameter corrosion-resistant
(galvanized or stainless steel) roofing nails (for tile finish)8 8 1/4 11/4 long, No. CRC § 1.5 High relevance — show source text
Lumber blocking, panel edge clips (one midway between each support, except two equally spaced between supports where span is 48 inches), tongue-and-groove panel
edges, or other approved type of edge support.
e. Includes Structural I panels in these grades.
f. Uniform load deflection limitation:1/180 of span under live load plus dead load,1/240 of span under live load only.
g. Maximum span 24 inches for15/32- and1/2-inch panels.
h. Maximum span 24 inches where3/4-inch wood finish flooring is installed at right angles to joists.
i. Maximum span 24 inches where 1.5 inches of lightweight concrete or approved cellular concrete is placed over the subfloor.
j. Unsupported edges shall have tongue-and-groove joints or shall be supported with blocking unless minimum nominal1/4-inch-thick wood panel-type underlayment, fiber-
cement underlayment with end and edge joints offset not less than 2 inches or 11/2 inches of lightweight concrete or approved cellular concrete is placed over the subfloor, or
3/4-inch wood finish flooring is installed at right angles to the supports. 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.
k. Unsupported edges shall have tongue-and-groove joints or shall be supported by blocking unless nominal1/4-inch-thick wood panel-type underlayment, fiber-cement under-
layment with end and edge joints offset not less than 2 inches or3/4-inch wood finish flooring is installed at right angles to the supports. 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, except panels with a span rating
of 48 on center are limited to 65 psf total uniform load at maximum span.
l. Allowable live load values at spans of 16 inches on center and 24 inches on center taken from referenced standard APA E30,APA Engineered Wood Construction Guide. Refer to
referenced standard for allowable spans not listed in the table.|TABLE R503.2.1.1(2)—ALLOWABLE SPANS FOR SANDED PLYWOOD COMBINATION SUBFLOOR UNDERLAYMENTa Col2 Col3 Col4 ** IDENTIFICATION** ** SPACING OF JOISTS**
(inches)** SPACING OF JOISTS**
(inches)** SPACING OF JOISTS**
(inches)** IDENTIFICATION** ** 16** ** 20** ** 24** Species groupb — — — 1 1/2 5/8 3/4 2, 3 5/8 3/4 7/8 4 3/4 7/8 1 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.CRC § 0.099 High relevance — show source text
099″ corrosion-resistant, ring shank nails (finished flooring other than tile)|3|6| |1/4|Staple 18 ga.7/8 long,1/4 crown (finished flooring other than tile)|3|6| |1/4|11/4 long × .121 shank × .375 head diameter corrosion-resistant
(galvanized or stainless steel) roofing nails (for tile finish)|8|8| |1/4|11/4 long, No. 8 × .375 head diameter, ribbed wafer-head screws (for tile finish)|8|8| |Plywood|Plywood|Plywood|Plywood| |1/4 and5/16|11/4 ring or screw shank nail-minimum 121/2 ga. (0.099″) shank diameter|3|6| |1/4 and5/16|Staple 18 ga.,7/8, 3/16 crown width|2|5| |11/32, 3/8, 15/32 and1/2|11/4 ring or screw shank nail-minimum 121/2 ga. (0.099″) shank diameter|6|8e| |19/32, 5/8, 23/32 and3/4|11/2 ring or screw shank nail-minimum 121/2 ga. (0.099″) shank diameter|6|8| |19/32, 5/8, 23/32 and3/4|Staple 16 ga. 11/2|6|8| |Hardboardf|Hardboardf|Hardboardf|Hardboardf| |0.200|11/2 long × 0.080″ ring-grooved shank underlayment nail|6|6| |0.200|13/8 long × 0.080″ polymer cement-coated sinker nail|6|6| |0.200|Staple 18 ga.,7/8 long (plastic coated)|3|6| |Particleboard|Particleboard|Particleboard|Particleboard| |1/4|11/2 long × 0.099″ ring-grooved shank underlayment nail|3|6| |1/4|Staple 18 ga.,7/8 long,3/16 crown|3|6| |3/8|2 long × 0.120″ ring-grooved shank underlayment nail|6|10| |3/8|Staple 16 ga., 11/8 long,3/8 crown|3|6| |1/2, 5/8|2 long × 0.120″ ring-grooved shank underlayment nail|6|10| |1/2, 5/8|Staple 16 ga., 15/8 long,3/8 crown|3|6| |For SI: 1 inch = 25.4 mm.
a. Nail is a general description and shall be permitted to be T-head, modified round head or round head.
b. Staples shall have a minimum crown width of7/16-inch except as noted.
c. Nails or staples shall be spaced at not more than 6 inches on center at all supports where spans are 48 inches or greater. Nails or staples shall be spaced at not more than 12
inches on center at intermediate supports for floors.
d.CRC § 0.099 High relevance — show source text
3(1)—continued Col2 Col3 Col4 NOMINAL MATE-
RIAL THICKNESS
(inches)DESCRIPTIONa, b OF FASTENER AND LENGTH
(inches)SPACINGc OF FASTENERS SPACINGc OF FASTENERS NOMINAL MATE-
RIAL THICKNESS
(inches)DESCRIPTIONa, b OF FASTENER AND LENGTH
(inches)Edges
(inches)Body of paneld
(inches)Floor underlayment; plywood-hardboard-particleboardf-fiber-cementh Floor underlayment; plywood-hardboard-particleboardf-fiber-cementh Floor underlayment; plywood-hardboard-particleboardf-fiber-cementh Floor underlayment; plywood-hardboard-particleboardf-fiber-cementh Fiber-cement Fiber-cement Fiber-cement Fiber-cement 1/4 11/4 long × 0.099″ corrosion-resistant, ring shank nails (finished flooring other than tile) 3 6 1/4 Staple 18 ga.7/8 long,1/4 crown (finished flooring other than tile) 3 6 1/4 11/4 long × .121 shank × .375 head diameter corrosion-resistant
(galvanized or stainless steel) roofing nails (for tile finish)8 8 1/4 11/4 long, No. 8 × .375 head diameter, ribbed wafer-head screws (for tile finish) 8 8 Plywood Plywood Plywood Plywood 1/4 and5/16 11/4 ring or screw shank nail-minimum 121/2 ga. (0.099″) shank diameter 3 6 1/4 and5/16 Staple 18 ga.,7/8, 3/16 crown width 2 5 11/32, 3/8, 15/32 and1/2 11/4 ring or screw shank nail-minimum 121/2 ga. (0.099″) shank diameter 6 8e 19/32, 5/8, 23/32 and3/4 11/2 ring or screw shank nail-minimum 121/2 ga. (0.099″) shank diameter 6 8 19/32, 5/8, 23/32 and3/4 Staple 16 ga. CRC § 1.1 High relevance — show source text
For roof slopes of 4 units vertical in 12 units horizontal (4:12)
or greater, underlayment shall be one layer applied in the
following manner: underlayment shall be applied shingle
fashion, parallel to and starting from the eave and lapped 2
inches. End laps shall be 4 inches and shall be offset by 6 feet.
3. A single layer of self-adhering polymer modified bitumen
underlayment complying with ASTM D1970, installed in
accordance with the underlayment and roof covering manu-
facturer’s installation instructions for the deck material, roof
ventilation configuration and climate exposure of the roof
covering.|Underlayment shall be one of the following:
1. Two layers of mechanically fastened underlayment applied in
the following manner: Apply a strip of underlayment felt that is
half the width of a full sheet parallel to and starting at the
eaves, fastened sufficiently to hold in place. Starting at the
eave, apply full width sheets of underlayment, overlapping
successive sheets half the width of a full sheet plus 2 inches.
Distortionsin the underlayment shall not interferewith the abil-
ity of the shingles to seal. End laps shall be 4 inches and shall
be offset by 6 feet.
2. A minimum 4-inch-wide strip of self-adhering polymer modi-
fied bitumen underlayment complying with ASTM D1970,
installed in accordance with the manufacturer’s installation
instructions for the deck material, shall be applied over all
joints in the roof decking. An approved underlayment comply-
ing with Table R905.1.1(1) for the applicable roof covering shall
be applied over the entire roof over the 4-inch-wide
membrane strips.
3. A single layer of self-adhering polymer modified bitumen
underlayment complying with ASTM D1970, installed in accor-
dance
with
the
underlayment
and
roof
covering
manufacturer’s installation instructions for the deck material,
roof ventilation configuration and climate exposure of the roof
covering.| |Metal roof
shingles|R905.4|Apply in accordance with the manufacturer’s installation
instructions.|Underlayment shall be one of the following:
1. Two layers of mechanically fastened underlayment applied in
the following manner: Apply a strip of underlayment that is
half the width of a full sheet parallel to and starting at the
eaves, fastened sufficiently to hold in place. Starting at the
eave, apply full width sheets of underlayment, overlapping
successive sheets half the width of a full sheet plus 2 inches.
End laps shall be 4 inches and shall be offset by 6 feet.
2. A minimum 4-inch-wide strip of self-adhering polymer modified
bitumen underlayment complying with ASTM D1970, installed
in accordance with the manufacturer’s installation instructions
for the deck material, shall be applied over all joints in the roof
decking. An approved underlayment complying with Table
R905.1.1(1) for the applicable roof covering shall be applied
over the entire roof over the 4-inch-wide membrane strips.
3.
Frequently asked questions
How strict is the code about using corrosion‑resistant fasteners?
Very strict where the table specifies it. If the table entry includes the words corrosion‑resistant, use galvanized or stainless fasteners as required by § R602.3(2).
Can I use staples instead of nails?
Only where the table shows staples as an alternate and the table specifies the staple gauge and crown width (and penetration). Staples with equivalent holding power and penetration are allowed as alternates per the table notes.
Do I follow the same spacing if I’m installing tile over the underlayment?
No. Table R602.3(2) includes specific rows for tile finishes that often require different fasteners (roofing nails or screws) and different spacing (commonly wider); follow the tile row for that material/thickness.
What if my joist spacing is 48" o.c.?
Panels spanning 48" or greater are subject to special spacing limits (nails/staples at not more than 6" o.c. at all supports) and load/span restrictions shown in the table notes — confirm the panel span rating and follow the footnotes.
Where do I find the full table and all footnotes?
The full Table R602.3(2) and its footnotes are in the California Residential Code; see § R602.3(2) in the CRC document for every material/thickness row and explanatory notes.
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