CRC · California Residential Code
Sheathing span, diaphragm action and deflection considerations
For homeowners: the Residential Code requires you use the prescriptive span tables to pick plywood or lumber sheathing thickness and spacing (see §§ R503.1 and R503.2.1.1); ceiling and roof diaphragm lengths and fastening are set in R804. If a diaphragm deflection check is needed, the CRC references the structural standards (AWC/CBC) for the calculation.
Last reviewed: July 6, 2026
What the code requires — plain English
The California Residential Code requires that floor sheathing spans and minimum sheathing thicknesses be taken directly from the prescriptive tables: Table R503.1 for lumber sheathing and Table R503.2.1.1(1) (and related tables) for wood structural panel sheathing — see § R503.1 and § R503.2.1.1.
For diaphragm behavior (ceiling/roof diaphragms) the CRC prescribes minimum diaphragm lengths, fastening/edge blocking and maximum aspect ratios in the roof/ceiling chapters (for example ceiling diaphragm provisions and required lengths) — see § R804.3.7.1 and § R804.3.7.2. The CRC itself does not print the full engineering equations used to compute diaphragm mid‑span deflection; for that you must use the referenced wood‑diaphragm standards (AWC SDPWS) or the CBC Chapter 23 procedure.
The single most important practical rule: use the CRC span tables for sheathing thickness and maximum spans (R503 tables) and follow the CRC diaphragm minimum length and fastening rules for ceiling/roof diaphragms — if you need diaphragm deflection calculations, the CRC directs you to the referenced standards (AWC SDPWS / CBC Chapter 23).
Requirements in detail
1) Which tables control “how thick” and “how far”
The prescriptive spans and minimum thicknesses for lumber floor sheathing are set in Table R503.1 (see § R503.1). Examples: 16 in. o.c. → 5/8 in. minimum net thickness (perpendicular or diagonal per table); 24 in. o.c. → 11/16 in. perpendicular or 3/4 in. diagonal.
The prescriptive spans for wood structural panel sheathing (plywood/OSB) are in Table R503.2.1.1(1) (roof & subfloor panel span ratings and allowable loads). Use the exact panel span rating and thickness row to pick the maximum span for your application (roof vs subfloor, edge‑supported vs field support; see the table footnotes).
Below is a condensed, decision‑focused summary (always confirm the exact product/grade in the full table and footnotes):
| Condition / decision | Key values (typical) | Code Reference |
|---|---|---|
| Lumber floor sheathing — joist spacing 16 in. o.c. | Min net thickness 5/8 in. (perpendicular & diagonal per table) | § R503.1 |
| Lumber floor sheathing — joist spacing 24 in. o.c. | Min net thickness 11/16 in. (perpendicular), 3/4 in. (diagonal) | § R503.1 |
| Lumber sheathing — wide spans (48 in. or more) | 1‑1/2 in. T&G or engineered products (see table notes for Fb/E requirements) | § R503.1 |
| Wood structural panel sheathing example rows | e.g. 7/16 in. panel (span rating 24/16) → max span ≈ 24 in. (see table for roof vs subfloor and edge conditions) | § R503.2.1.1 |
| Wood structural panel sheathing thicker examples | 23/32 or 3/4 in. panel (span rating 48/24) gives larger allowable spans — consult table row for live/total load and subfloor values | § R503.2.1.1 |
Notes in the full tables control final selection (edge support, tongue‑and‑groove, underlayment, and allowable live/total loads) — always read the associated footnotes in Table R503.2.1.1.
2) Fastening, edges, blocking and “blocked diaphragm” requirements
Panel edges must be supported by tongue‑and‑groove, panel edge clips, lumber blocking, or acceptable underlayment/finish floor—the table footnotes spell which methods satisfy the edge‑support requirement and what satisfies a blocked diaphragm. Only lumber blocking is allowed to meet the blocked diaphragm requirement in many table notes. See the table footnotes referenced from § R503.2.1.1.
For ceiling diaphragms (gypsum ceiling panels), the CRC gives explicit fastener spacing and required lengths at gable endwalls; when edges are blocked or fastening spacing is increased, the required diaphragm length can be multiplied by a factor per the table notes (for example multiplying by 0.35 when all edges are blocked or 0.9 if edges are fastened more densely). See § R804.3.7.1 and its notes.
3) Diaphragm action and deflection — what CRC prescribes vs what it references
The CRC prescribes minimum diaphragm lengths, maximum aspect ratios (2:1) for certain ceiling diaphragms, and fastening/blocking patterns for ceiling and roof diaphragms (see § R804.3.7.1 and § R804.3.7.2). These are prescriptive, construction‑level rules for providing a functioning diaphragm.
The CRC does not include the full elastic‑analysis equations for diaphragm mid‑span deflection in the R502/R503 sections. For engineering calculations of diaphragm deflection (Δdia) and span‑depth limitations based on diaphragm flexibility, the accepted procedures are in the AWC SDPWS and CBC Chapter 23 (see CBC § 2305.2 and Equation 23‑1). If you must compute diaphragm elastic deflection or evaluate span‑to‑depth limits (flexible vs rigid diaphragms), follow those referenced standards.
Exceptions & special cases
Tongue‑and‑groove or approved edge‑support substitutes: some larger support spacings require T&G or higher grade lumber (see table footnotes for required Fb and E). Check the footnote for required material values when using wide spans (e.g., 48 in. support spacings). § R503.1 and the table footnotes control this.
Underlayment or finished floor changes allowable edge support conditions — the table footnotes allow alternate support where 1/4 in. underlayment or 3/4 in. wood strip finish or 1‑1/2 in. cellular/ lightweight concrete is applied over panels. See Table R503.2.1.1 footnotes.
Ceiling diaphragms: required diaphragm lengths are reduced when panel edges are blocked or when edge fastener spacing is increased — the table notes provide multiplicative factors (e.g., ×0.35) for blocked edges; verify your ceiling type against § R804.3.7.1.
Cantilevers and span‑depth ratios: when applying span‑to‑depth limitations to cantilevered diaphragms the allowable span‑to‑depth ratio is halved — see the span‑depth discussion in the referenced structural provisions (CBC / AWC) if you must analyze diaphragms for large cantilevers.
If a detail, geometry or loading falls outside the prescriptive tables, the CRC requires design by a registered design professional (RDP) — do not invent a span.
Common mistakes
Reading a panel “span rating” row without observing the associated footnotes (edge support, live/total load and blocking requirements). The table footnotes change allowable spans. See § R503.2.1.1.
Confusing lumber sheathing (Table R503.1) and panel sheathing (Table R503.2.1.1). They are different tables with different thickness/span rules — use the correct table for the material you specify.
Assuming CRC gives diaphragm elastic deflection equations in R502/R503. It does not; for Δdia calculations use the standards referenced (AWC SDPWS or CBC Chapter 23), not only the prescriptive diaphragm length/fastening rules in R804.
Using under‑thickness or non‑rated panels without confirming grade/face rating — Table R503.2.1.1 presumes specific panel grades and includes footnotes on increase/decrease factors for non‑Structural I panels. Always verify panel grade marks.
Assuming blocked diaphragms can be satisfied by any material: the tables/notes are specific that lumber blocking (not just any filler) satisfies some blocked diaphragm requirements — check the table notes.
Worked example — quick decision path (prescriptive case)
Scenario: You are specifying floor sheathing for a one‑story house. Floor joists are spaced 24 in. o.c. and the subfloor will be plywood panels. What thickness and rules apply?
Use Table R503.1 for lumber sheathing if you intended to use solid lumber sheathing; for plywood/OSB use Table R503.2.1.1(1) — the CRC separates these materials. § R503.1 tells you to follow those tables.
If you choose lumber sheathing: Table R503.1 requires minimum net thickness 11/16 in. when laid perpendicular to the joists at 24 in. o.c. (if diagonal, the table shows 3/4 in.) — so specify 11/16 in. (or 3/4" if you plan diagonal application). § R503.1
If you choose panel sheathing (plywood/OSB): identify the panel's span rating printed on the panel (for example a panel marked 24/16) and then read Table R503.2.1.1 to get maximum spans and allowable loads for subfloor service and for 16 in. o.c. vs 24 in. o.c. supports. If your panel is 7/16 in. with 24/16 span rating, the table row for the 24/16 rating governs and shows the maximum span for subfloor use — use that exact table row and footnotes to confirm acceptance. § R503.2.1.1
Fastening and edge support: confirm edge support (tongue‑and‑groove, blocking, or underlayment/finish) per the table footnotes. If you rely on blocked diaphragm behavior for lateral load transfer, use lumber blocking as required. § R503.2.1.1
If any of these items do not match the product marking, or you need to assess diaphragm deflection under seismic/shear demands, engage an RDP and use AWC SDPWS / CBC § 2305.2 analytics.
Related provisions (quick list)
- § R503.1 — Lumber sheathing; follow Tables R503.1 and related tables.
- § R503.2.1.1 — Allowable spans and loads for wood structural panels (Table R503.2.1.1(1)).
- § R502 — Wood floor framing requirements (general framing provisions and truss requirements) — see R502 chapter text for framing rules that interact with sheathing.
- § R804.3.7.1 / § R804.3.7.2 — Ceiling and roof diaphragm required lengths, fastening, aspect ratio and blocking details.
- CBC § 2305.2 (referenced for diaphragm deflection) — gives the AWC SDPWS‑based formula (Equation 23‑1) for blocked wood structural panel diaphragm deflection; use when an elastic deflection calculation is required.
- CBC 1604A / span‑depth tables — for diaphragm span‑to‑depth (flexibility) considerations used in lateral analysis if a diaphragm must be evaluated as flexible vs rigid.
Code references
Grounded in the retrieved California Residential Code — click a citation to read the verbatim passage:
CRC § 25.4 High relevance — show source text
Span is permitted to be 24 inches on center where3/4-inch wood strip flooring is installed at right angles to joist.
h. Span is permitted to be 24 inches on center for floors where 11/2 inches of cellular or lightweight concrete is applied over the panels.|For SI: 1 inch = 25.4 mm, 1 pound per square foot = 0.0479 kN/m2.
a. Applies to panels 24 inches or wider.
b. Uniform load deflection limitations1/180 of span under live load plus dead load,1/240 under live load only.
c. Panel edges shall have approved tongue-and-groove joints or shall be supported with blocking unless1/4-inch minimum thickness underlayment or 11/2 inches of approved
cellular or lightweight concrete is placed over the subfloor, or finish floor is3/4-inch wood strip. Allowable uniform load based on deflection of1/360 of span is 100 pounds per
square foot except the span rating of 48 inches on center is based on a total load of 65 pounds per square foot.
d. Allowable load at maximum span. Where the total load includes snow, use allowable stress design snow loads.
e. Tongue-and-groove edges, panel edge clips (one midway between each support, except two equally spaced between supports 48 inches on center), lumber blocking or
other. Only lumber blocking shall satisfy blocked diaphragm requirements.
f. For1/2-inch panel, maximum span shall be 24 inches.
g. Span is permitted to be 24 inches on center where3/4-inch wood strip flooring is installed at right angles to joist.
h. Span is permitted to be 24 inches on center for floors where 11/2 inches of cellular or lightweight concrete is applied over the panels.|For SI: 1 inch = 25.4 mm, 1 pound per square foot = 0.0479 kN/m2.
a. Applies to panels 24 inches or wider.
b. Uniform load deflection limitations1/180 of span under live load plus dead load,1/240 under live load only.
c. Panel edges shall have approved tongue-and-groove joints or shall be supported with blocking unless1/4-inch minimum thickness underlayment or 11/2 inches of approved
cellular or lightweight concrete is placed over the subfloor, or finish floor is3/4-inch wood strip. Allowable uniform load based on deflection of1/360 of span is 100 pounds per
square foot except the span rating of 48 inches on center is based on a total load of 65 pounds per square foot.
d. Allowable load at maximum span. Where the total load includes snow, use allowable stress design snow loads.
e. Tongue-and-groove edges, panel edge clips (one midway between each support, except two equally spaced between supports 48 inches on center), lumber blocking or
other. Only lumber blocking shall satisfy blocked diaphragm requirements.
f. For1/2-inch panel, maximum span shall be 24 inches.
g. Span is permitted to be 24 inches on center where3/4-inch wood strip flooring is installed at right angles to joist.
h. Span is permitted to be 24 inches on center for floors where 11/2 inches of cellular or lightweight concrete is applied over the panels.|For SI: 1 inch = 25.4 mm, 1 pound per square foot = 0.0479 kN/m2.
a.CRC § 1604A.3.9 High relevance — show source text
Diaphragms shall satisfy span-depth limitations based on flexibility.
2. Flexibility factor (F) is the average deflection in micro inches (10-6) or_μ_m of the diaphragm web per foot (m) of span stressed with a shear of 1 pound per foot (N/m).
3.
The total deflection_Δ of the diaphragm may be computed from the equation:Δ =Δf +Δw.
Where:
Δ_f = Flexural deflection of the diaphragm determined in the same manner as the deflection of beams. The flexural stiffness of the web of diaphragms consisting of bare steel decking_
shall be neglected.
Δ_w = Web deflection of the diaphragm may be determined solving the following equation:
Where:
L = Distance in feet (m) between the vertical resisting element (such as a shear wall) and the point to which the deflection is to be determined.
q_ave = Average shear in the diaphragm in pounds per foot (N/m) over length L.
4. When applying these limitations to cantilevered diaphragms, the allowable span-depth ratio will be half of that shown.
F
Δ_wx106_
qaveL
-----------------
=|1604A.3.9 Deflections. Deflection criteria for materials not specified shall be developed by the project architect or structural engi- neer in a manner consistent with the provisions of this section and approved by the enforcement agency.
1604 A .4 Analysis. Load effects on structural members and their connections shall be determined by methods of structural analysis that take into account equilibrium, general stability, geometric compatibility and both short- and long-term material properties.
Members that tend to accumulate residual deformations under repeated service loads shall have included in their analysis the effects of added deformations expected to occur during their service life.
Any system or method of construction to be used shall be based on a rational analysis in accordance with well-established principles of mechanics. Such analysis shall result in a system that provides a complete load path capable of transferring loads from their point of origin to the load-resisting elements.
The total lateral force shall be distributed to the various vertical elements of the lateral force-resisting system in proportion to their rigidities, considering the rigidity of the horizontal bracing system or diaphragm. Rigid elements assumed not to be a part of the lateral force-resisting system are permitted to be incorporated into buildings provided that their effect on the action of the system is considered and provided for in the design. Where a diaphragm is not permitted to be idealized as either flexible or rigid in accordance with ASCE 7 or for wood diaphragms in accordance with AWC SDPWS, the structure shall be analyzed and designed utilizing one of the following procedures:
An envelope analysis of the structure using a flexible and rigid diaphragm analysis separately and designing each component for the more severe load condition.
A semirigid diaphragm analysis and design.
Where required by ASCE 7, provisions shall be made for the increased forces induced on resisting elements of the structural system resulting from torsion due to eccentricity between the center of application of the lateral forces and the center of rigidity of the lateral force-resisting system.
2025 CALIFORNIA BUILDING CODE 16A-7
CRC § 25.4 High relevance — show source text
Applies to panels 24 inches or wider.
b. Uniform load deflection limitations1/180 of span under live load plus dead load,1/240 under live load only.
c. Panel edges shall have approved tongue-and-groove joints or shall be supported with blocking unless1/4-inch minimum thickness underlayment or 11/2 inches of approved
cellular or lightweight concrete is placed over the subfloor, or finish floor is3/4-inch wood strip. Allowable uniform load based on deflection of1/360 of span is 100 pounds per
square foot except the span rating of 48 inches on center is based on a total load of 65 pounds per square foot.
d. Allowable load at maximum span. Where the total load includes snow, use allowable stress design snow loads.
e. Tongue-and-groove edges, panel edge clips (one midway between each support, except two equally spaced between supports 48 inches on center), lumber blocking or
other. Only lumber blocking shall satisfy blocked diaphragm requirements.
f. For1/2-inch panel, maximum span shall be 24 inches.
g. Span is permitted to be 24 inches on center where3/4-inch wood strip flooring is installed at right angles to joist.
h. Span is permitted to be 24 inches on center for floors where 11/2 inches of cellular or lightweight concrete is applied over the panels.|For SI: 1 inch = 25.4 mm, 1 pound per square foot = 0.0479 kN/m2.
a. Applies to panels 24 inches or wider.
b. Uniform load deflection limitations1/180 of span under live load plus dead load,1/240 under live load only.
c. Panel edges shall have approved tongue-and-groove joints or shall be supported with blocking unless1/4-inch minimum thickness underlayment or 11/2 inches of approved
cellular or lightweight concrete is placed over the subfloor, or finish floor is3/4-inch wood strip. Allowable uniform load based on deflection of1/360 of span is 100 pounds per
square foot except the span rating of 48 inches on center is based on a total load of 65 pounds per square foot.
d. Allowable load at maximum span. Where the total load includes snow, use allowable stress design snow loads.
e. Tongue-and-groove edges, panel edge clips (one midway between each support, except two equally spaced between supports 48 inches on center), lumber blocking or
other. Only lumber blocking shall satisfy blocked diaphragm requirements.
f. For1/2-inch panel, maximum span shall be 24 inches.
g. Span is permitted to be 24 inches on center where3/4-inch wood strip flooring is installed at right angles to joist.
h. Span is permitted to be 24 inches on center for floors where 11/2 inches of cellular or lightweight concrete is applied over the panels.|For SI: 1 inch = 25.4 mm, 1 pound per square foot = 0.0479 kN/m2.
a. Applies to panels 24 inches or wider.
b. Uniform load deflection limitations1/180 of span under live load plus dead load,1/240 under live load only.
c.CRC § 25.4 Medium relevance — show source text
limitation_|3:1 or as required for deflection|5:1|As required for deflection|3:1| |Less than 1|No limitation|As required for deflection|No limitation|As required for deflection|31/2:1| |For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 plf = 14.594 N/m, 1 psi = 6894 Pa
1. Diaphragms shall satisfy span-depth limitations based on flexibility.
2. Flexibility factor (F) is the average deflection in micro inches (10-6_) or_μ_m of the diaphragm web per foot (m) of span stressed with a shear of 1 pound per foot (N/m).
3.
The total deflection_Δ of the diaphragm may be computed from the equation:Δ =Δf +Δw.
Where:
Δ_f = Flexural deflection of the diaphragm determined in the same manner as the deflection of beams. The flexural stiffness of the web of diaphragms consisting of bare steel decking_
shall be neglected.
Δ_w = Web deflection of the diaphragm may be determined solving the following equation:
Where:
L = Distance in feet (m) between the vertical resisting element (such as a shear wall) and the point to which the deflection is to be determined.
q_ave = Average shear in the diaphragm in pounds per foot (N/m) over length L.
4. When applying these limitations to cantilevered diaphragms, the allowable span-depth ratio will be half of that shown.
F
Δ_wx106_
qaveL
-----------------
=|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 plf = 14.594 N/m, 1 psi = 6894 Pa
1. Diaphragms shall satisfy span-depth limitations based on flexibility.
2. Flexibility factor (F) is the average deflection in micro inches (10-6_) or_μ_m of the diaphragm web per foot (m) of span stressed with a shear of 1 pound per foot (N/m).
3.
The total deflection_Δ of the diaphragm may be computed from the equation:Δ =Δf +Δw.
Where:
Δ_f = Flexural deflection of the diaphragm determined in the same manner as the deflection of beams. The flexural stiffness of the web of diaphragms consisting of bare steel decking_
shall be neglected.
Δ_w = Web deflection of the diaphragm may be determined solving the following equation:
Where:
L = Distance in feet (m) between the vertical resisting element (such as a shear wall) and the point to which the deflection is to be determined.
q_ave = Average shear in the diaphragm in pounds per foot (N/m) over length L.
_4.CRC § 25.4 Medium relevance — show source text
When applying these limitations to cantilevered diaphragms, the allowable span-depth ratio will be half of that shown.
F
Δ_wx106
qaveL
-----------------
=|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 plf = 14.594 N/m, 1 psi = 6894 Pa
1. Diaphragms shall satisfy span-depth limitations based on flexibility.
2. Flexibility factor (F) is the average deflection in micro inches (10-6_) or_μ_m of the diaphragm web per foot (m) of span stressed with a shear of 1 pound per foot (N/m).
3.
The total deflection_Δ of the diaphragm may be computed from the equation:Δ =Δf +Δw.
Where:
Δ_f = Flexural deflection of the diaphragm determined in the same manner as the deflection of beams. The flexural stiffness of the web of diaphragms consisting of bare steel decking_
shall be neglected.
Δ_w = Web deflection of the diaphragm may be determined solving the following equation:
Where:
L = Distance in feet (m) between the vertical resisting element (such as a shear wall) and the point to which the deflection is to be determined.
q_ave = Average shear in the diaphragm in pounds per foot (N/m) over length L.
4. When applying these limitations to cantilevered diaphragms, the allowable span-depth ratio will be half of that shown.
F
Δ_wx106_
qaveL
-----------------
=|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 plf = 14.594 N/m, 1 psi = 6894 Pa
1. Diaphragms shall satisfy span-depth limitations based on flexibility.
2. Flexibility factor (F) is the average deflection in micro inches (10-6_) or_μ_m of the diaphragm web per foot (m) of span stressed with a shear of 1 pound per foot (N/m).
3.
The total deflection_Δ of the diaphragm may be computed from the equation:Δ =Δf +Δw.
Where:
Δ_f = Flexural deflection of the diaphragm determined in the same manner as the deflection of beams. The flexural stiffness of the web of diaphragms consisting of bare steel decking_
shall be neglected.
Δ_w = Web deflection of the diaphragm may be determined solving the following equation:
Where:
L = Distance in feet (m) between the vertical resisting element (such as a shear wall) and the point to which the deflection is to be determined.
q_ave = Average shear in the diaphragm in pounds per foot (N/m) over length L.
4. When applying these limitations to cantilevered diaphragms, the allowable span-depth ratio will be half of that shown.
F
Δ_wx106_
qaveL
-----------------
=|CRC § 25.4 Medium relevance — show source text
SPAN-DEPTH LIMITATION_**|DIAPHRAGM SPAN-DEPTH LIMITATION|DIAPHRAGM SPAN-DEPTH LIMITATION|DIAPHRAGM SPAN-DEPTH LIMITATION| |FLEXIBILITY
FACTOR(F)2|MAXIMUM DIAPHRAGM
SPAN FOR MASONRY OR
CONCRETE WALLS (feet)|Rotation (torsion) Not Considered in Diaphragm|Rotation (torsion) Not Considered in Diaphragm|Rotation (torsion) Considered in Diaphragm|Rotation (torsion) Considered in Diaphragm| |FLEXIBILITY
FACTOR(F)2|MAXIMUM DIAPHRAGM
SPAN FOR MASONRY OR
CONCRETE WALLS (feet)|Masonry or Concrete Walls|Flexible Walls|Masonry or Concrete Walls|Flexible Walls| |More than 150|Not to be used|Not to be used|2:1|Not to be used|11/2:1| |70−150|200|2:1 or as required for deflection|3:1|Not to be used|2:1| |10−70|400|21/2:1 or as required for deflection|4:1|As required for deflection|21/2:1| |1−10|No limitation|3:1 or as required for deflection|5:1|As required for deflection|3:1| |Less than 1|No limitation|As required for deflection|No limitation|As required for deflection|31/2:1| |For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 plf = 14.594 N/m, 1 psi = 6894 Pa
1. Diaphragms shall satisfy span-depth limitations based on flexibility.
2. Flexibility factor (F) is the average deflection in micro inches (10-6_) or_μ_m of the diaphragm web per foot (m) of span stressed with a shear of 1 pound per foot (N/m).
3.
The total deflection_Δ of the diaphragm may be computed from the equation:Δ =Δf +Δw.
Where:
Δ_f = Flexural deflection of the diaphragm determined in the same manner as the deflection of beams. The flexural stiffness of the web of diaphragms consisting of bare steel decking_
shall be neglected.
Δ_w = Web deflection of the diaphragm may be determined solving the following equation:
Where:
L = Distance in feet (m) between the vertical resisting element (such as a shear wall) and the point to which the deflection is to be determined.
q_ave = Average shear in the diaphragm in pounds per foot (N/m) over length L.
_4.CRC § 3-82 Medium relevance — show source text
R336 Large Family Day-Care Homes . . . . . . . . . . . . . . . . . . . 3-82
R337 Materials and Construction Methods for Exterior
Wildfire Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-83
R338 Electric Vehicle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-83
2025 CALIFORNIA RESIDENTIAL CODE xxi
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
CONTENTS
R339 Reserved . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-84
R340 Pollutant Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-84
CHAPTER 4 FOUNDATIONS . . . . . . . . . . . . . . . . . . . . . . . . . 4-3
R401 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-3
R402 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-5
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
CRC § 12.3 Medium relevance — show source text
R502.12.3 Alterations to trusses. Truss members and components shall not be cut, notched, spliced or otherwise altered in any way without the approval of a registered design professional. Alterations resulting in the addition of load that exceeds the design load for the truss, shall not be permitted without verification that the truss is capable of supporting the additional loading.
R502.12.4 Truss design drawings. Truss design drawings, prepared in compliance with Section R502.12.1, shall be submitted to the building official and approved prior to installation. Truss design drawings shall be provided with the shipment of trusses delivered to the job site. Truss design drawings shall include, at a minimum, the information specified as follows:
- Slope or depth, span and spacing.
- Location of all joints.
- Required bearing widths.
- Design loads as applicable: 4.1. Top chord live load. 4.2. Top chord dead load.
4.3. Bottom chord live load.
4.4. Bottom chord dead load.
2025 CALIFORNIA RESIDENTIAL CODE 5-11
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
FLOORS
4.5. Concentrated loads and their points of application. 4.6. Controlling wind and earthquake loads. 5. Adjustments to lumber and joint connector design values for conditions of use.
Each reaction force and direction.
Joint connector type and description, such as size, thickness or gage, and the dimensioned location of each joint connector except where symmetrically located relative to the joint interface.
Lumber size, species and grade for each member.
Connection requirements for: 9.1. Truss-to-girder-truss. 9.2. Truss ply-to-ply. 9.3. Field splices.
Calculated deflection ratio, maximum description for live and total load, or both.
Maximum axial compression forces in the truss members to enable the building designer to design the size, connections and anchorage of the permanent continuous lateral bracing. Forces shall be shown on the truss drawing or on supplemental documents.
Required permanent truss member bracing location.
R502.13 Draftstopping required. Draftstopping shall be provided in accordance with Section R302.12.
R502.14 Fireblocking required. Fireblocking shall be provided in accordance with Section R302.11.
SECTION R503—FLOOR SHEATHING
R503.1 Lumber sheathing. Maximum allowable spans for lumber used as floor sheathing shall conform to Tables R503.1, R503.2.1.1(1) and R503.2.1.1(2).
CRC § 23-25 Medium relevance — show source text
L = Diaphragm length (dimension perpendicular to the direction of the applied load), in feet (mm).
v = Induced unit shear in pounds per linear foot (plf) (N/mm).
W = Diaphragm width [in the direction of applied force, in feet (mm)].
x = Distance from chord splice to nearest support, in feet (mm).
Δ c = Diaphragm chord splice slip at the induced unit shear, in inches (mm). Δ dia = Maximum mid-span diaphragm deflection determined by elastic analysis, in inches (mm).
2025 CALIFORNIA BUILDING CODE 23-25
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
WOOD
TABLE 2305.2(1)—e VALUES (inches) FOR USE IN CALCULATING
n
DIAPHRAGM AND SHEAR WALL DEFLECTION DUE TO FASTENER SLIP (Structural I)a, cCol2 LOAD PER FASTENERb (pounds) FASTENER DESIGNATIONS LOAD PER FASTENERb (pounds) 14-Ga staple × 2 inches long 60 0.011 80 0.018 100 0.028 120 0.04 140 0.053 160 0.068 For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound = 4.448 N.
a. Increase_en_ values 20 percent for plywood grades other than Structural I.
b. Load per fastener = maximum shear per foot divided by the number of fasteners per foot at interior panel edges.
c. Decrease_en_ values 50 percent for seasoned lumber (moisture content < 19 percent).For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound = 4.448 N.
a. Increase_en_ values 20 percent for plywood grades other than Structural I.
b. Load per fastener = maximum shear per foot divided by the number of fasteners per foot at interior panel edges.
c. Decrease_en_ values 50 percent for seasoned lumber (moisture content < 19 percent).TABLE 2305.2(2)—VALUES OF Gt FOR USE IN CALCULATING
DEFLECTION OF WOOD STRUCTURAL PANEL SHEAR WALLS AND DIAPHRAGMSCol2 Col3 Col4 Col5 Col6 Col7 Col8 Col9 Col10 PANEL
TYPESPAN
RATINGVALUES OFGt (lb/in. panel depth or width) VALUES OFGt (lb/in. panel depth or width) VALUES OFGt (lb/in. panel depth or width) VALUES OFGt (lb/in. panel depth or width) VALUES OFGt (lb/in. panel depth or width) VALUES OFGt (lb/in. panel depth or width) VALUES OFGt (lb/in. panel depth or width) VALUES OF**Gt (lb/in. CRC § 2305.1.3 Medium relevance — show source text
2305.1.3 Additional requirements. [DSA-SS, DSA-SS/CC, OSHPD 1, 1R, 2, 4 & 5] See Section 2301.1.5 for modifications to AWC SDPWS.
2305.2 Diaphragm deflection. The deflection of wood-frame diaphragms shall be determined in accordance with AWC SDPWS. The deflection (Δ dia ) of a blocked wood structural panel diaphragm uniformly fastened throughout with staples is permitted to be calculated in accordance with Equation 23-1. If not uniformly fastened, the constant 0.188 (For SI: 1/1627) in the third term shall be modified by an approved method. Equation 23-1 Δ dia = 5 vL [3] /8 EAW + vL /4 Gt + 0.188 Le n + Σ( x Δ c )/2 W For SI: Δ dia = 0.052 vL [3] / EAW + vL /4 Gt + Le n /1627 + Σ( x Δ c )/2 W
where:
A = Area of chord cross section, in square inches (mm [2] ). E = Modulus of elasticity of diaphragm chords, in pounds per square inch (N/mm [2] ).
e n = Staple slip, in inches (mm) [see Table 2305.2(1)].
Gt = Panel rigidity through the thickness, in pounds per inch (N/mm) of panel width or depth [see Table 2305.2(2)].
L = Diaphragm length (dimension perpendicular to the direction of the applied load), in feet (mm).
v = Induced unit shear in pounds per linear foot (plf) (N/mm).
W = Diaphragm width [in the direction of applied force, in feet (mm)].
x = Distance from chord splice to nearest support, in feet (mm).
Δ c = Diaphragm chord splice slip at the induced unit shear, in inches (mm). Δ dia = Maximum mid-span diaphragm deflection determined by elastic analysis, in inches (mm).
2025 CALIFORNIA BUILDING CODE 23-25
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
WOOD
TABLE 2305.2(1)—e VALUES (inches) FOR USE IN CALCULATING
n
DIAPHRAGM AND SHEAR WALL DEFLECTION DUE TO FASTENER SLIP (Structural I)a, cCol2 LOAD PER FASTENERb (pounds) FASTENER DESIGNATIONS LOAD PER FASTENERb (pounds) 14-Ga staple × 2 inches long 60 0.011 80 0.018 100 0.028 120 0.04 140 0.053 160 0.068 For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound = 4.448 N.
a. Increase_en_ values 20 percent for plywood grades other than Structural I.
b.CRC § 2.1 Medium relevance — show source text
DCR = 2.1 S D1 W d /Σ v u D + V cb
APPENDIX A-12 2025 CALIFORNIA EXISTING BUILDING CODE
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
APPENDIX A—GUIDELINES FOR THE SEISMIC RETROFIT OF EXISTING BUILDINGS
- For diaphragms in a multiple-story building with qualifying crosswalls in all levels:
Equation A1-9 DCR = 2.1 S D1 ΣW d / ΣΣ( v u D + V cb )
DCR shall be calculated at each level for the set of diaphragms at and above the level under consideration. In addition, the roof diaphragm shall meet the requirements of Equation A1-10. 4. For a roof diaphragm and the diaphragm directly below, if coupled by crosswalls:
Equation A1-10 DCR = 2.1 S D1 Σ W d /ΣΣ v u D
[BS] A111.4.3 Chords. An analysis for diaphragm flexure need not be made, and chords need not be provided.
[BS] A111.4.4 Collectors. An analysis of diaphragm collector forces shall be made for the transfer of diaphragm edge shears into vertical elements of the lateral force-resisting system. Collector forces may be resisted by new or existing elements.
[BS] A111.4.5 Diaphragm openings.
- Diaphragm forces at corners of openings shall be investigated and shall be developed into the diaphragm by new or existing materials.
- In addition to the demand-capacity ratios of Section A111.4.2, the demand-capacity ratio of the portion of the diaphragm adjacent to an opening shall be calculated using the opening dimension as the span.
- Where an opening occurs in the end quarter of the diaphragm span, the calculation of v u D for the demand-capacity ratio shall be based on the net depth of the diaphragm.
[BS] A111.5 Diaphragm shear transfer. Diaphragms shall be connected to shear walls and new vertical seismic force-resisting elements with connections capable of developing the diaphragm-loading tributary to the shear wall or new seismic force-resisting elements given by the lesser of the following formulas: Equation A1-11 V = 1.2 S D1 C p W d using the C p values in Table A111.5, or Equation A1-12 V = v u D
DCR = 2.1 S D1 ΣW d / ΣΣ( v u D + V cb )
DCR = 2.1 S D1 Σ W d /ΣΣ v u D
V = 1.2 S D1 C p W d
V = v u D
CRC § 0.84 Medium relevance — show source text
Flat blocking shall consist of C-shaped or track section with a minimum thickness of 33 mils (0.84 mm). For a gypsum board sheathed ceiling, the diaphragm length shall be in accordance with Table R804.3.7.1. For a wood structural panel sheathed ceiling, the diaphragm length shall be not less than 12 feet (3658 mm) for building widths less than 36 feet (10 973 mm), or not less than 14 feet (4267 mm) for building widths greater than or equal to 36 feet (10 973 mm).
The ceiling diaphragm shall be secured with screws spaced at a maximum 6 inches (152 mm) o.c. at panel edges and a maximum 12 inches (305 mm) o.c. in the field. The required lengths in Table R804.3.7.1 for gypsum board sheathed ceiling diaphragms shall be permitted to be multiplied by 0.35 if all panel edges are blocked. Multiplying the required lengths in Table R804.3.7.1 for gypsum board sheathed ceiling diaphragms by 0.9 shall be permitted if all panel edges are secured with screws spaced at 4 inches (102 mm) o.c.
2025 CALIFORNIA RESIDENTIAL CODE 8-41
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
ROOF-CEILING CONSTRUCTION
TABLE R804.3.7.1—REQUIRED LENGTHS FOR CEILING DIAPHRAGMS AT
GABLE ENDWALLS GYPSUM BOARD SHEATHED, CEILING HEIGHT = 8 FEETa, b, c, d, e, f, gCol2 Col3 Col4 Col5 Col6 Col7 Col8 ** EXPOSURE CATEGORY** ** EXPOSURE CATEGORY** ** ULTIMATE DESIGN WIND SPEED (mph)** ** ULTIMATE DESIGN WIND SPEED (mph)** ** ULTIMATE DESIGN WIND SPEED (mph)** ** ULTIMATE DESIGN WIND SPEED (mph)** ** ULTIMATE DESIGN WIND SPEED (mph)** ** ULTIMATE DESIGN WIND SPEED (mph)** ** B** ** B** ** 115** ** 120** ** 130** ** < 140** ** —** ** —** ** C** ** C** ** —** ** —** ** 115** ** 120** ** 130** ** < 140** ** Roof pitch** ** Building endwall width**
(feet)** Minimum diaphragm length**
(feet)** Minimum diaphragm length**
(feet)** Minimum diaphragm length**
(feet)** Minimum diaphragm length**
(feet)3:12 to 6:12 24–28 16 18 24 26 30 34 3:12 to 6:12 > 28–32 20 20 26 32 34 40 3:12 to 6:12 > 32–36 24 26 30 36 42 46 3:12 to 6:12 > 36–40 26 28 36 40 48 52 6:12 to 9:12 > CRC § 1604.3.1 Medium relevance — show source text
1604.3.1 Deflections. The deflections of structural members shall not exceed the more restrictive of the limitations of Sections 1604.3.2 through 1604.3.5 or that permitted by Table 1604.3.
1604.3.2 Reinforced concrete. The deflection of reinforced concrete structural members shall not exceed that permitted by ACI 318.
1604.3.3 Steel. The deflection of steel structural members shall not exceed that permitted by AISC 360, AISI S100, ASCE 8, SJI 100 or SJI 200, as applicable.
1604.3.4 Masonry. The deflection of masonry structural members shall not exceed that permitted by TMS 402.
1604.3.5 Aluminum. The deflection of aluminum structural members shall not exceed that permitted by AA ADM.
1604.3.6 Limits. The deflection limits of Section 1604.3.1 shall be used unless more restrictive deflection limits are required by a referenced standard for the element or finish material.
1604.3.7 Framing supporting glass. The deflection of framing members supporting glass subjected to 0.6 times the “component and cladding” wind loads shall not exceed either of the following:
- 1 / 175 of the length of span of the framing member, for framing members having a length not more than 13 feet 6 inches (4115 mm).
- 1 / 240 of the length of span of the framing member + 1 / 4 inch (6.4 mm), for framing members having a length greater than 13 feet 6 inches (4115 mm).
1604.4 Analysis. Load effects on structural members and their connections shall be determined by methods of structural analysis that take into account equilibrium, general stability, geometric compatibility and both short- and long-term material properties.
Members that tend to accumulate residual deformations under repeated service loads shall have included in their analysis the effects of added deformations expected to occur during their service life.
Any system or method of construction to be used shall be based on a rational analysis in accordance with well-established principles of mechanics. Such analysis shall result in a system that provides a complete load path capable of transferring loads from their point of origin to the load-resisting elements.
The total lateral force shall be distributed to the various vertical elements of the lateral force-resisting system in proportion to their rigidities, considering the rigidity of the horizontal bracing system or diaphragm. Rigid elements assumed not to be a part of the lateral forceresisting system are permitted to be incorporated into buildings provided that their effect on the action of the system is considered and provided for in the design. Where a diaphragm is not permitted to be idealized as either flexible or rigid in accordance with ASCE 7 or for wood diaphragms in accordance with AWC SDPWS, the structure shall be analyzed and designed utilizing one of the following procedures:
An envelope analysis of the structure using a flexible and rigid diaphragm analysis separately and designing each component for the more severe load condition.
A semirigid diaphragm analysis and design.
Where required by ASCE 7, provisions shall be made for the increased forces induced on resisting elements of the structural system resulting from torsion due to eccentricity between the center of application of the lateral forces and the center of rigidity of the lateral force-resisting system.
16-6 2025 CALIFORNIA BUILDING CODE
CRC § 0.35 Medium relevance — show source text
Required diaphragm lengths are to be provided at each end of the structure.
e. Multiplying required diaphragm lengths by 0.35 is permitted if all panel edges are blocked.
f. Multiplying required diaphragm lengths by 0.9 is permitted if all panel edges are secured with screws spaced at 4 inches o.c.
g. To determine the minimum diaphragm length for buildings with ceiling heights of 9 feet or 10 feet values in this table shall be multiplied by 1.15.|For SI: 1 inch = 25.4 mm, 1 mile per hour = 0.447 m/s, 1 foot = 304.8 mm, 1 mil = 0.0254 mm.
a. Ceiling diaphragm is composed of1/2-inch gypsum board (min. thickness) secured with screws spaced at 6 inches o.c. at panel edges and 12 inches o.c. infield. Use No. 8
screws (min.) where framing members have a designation thickness of 54 mils or less and No. 10 screws (min.) where framing members have a designation thickness greater
than 54 mils.
b. Maximum aspect ratio (length/width) of diaphragms is 2:1.
c. Building width is in the direction of horizontal framing members supported by the wall studs.
d. Required diaphragm lengths are to be provided at each end of the structure.
e. Multiplying required diaphragm lengths by 0.35 is permitted if all panel edges are blocked.
f. Multiplying required diaphragm lengths by 0.9 is permitted if all panel edges are secured with screws spaced at 4 inches o.c.
g. To determine the minimum diaphragm length for buildings with ceiling heights of 9 feet or 10 feet values in this table shall be multiplied by 1.15.|For SI: 1 inch = 25.4 mm, 1 mile per hour = 0.447 m/s, 1 foot = 304.8 mm, 1 mil = 0.0254 mm.
a. Ceiling diaphragm is composed of1/2-inch gypsum board (min. thickness) secured with screws spaced at 6 inches o.c. at panel edges and 12 inches o.c. infield. Use No. 8
screws (min.) where framing members have a designation thickness of 54 mils or less and No. 10 screws (min.) where framing members have a designation thickness greater
than 54 mils.
b. Maximum aspect ratio (length/width) of diaphragms is 2:1.
c. Building width is in the direction of horizontal framing members supported by the wall studs.
d. Required diaphragm lengths are to be provided at each end of the structure.
e. Multiplying required diaphragm lengths by 0.35 is permitted if all panel edges are blocked.
f. Multiplying required diaphragm lengths by 0.9 is permitted if all panel edges are secured with screws spaced at 4 inches o.c.
g. To determine the minimum diaphragm length for buildings with ceiling heights of 9 feet or 10 feet values in this table shall be multiplied by 1.15.|For SI: 1 inch = 25.4 mm, 1 mile per hour = 0.447 m/s, 1 foot = 304.8 mm, 1 mil = 0.0254 mm.
a.
Frequently asked questions
Do I always have to use the tables in R503, or can I engineer a thinner panel?
You may use a design by a registered design professional (engineered solution) if the prescriptive tables do not cover your product or loading. For prescriptive work you must use the tables in § R503.1 and § R503.2.1.1.
Where do I get the diaphragm mid‑span deflection formula?
The CRC points designers to the AWC SDPWS method; the CBC Chapter 23 provides Equation 23‑1 for blocked wood structural panel diaphragms — see CBC § 2305.2 for the full equation and table values.
If my panels are edge‑blocked, how does that change diaphragm requirements?
Blocked edges may reduce required diaphragm lengths (see multiplying factors in the CRC ceiling diaphragm notes — e.g., multiply required lengths by 0.35 if all edges are blocked). See § R804.3.7.1 and table notes.
Can I use panel edge clips instead of blocking?
Yes — panel edge clips, tongue‑and‑groove, or lumber blocking are accepted edge supports per the table footnotes, but note only lumber blocking may satisfy certain blocked diaphragm requirements in the table notes. Always read the specific table footnote in § R503.2.1.1.
What deflection limits should I check for a floor sheathing application?
Prescriptive spans in the tables are established with deflection and load criteria in mind. If you need the explicit deflection limit (L/180, L/240, etc.) or are evaluating non‑standard loads (snow, concentrated loads) consult the table footnotes and the referenced structural deflection provisions (CBC structural deflection rules or AWC). The tables and their footnotes in § R503.2.1.1 are the starting point.
My building has a long cantilevered roof diaphragm — how do I treat span‑to‑depth?
Cantilevers require special treatment; span‑to‑depth limits for diaphragms (and halved ratios for cantilevers) and flexibility factors are handled in the referenced structural provisions (CBC/AWC). For prescriptive ceiling/roof diaphragm lengths see CRC R804, but for span‑depth engineering consult CBC/AWC guidance.
More in California Residential Code
Ask about the CRC
Get cited, plain-English answers on the California Residential Code for your project — any code section, any scenario.
Start Free Trial