CRC · California Residential Code
How must a roof diaphragm be constructed and how is blocking required?
For homeowners: the roof diaphragm is the sheathing that helps the roof act as a single unit to resist wind and seismic forces. The CRC requires at least 3/8‑inch wood structural panel attached to the rafters for the roof diaphragm and calls for blocking of rafters/ceiling joists when the house plan is long and narrow (aspect ratio 3:1 or greater) and the roof is steep (pitch 9:12 or steeper). Ceiling diaphragms at gable ends can use 1/2‑inch gypsum or 3/8‑inch wood panels with specific edge blocking and fastener spacing rules that may reduce required diaphragm lengths when properly blocked. file
Last reviewed: July 6, 2026
What the code requires — 2-4 sentences
A roof diaphragm must be formed by attaching at least 3/8‑inch (9.5 mm) wood structural panel sheathing to roof rafters or truss top chords in accordance with the roof sheathing/fastening schedules (see § R804.3.7.2). Ceiling diaphragms at gable endwalls are covered by § R804.3.7.1 and require either 1/2‑inch gypsum board or 3/8‑inch wood structural panel attached to the underside of joists/trusses with specified blocking and fastener spacing. Where the building has a plan aspect ratio of 3:1 or larger and roof pitch is 9:12 or steeper, the code requires blocking of roof rafters and ceiling joists in accordance with the roof‑blocking detail (see § R804.3.7.2).
The single most important rule: attach the minimum required sheathing thickness to framing (roof: 3/8‑inch wood structural panel) and provide blocking where the code calls for it (e.g., 3:1 plan aspect ratio with ≥9:12 pitch), because the diaphragm only works when edges and panels are properly supported and fastened.
Requirements in detail
Which panels (what creates the diaphragm)
- For a roof diaphragm the CRC requires attaching not less than 3/8‑inch (9.5 mm) wood structural panel sheathing to roof rafters or truss top chords. This is the baseline roof diaphragm requirement. § R804.3.7.2.
- For a ceiling diaphragm at gable endwalls, the code permits either 1/2‑inch (12.7 mm) gypsum board or 3/8‑inch (9.5 mm) wood structural panel attached to the underside of joists/trusses. § R804.3.7.1.
Blocking — when it is required
- Flat blocking (C‑shaped or track section) of a specified minimum thickness is required for the ceiling diaphragm details at gable endwalls (see gable end details). The code specifies flat blocking of C‑shaped/track with minimum thickness of 33 mils (0.84 mm) for those ceiling details. § R804.3.7.1.
- Roof blocking (blocking of rafters and ceiling joists) is required where the building plan aspect ratio is 3:1 or larger AND the roof rafter slope (pitch) is 9:12 or larger — blocking must follow Figure R804.3.7.2. § R804.3.7.2.
Fastening and edge support rules that affect blocking needs
- The ceiling diaphragm (gypsum board) shall be secured with screws spaced a maximum of 6 inches on center at panel edges and 12 inches on center in the field. Multipliers to required diaphragm lengths are allowed when edges are blocked or when edge screw spacing is increased (.35 multiplier if all panel edges are blocked; .9 multiplier if all panel edges are secured with screws at 4 inches o.c.). § R804.3.7.1.
- For roof sheathing and other fastener spacing and nailing schedules, see the roof sheathing tables; edge nailing, and where panels have edges perpendicular to framing, those edges must be backed by blocking and fastened at the edge spacing. (See related Tables and fastening provisions in Chapter 8.)
Key defined terms (first mention bolded)
- Diaphragm — a horizontal or sloped system that transfers lateral forces to vertical elements; in light‑frame construction this includes the ceiling/roof sheathing system. § R804.3.7 and definitions.
- Diaphragm, blocked — diaphragm in which all sheathing edges not on framing members are supported on and fastened to blocking. Defined in the code.
Quick reference table — decision‑relevant dimensions/values
| Requirement / value | Typical numeric value(s) | Where it appears (Code Reference) |
|---|---|---|
| Minimum roof diaphragm sheathing | 3/8‑inch (9.5 mm) wood structural panel | § R804.3.7.2 |
| Ceiling diaphragm sheathing options (gable endwalls) | 1/2‑inch gypsum board OR 3/8‑inch wood structural panel | § R804.3.7.1 |
| Minimum wood structural panel diaphragm length (ceiling) | 12 ft if building width < 36 ft; 14 ft if width ≥ 36 ft | § R804.3.7.1 |
| Edge screw spacing for gypsum ceiling | 6 in. o.c. at panel edges; 12 in. o.c. in field | § R804.3.7.1 |
| Alternate edge fastening (reduces required diaphragm length) | 4 in. o.c. screws at all panel edges → multiply required length by 0.9 | § R804.3.7.1 |
| Edge blocking effect on required length | If all panel edges are blocked, required length for gypsum board diaphragms may be multiplied by 0.35 | § R804.3.7.1 |
| Flat blocking material thickness | 33 mil (0.84 mm) C‑shaped / track section | § R804.3.7.1 |
| Trigger for roof/ceiling blocking (plan and slope) | Plan aspect ratio ≥ 3:1 AND roof pitch ≥ 9:12 → blocking required per Figure R804.3.7.2 | § R804.3.7.2 |
(See the referenced figures and tables in § R804.3.7.1 and § R804.3.7.2 for connection/sketch detail.)
Exceptions & special cases
- The code allows multiplying the required gypsum board diaphragm lengths by 0.35 when all panel edges are blocked, and by 0.9 when all panel edges are fastened with screws at 4 in. o.c. — these provide prescriptive reductions only for gypsum‑sheathed ceilings (not a general relaxation for roof sheathing). § R804.3.7.1.
- Wood structural panel sheathed ceilings use a simpler minimum length rule (12 ft / 14 ft depending on building width) rather than the table used for gypsum board; follow that instead of attempting to apply gypsum multipliers. § R804.3.7.1.
- Where other code sections (e.g., seismic provisions) impose more restrictive diaphragm blocking/fastening requirements, those sections control. For example, Seismic Design Category rules require panel edge backing and different fastener spacing in some cases (see Chapter R606 references). If your project falls in a seismic category with additional requirements, follow those provisions in addition to § R804.3.7.x.
Common mistakes
- Assuming the roof diaphragm can be made with thinner or different materials than the code minimum. The roof diaphragm minimum is 3/8‑inch wood structural panel unless engineered otherwise. § R804.3.7.2.
- Forgetting to provide blocking for panel edges when the prescriptive exceptions or multipliers are being applied (for example, using the 0.35 multiplier without actually blocking every panel edge). § R804.3.7.1.
- Incorrect fastener spacing at panel edges or in the field (e.g., exceeding 6 in. o.c. at edges for gypsum ceilings or not meeting the required field spacing). § R804.3.7.1.
- Missing the roof‑blocking trigger: not providing blocking when plan aspect ratio ≥ 3:1 and pitch ≥ 9:12, which the code requires. § R804.3.7.2.
Worked example — simple, concrete scenario
Scenario: A single‑story house with a gable endwall. Building plan is rectangular with length 48 ft and width 16 ft (plan aspect ratio = 48/16 = 3:1). Roof slope is 10:12. Ceiling is to be wood‑panel sheathed.
Does blocking of rafters/ceiling joists at ridge/edges have to be provided?
- Yes — because the plan aspect ratio is 3:1 and roof pitch is 10:12 (≥ 9:12), roof rafters and ceiling joists must be blocked per Figure R804.3.7.2. § R804.3.7.2.
What diaphragm sheathing thickness is required on the roof?
- The roof diaphragm must be formed with not less than 3/8‑inch wood structural panel attached to rafters/truss top chords. § R804.3.7.2.
If the ceiling is sheathed with wood structural panels (not gypsum), what minimum diaphragm length is required at the gable endwall?
- For wood structural panel sheathed ceiling: minimum 12 ft diaphragm length for building widths less than 36 ft. Here the width is 16 ft → 12 ft required. § R804.3.7.1.
If instead the ceiling used 1/2‑inch gypsum board, could you reduce the required length by blocking edges?
- Yes — for gypsum board you must consult Table R804.3.7.1 for the base required length, but if all panel edges are blocked you may multiply the value from the table by 0.35; alternately, if all panel edges are screwed at 4 in. o.c., multiply by 0.9. § R804.3.7.1.
Related provisions
- § R804.3.7.1 — Ceiling diaphragms (minimum sheathing types, blocking, fastener spacings and length multipliers)
- § R804.3.7.2 — Roof diaphragm (minimum roof sheathing thickness, roof blocking trigger and figure)
- Chapter R803 — Roof sheathing materials and compliance (referenced for wood structural panel compliance)
- Tables and fastener schedules in Chapter 8 (roof sheathing nail/screw spacing) — these affect diaphragm attachment details and must be followed for specific wind/seismic conditions.
- Definitions: Diaphragm, blocked/unblocked — code definitions that explain the effect of blocking on diaphragm behavior.
Code references
Grounded in the retrieved California Residential Code — click a citation to read the verbatim passage:
CRC § 0.35 High relevance — show source text
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.|FIGURE R804.3.7.1(1)—CEILING DIAPHRAGM TO GABLE ENDWALL DETAIL
CEILING JOISTS
FLAT STUD OR TRACK BLOCKING AT 48 IN. O.C. AT FIRST TWO JOIST SPACES
3 NO. 8 SCREWS AT EACH STUD
STUD BLOCKING AT CEILING ELEVATION
GABLE
ENDWALL
STUDS
For SI: 1 inch = 25.4 mm.
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FIGURE R804.3.7.1(2)—CEILING DIAPHRAGM TO SIDEWALL DETAIL
ROOF-CEILING CONSTRUCTION
ROOF RAFTER (OR TRUSS TOP CHORD)
CEILING JOIST (OR TRUSS BOTTOM CHORD)
WOOD STRUCTURAL
PANEL OR GYPSUM
BOARD DIAPHRAGM
350T125-33 TRACK
ROOF SHEATHING
ROOF BLOCKING
NO. 8 SCREWS AT 6 IN. O.C.
STRUCTURAL WALL
For SI: 1 inch = 25.4 mm.
R804.3.7.2 Roof diaphragm. A roof diaphragm shall be provided by attaching not less than [3] / 8 -inch (9.5 mm) wood structural panel that complies with Section R803 to roof rafters or truss top chords in accordance with Table R804.3. Buildings with 3:1 or larger plan aspect ratio and with roof rafter slope (pitch) of 9:12 or larger shall have the roof rafters and ceiling joists blocked in accordance with Figure R804.3.7.2.
FIGURE R804.3.7.2—ROOF BLOCKING DETAIL
BE ATTACHED TO OUTSIDE FACE OF SHEATHING OR PREFERABLY DIRECTLY TO TRACK FLANGE PRIOR TO SHEATHING PLACEMENT AS SHOWN
For SI: 1 mil = 0.0254 mm, 1 inch = 25.4 mm, 1 foot = 304.8 mm.
2025 CALIFORNIA RESIDENTIAL CODE 8-43
CRC § 3.7.1 High relevance — show source text
FIGURE R804.3.7.1(2)—CEILING DIAPHRAGM TO SIDEWALL DETAIL
ROOF-CEILING CONSTRUCTION
ROOF RAFTER (OR TRUSS TOP CHORD)
CEILING JOIST (OR TRUSS BOTTOM CHORD)
WOOD STRUCTURAL
PANEL OR GYPSUM
BOARD DIAPHRAGM
350T125-33 TRACK
ROOF SHEATHING
ROOF BLOCKING
NO. 8 SCREWS AT 6 IN. O.C.
STRUCTURAL WALL
For SI: 1 inch = 25.4 mm.
R804.3.7.2 Roof diaphragm. A roof diaphragm shall be provided by attaching not less than [3] / 8 -inch (9.5 mm) wood structural panel that complies with Section R803 to roof rafters or truss top chords in accordance with Table R804.3. Buildings with 3:1 or larger plan aspect ratio and with roof rafter slope (pitch) of 9:12 or larger shall have the roof rafters and ceiling joists blocked in accordance with Figure R804.3.7.2.
FIGURE R804.3.7.2—ROOF BLOCKING DETAIL
BE ATTACHED TO OUTSIDE FACE OF SHEATHING OR PREFERABLY DIRECTLY TO TRACK FLANGE PRIOR TO SHEATHING PLACEMENT AS SHOWN
For SI: 1 mil = 0.0254 mm, 1 inch = 25.4 mm, 1 foot = 304.8 mm.
2025 CALIFORNIA RESIDENTIAL CODE 8-43
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ROOF-CEILING CONSTRUCTION
R804.3.8 Roof tie-down. Roof assemblies shall be connected to walls below in accordance with Table R804.3. A continuous load path shall be provided to transfer uplift loads to the foundation.
SECTION R805—CEILING FINISHES
R805.1 Ceiling installation. Ceilings shall be installed in accordance with the requirements for interior wall finishes as provided in Sections R702.1 through R702.6.
SECTION R806—ROOF VENTILATION
Note: The IECC climate zones used by this section differ from those used by the California Energy Code to determine applicability of energy efficiency measures. Comparison of IECC and California Energy Code climate zones is shown in Table R702.7.3.
R806.1 Ventilation required. Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. Ventilation openings shall have a least dimension of [1] / 16 inch (1.6 mm) minimum and [1] / 4 inch (6.4 mm) maximum. Ventilation openings having a least dimension larger than [1] / 4 inch (6.4 mm) shall be provided with corrosion-resistant wire cloth screening, hardware cloth, perforated vinyl or similar material with openings having a least dimension of [1] / 16 inch (1.6 mm) minimum and [1] / 4 inch (6.4 mm) maximum. Openings in roof framing members shall conform to the requirements of Section R802.7.
CRC § 0.84 High 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 § 3.5 High relevance — show source text
FIGURE R804.3.5(1)—ROOF OR CEILING OPENING
For SI: 1 foot = 304.8 mm.
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ROOF-CEILING CONSTRUCTION
FIGURE R804.3.5(2)—HEADER TO TRIMMER CONNECTION
MINIMUM 2 IN. × 2 IN. CLIP ANGLE WITH 4 NO. 8 SCREWS THROUGH EACH LEG, BOTH SIDES OF CONNECTION
For SI: 1 inch = 25.4 mm.
R804.3.6 Roof trusses. Cold-formed steel trusses shall be designed and installed in accordance with AISI S230, Section F6. In the absence of specific bracing requirements, trusses shall be braced in accordance with accepted industry practices, such as the SBCA Cold-Formed Steel Building Component Safety Information (CFSBCSI) Guide to Good Practice for Handling, Installing & Bracing of Cold-Formed Steel Trusses. Trusses shall be connected to the top track of the load-bearing wall in accordance with Table R804.3, either with the required number of No. 10 screws applied through the flange of the truss or by using a 54-mil (1.37 mm) clip angle with the required number of No. 10 screws in each leg.
R804.3.7 Ceiling and roof diaphragms. Ceiling and roof diaphragms shall be in accordance with this section. R804.3.7.1 Ceiling diaphragms. At gable endwalls a ceiling diaphragm shall be provided by attaching a minimum [1] / 2 -inch (12.7 mm) gypsum board or a minimum [3] / 8 -inch (9.5 mm) wood structural panel sheathing, that complies with Section R803, to the bottom of ceiling joists or roof trusses and connected to wall framing in accordance with Figures R804.3.7.1(1) and R804.3.7.1(2), unless studs are designed as full height without bracing at the ceiling. 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
CRC § 0.42 High relevance — show source text
Spacing of fasteners not included in this table shall be based on Table R602.3(2).
f. For wood structural panel roof sheathing attached to gable end roof framing and to intermediate supports within 48 inches of roof edges and ridges, nails shall be spaced at 4
inches on center where the ultimate design wind speed is greater than 130 mph in Exposure B or greater than 110 mph in Exposure C. Fastener spacing applies where roof
framing specific gravity is 0.42 or larger. Where roof framing specific gravity is greater than or equal to 0.35 but less than 0.42 in accordance with AWC NDS, fastening of roof
sheathing shall be with RSRS-03 (21/2″ × 0.131″ × 0.281″ head) nails.
g. Gypsum sheathing shall conform to ASTM C1396 and shall be installed in accordance with ASTM C1280 or GA 253 . Fiberboard sheathing shall conform to ASTM C208.
h. Spacing of fasteners on floor sheathing panel edges applies to panel edges supported by framing members and required blocking and at floor perimeters only. Spacing of fasteners
on roof sheathing panel edges applies to panel edges supported by framing members and required blocking. Blocking of roof or floor sheathing panel edges perpendicular to the
framing members need not be provided except as required by other provisions of this code. Floor perimeter shall be supported by framing members or solid blocking.
i. Where a rafter is fastened to an adjacent parallel ceiling joist in accordance with this schedule, provide two toe nails on one side of the rafter and toe nails from the ceiling joist
to top plate in accordance with this schedule. The toe nail on the opposite side of the rafter shall not be required.|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 mile per hour = 0.447 m/s; 1 ksi = 6.895 MPa.
a. Nails are smooth-common, box or deformed shanks except where otherwise stated. Nails used for framing and sheathing connections are carbon steel and shall have mini-
mum average bending yield strengths as shown: 80 ksi for shank diameter of 0.192 inch (20d common nail), 90 ksi for shank diameters larger than 0.142 inch but not larger than
0.177 inch, and 100 ksi for shank diameters of 0.142 inch or less. Connections using nails and staples of other materials, such as stainless steel, shall be designed by accepted
engineering practice or approved under Section R104.2.2.
b. RSRS-01 is a Roof Sheathing Ring Shank nail meeting the specifications in ASTM F1667.
c. Nails shall be spaced at not more than 6 inches on center at all supports where spans are 48 inches or greater.
d. Four-foot by 8-foot or 4-foot by 9-foot panels shall be applied vertically.
e. Spacing of fasteners not included in this table shall be based on Table R602.3(2).
f. For wood structural panel roof sheathing attached to gable end roof framing and to intermediate supports within 48 inches of roof edges and ridges, nails shall be spaced at 4
inches on center where the ultimate design wind speed is greater than 130 mph in Exposure B or greater than 110 mph in Exposure C. Fastener spacing applies where roof
framing specific gravity is 0.42 or larger.CRC § 10.8.2 High relevance — show source text
Exception: Where the outside edge of truss vertical web members aligns with the outside face of the wall studs below, wood structural panel sheathing extending above the top plate as shown in Figure R602.10.8.2(3) shall be permitted to be fastened to each truss web with three-8d nails (2 [1] / 2 inches × 0.131 inch) and blocking between the trusses shall not be required. 2. For Seismic Design Categories D 0, D 1 and D 2, where the distance from the top of the braced wall panel to the top of the rafters or roof trusses is 15 [1] / 4 inches (387 mm) or less, blocking between rafters or roof trusses shall be provided above the braced wall panel in accordance with Figure R602.10.8.2(1). 3. Where the distance from the top of the braced wall panel to the top of rafters or roof trusses exceeds 15 [1] / 4 inches (387 mm), the top plates of the braced wall panel shall be connected to perpendicular rafters or roof trusses above in accordance with one or more of the following methods: 3.1. Soffit blocking panels constructed in accordance with Figure R602.10.8.2(2). 3.2. Vertical blocking panels constructed in accordance with Figure R602.10.8.2(3). 3.3. Blocking panels provided by the roof truss manufacturer and designed in accordance with Section R802. 3.4. Blocking, blocking panels or other methods of lateral load transfer designed in accordance with the AWC
FIGURE R602.10.8.2(1)—BRACED WALL PANEL CONNECTION TO PERPENDICULAR RAFTERS
SOLID BLOCKING BETWEEN RAFTERS ATTACHED TO TOP PLATES WITH 8d @ 6 ″ O.C. ALONG LENGTH OF BRACED WALL PANEL
For SI: 1 inch = 25.4 mm.
2025 CALIFORNIA RESIDENTIAL CODE 6-47
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WALL CONSTRUCTION
FIGURE R602.10.8.2(2)—BRACED WALL PANEL CONNECTION OPTION TO PERPENDICULAR RAFTERS OR ROOF TRUSSES
For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm. a. Methods of bracing shall be as described in Section R602.10.4.
FIGURE R602.10.8.2(3)—BRACED WALL PANEL CONNECTION OPTION TO PERPENDICULAR RAFTERS OR ROOF TRUSSES
WHERE AIR GAP AT TOP IS NOT USED, CENTER W/3 AVAILABLE FOR VENT HOLES
Col1 Col2 Col3 Col4 Col5 W Col7 Col8 Col9 Col10 Col11 W/3ROOF SHEATHING Col2 Col3 Col4 Col5 Col6 Col7 BRACED WALL
PANEL
2x
BLK’G
VENTING
A
BRACING
a
EDGE NAILING PER
TABLE R602.CRC § 25.4 High relevance — show source text
Values are not cumulative with other horizontal diaphragm values and are for short-term wind or seismic loading. Values shall be reduced 25 percent for normal loading.
b. Values shall be reduced 50 percent in Seismic Design Categories D, E and F.
c. 11/4-inch, No. 6 Type S or W screws are permitted to be substituted for the listed nails.|For SI: 1 inch = 25.4 mm, 1 pound per foot = 14.59 N/m.
a. Values are not cumulative with other horizontal diaphragm values and are for short-term wind or seismic loading. Values shall be reduced 25 percent for normal loading.
b. Values shall be reduced 50 percent in Seismic Design Categories D, E and F.
c. 11/4-inch, No. 6 Type S or W screws are permitted to be substituted for the listed nails.|2508.6.1 Diaphragm proportions. The maximum allowable diaphragm proportions shall be 1 [1] / 2 :1 between shear resisting elements. Rotation or cantilever conditions shall not be permitted.
2508.6.2 Installation. Gypsum panel products used in a horizontal diaphragm ceiling shall be installed perpendicular to ceiling framing members. End joints of adjacent courses of gypsum board shall not occur on the same joist.
2508.6.3 Blocking of perimeter edges. Perimeter edges shall be blocked using a wood member not less than 2-inch by 6-inch (51 mm by 152 mm) nominal dimension. Blocking material shall be installed flat over the top plate of the wall to provide a nailing surface not less than 2 inches (51 mm) in width for the attachment of the gypsum panel product.
2508.6.4 Fasteners. Fasteners used for the attachment of gypsum panel products to a horizontal diaphragm ceiling shall be as defined in Table 2508.6. Fasteners shall be spaced not more than 7 inches (178 mm) on center at all supports, including perimeter blocking, and not more than [3] / 8 inch (9.5 mm) from the edges and ends of the gypsum panel product.
2508.6.5 Lateral force restrictions. Gypsum panel products shall not be used in diaphragm ceilings to resist lateral forces imposed by masonry or concrete construction.
2508.6.6 Diaphragm ceiling connection to partitions. [DSA-SS & DSA-SS/CC and OSHPD 1, 1R, 2, 4 & 5] Gypsum board shall not be used in diaphragm ceilings to resist lateral forces imposed by partitions. Connection of diaphragm ceiling to the vertical lateral force resisting elements shall be designed and detailed to transfer lateral forces.
Exception: [OSHPD 2] Single-story Type V skilled nursing or intermediate care facilities utilizing wood-frame or light-steel-frame construction.
SECTION 2509—SHOWERS AND WATER CLOSETS
2509.1 Wet areas. Showers and public toilet walls shall conform to Section 1210.2.
CRC § 12.2. Medium relevance — show source text
Note: A full bed joint must be provided. Cells containing vertical bars are to be filled to the top of wall and provide inspection opening as shown on detail “A.” Horizontal bars are to be laid as shown on detail “B.” Lintel bars are to be laid as shown on Section C.
R606.12 Seismic requirements. The seismic requirements of this section shall apply to the design of masonry and the construction of masonry building elements located in Seismic Design Category D 0, D 1 or D 2 . Townhouses in Seismic Design Category C shall comply with the requirements of Section R606.12.2. These requirements shall not apply to glass unit masonry conforming to Section R607, anchored masonry veneer conforming to Section R703.8 or adhered masonry veneer conforming to Section R703.12.
R606.12.1 General. Masonry structures and masonry elements shall comply with the requirements of Sections R606.12.2 through R606.12.4 based on the seismic design category established in Table R301.2.1(1). Masonry structures and masonry elements shall comply with the requirements of Section R606.12 and Figures R606.11(1), R606.11(2) and R606.11(3) or shall be designed in accordance with TMS 402 or TMS 403.
R606.12.1.1 Floor and roof diaphragm construction. Floor and roof diaphragms shall be constructed of wood structural panels attached to wood framing in accordance with Table R602.3(1) or to cold-formed steel floor framing in accordance with Table R505.3.1(2) or to cold-formed steel roof framing in accordance with Table R804.3. Additionally, sheathing panel edges
6-102 2025 CALIFORNIA RESIDENTIAL CODE
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WALL CONSTRUCTION
perpendicular to framing members shall be backed by blocking, and sheathing shall be connected to the blocking with fasteners at the edge spacing. For Seismic Design Categories C, D 0, D 1 and D 2, where the width-to-thickness dimension of the diaphragm exceeds 2-to-1, edge spacing of fasteners shall be 4 inches (102 mm) on center.
R606.12.2 Seismic Design Category C. Townhouses located in Seismic Design Category C shall comply with the requirements of this section.
R606.12.2.1 Minimum length of wall without openings. Table R606.12.2.1 shall be used to determine the minimum required solid wall length without openings at each masonry exterior wall. The provided percentage of solid wall length shall include only those wall segments that are 3 feet (914 mm) or longer. The maximum clear distance between wall segments included in determining the solid wall length shall not exceed 18 feet (5486 mm). Shear wall segments required to meet the minimum wall length shall be in accordance with Section R606.12.2.2.3.
CRC § 2.3.1 Medium relevance — show source text
2025 CALIFORNIA RESIDENTIAL CODE APPENDIX CK-7
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APPENDIX CK-8 2025 CALIFORNIA RESIDENTIAL CODE
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2025 CALIFORNIA RESIDENTIAL CODE APPENDIX NA-1
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APPENDIX NA-2 2025 CALIFORNIA RESIDENTIAL CODE
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Concrete Capillary break R506.2.3.1 Compressive strength R402.2 Floors (on ground) R506 Tile (roof) R905.3 Weathering Figure R301.2(1), R402.2 Construction
3D-printed Appendix BM Cavity wall masonry R608 Cob Appendix BK Extended plate walls Appendix BN Flood-resistant R306
Floors Chapter 5 Footings R403 Foundation material R402
Foundation walls R404
Foundations Chapter 4 Hemp-lime Appendix BL Masonry R606, R607, R608, R610 Pollutant control R340 Roofs Chapter 8 Steel framing R505, R603, R804 Strawbale Appendix BJ Straw-clay Appendix BI Walls Chapter 6 Waste R334.1 Wood framing R502, R602, R802 Construction Documents R106, R306.3.9
Court Emergency escape and rescue openings R319.1, R319.2.4 Means of egress R318.1 Natural light R325.1.1 Covering Exterior R703
Interior R702 Roof Chapter 9 Wall Chapter 7 Crawl Space R408
Access R408.4
Damper Fireplace R1001.7.1 Dampproofing R406 Day Care Appendix BD Defined R202 Large family day-care homes R336 Dead Loads R301.4
Decay Protection against R304 Deck
Attachment R318.5 Supported by exterior wall R507 Wood/plastic composite boards R507.2.2
Definitions Building R202 Mechanical system R202 Plumbing R202 Design Criteria R301 Alternative provisions R301.1.1 Application R301.1 Climatic and geographic R301.2 Dead load R301.4
Deflection R301.6 Engineered design R301.1.3
Accessibility R322 Accessory Dwelling Unit R202 Authority to Enforce Automatic Fire Sprinkler Systems 1.8.9.1, R313.2 Accessory Structure Elevations R306.3.2
Energy R330.5 Exterior walls R302.1
Scope R101.2 Addition (see also Existing Buildings) Carbon monoxide alarms R311.2.2 Change of occupancy R110.1 Construction R302.2.3
CRC § 12.1.1 Medium relevance — show source text
R606.12.1.1 Floor and roof diaphragm construction. Floor and roof diaphragms shall be constructed of wood structural panels attached to wood framing in accordance with Table R602.3(1) or to cold-formed steel floor framing in accordance with Table R505.3.1(2) or to cold-formed steel roof framing in accordance with Table R804.3. Additionally, sheathing panel edges
6-102 2025 CALIFORNIA RESIDENTIAL CODE
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WALL CONSTRUCTION
perpendicular to framing members shall be backed by blocking, and sheathing shall be connected to the blocking with fasteners at the edge spacing. For Seismic Design Categories C, D 0, D 1 and D 2, where the width-to-thickness dimension of the diaphragm exceeds 2-to-1, edge spacing of fasteners shall be 4 inches (102 mm) on center.
R606.12.2 Seismic Design Category C. Townhouses located in Seismic Design Category C shall comply with the requirements of this section.
R606.12.2.1 Minimum length of wall without openings. Table R606.12.2.1 shall be used to determine the minimum required solid wall length without openings at each masonry exterior wall. The provided percentage of solid wall length shall include only those wall segments that are 3 feet (914 mm) or longer. The maximum clear distance between wall segments included in determining the solid wall length shall not exceed 18 feet (5486 mm). Shear wall segments required to meet the minimum wall length shall be in accordance with Section R606.12.2.2.3.
TABLE R606.12.2.1—MINIMUM SOLID WALL LENGTH ALONG EXTERIOR WALL LINES Col2 Col3 Col4 ** SESIMIC**
DESIGN CATEGORY** MINIMUM SOLID WALL LENGTH**
(percent)a** MINIMUM SOLID WALL LENGTH**
(percent)a** MINIMUM SOLID WALL LENGTH**
(percent)a** SESIMIC**
DESIGN CATEGORY** One story or**
top story of two story** Wall supporting**
light-frame second story and roof** Wall supporting**
masonry second story and roofTownhouses in C 20 25 35 D0 or D1 25 NP NP D2 30 NP NP NP = Not Permitted, except with design in accordance with the_California Building Code_.
a. For all walls, the minimum required length of solid walls shall be based on the table percent multiplied by the dimension, parallel to the wall direction under consideration, of
a rectangle inscribing the overall building plan.NP = Not Permitted, except with design in accordance with the_California Building Code_.
a. For all walls, the minimum required length of solid walls shall be based on the table percent multiplied by the dimension, parallel to the wall direction under consideration, of
a rectangle inscribing the overall building plan.NP = Not Permitted, except with design in accordance with the_California Building Code_.
a.CRC § 13.1.3 Medium relevance — show source text
[BS] DESIGNATED SEISMIC SYSTEM. Those nonstructural components that require design in accordance with Chapter 13 of ASCE 7 and for which the component importance factor, I p, is greater than 1 in accordance with Section 13.1.3 of ASCE 7. DESTINATION-ORIENTED ELEVATOR. [DSA-AC] Destination-oriented elevators are operated by the user selecting a destination floor at a hall call console located at or near an elevator landing. The destination-oriented elevator system then assigns an elevator car which trans- ports the user to the selected destination floor. Destination-oriented elevators do not provide floor selection within elevator cars.
[F] DETACHED BUILDING. A separate single-story building, without a basement or crawl space, used for the storage or use of hazardous materials and located an approved distance from all structures.
DETACHED SINGLE-FAMILY DWELLING. [HCD 1 & HCD 2] Any single-family dwelling which is separated (detached) from adjacen t buildings.
[BS] DETAILED PLAIN CONCRETE STRUCTURAL WALL. See Section 1905.2.
[BE] DETECTABLE WARNING. A standardized surface feature built in or applied to walking surfaces or other elements to warn visually impaired persons of hazards on a circulation path.
[F] DETECTOR, HEAT. A fire detector that senses heat—either abnormally high temperature or rate of rise, or both.
DETENTION ELEVATOR. [SFM] Detention elevator shall mean an elevator which moves in-custody individuals within a secure and restrained environment.
DETENTION PROGRAM SUITE. Within a state prison, correctional treatment facility, local detention facility or juvenile facility, a detention program suite shall be a group of program related spaces, not classified as group F uses, located around shared circulation.
DETENTION TREATMENT ROOM. [SFM] Detention treatment room shall mean a lockable room or rooms within Group I-3 occupancies used for recreational therapy, group rooms, interdisciplinary treatment team rooms and interview rooms not classified solely as a Group I-2 occupancy.
[F] DETONATION. An exothermic reaction characterized by the presence of a shock wave in the material which establishes and maintains the reaction. The reaction zone progresses through the material at a rate greater than the velocity of sound. The principal heating mechanism is one of shock compression. Detonations have an explosive effect.
[BG] DETOXIFICATION FACILITIES. Facilities that provide treatment for substance abuse, serving care recipients who are incapable of self-preservation or who are harmful to themselves or others.
[BS] DIAPHRAGM. A horizontal or sloped system acting to transmit lateral forces to vertical elements of the lateral force-resisting system. When the term “diaphragm” is used, it shall include horizontal bracing systems.
Diaphragm, blocked. In light-frame construction, a diaphragm in which all sheathing edges not occurring on a framing member are supported on and fastened to blocking.
Diaphragm, unblocked. A diaphragm that has edge nailing at supporting members only. Blocking between supporting structural members at panel edges is not included. Diaphragm panels are field nailed to supporting members.
Diaphragm boundary. In light-frame construction, a location where shear is transferred into or out of the diaphragm sheathing. Transfer is either to a boundary element or to another force resisting element.
CRC § 13.1.2 Medium relevance — show source text
General R606
Hollow unit R606.13.1.2, R606.3.2.2 Inspection R109.1.4 Seismic requirements R606.12 Veneer R703.8
Veneer attachment R703.3
Materials
Alternative 1.8.7, R104.2.2 Combustible R1001.11, R1001.12, R1003.18, R1003.19 For flue liners R1003.11
For hearth extension R1001.9 For siding R703.5 Means of Appeal R112 Board of appeals Appendix AA Means of Egress R318 Attachment of landings, decks, balconies and stairs R318.5
Door R318.2 Floors and landings at exterior doors R318.3 Hallways R318.6 Mezzanines R314.4
Ramps R318.8 Sleeping lofts R315.5 Stairways R318.7 Vertical egress R318.4 Mechanical
Inspection R109.1.2 Membrane Polyethylene R504.2.2, R506.3.3 Waterproofing R406.2 Water-resistive R703.2
Metal Roof panels R905.10
2025 CALIFORNIA RESIDENTIAL CODE INDEX-3
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INDEX
Purpose R1.1.2, R101.3
Radon Control methods Appendix BE Rafters
Grade of lumber R802.1
Spans R802.1.1, R802.4.1 Ramps R318.8 Glazing R324.4.5 Guards R321
Handrails R318.8.3, R320 Relocation (see Existing Buildings) Repair (see Existing Buildings) Resistant Siding Material (see Materials) Ridge Board R802.4 Roof
Coverings R905 Coverings fire hazard severity zones R902.1.1
Drainage R903.4 Fire classification R902 Flashing R703.4, R903.2, R905 Materials R904 Reroofing R908 Rooftop-mounted solar energy systems R329.3, R329.6 Steel framing R804 Vents in the WUI R806.1.1 Wood framing R802 Roofing Built-up R905.9 Liquid-applied coating R905.14 Modified bitumen R905.11 Sprayed polyurethane foam R905.13 Thermoplastic single-ply R905.13 Thermoset single-ply R905.12 Room Area, height and dimensions R312, R313
Minimum Sizes R312
Sanitation R326
Seismic Loads
Design criteria R301.1, R301.2.2, R301.3
Service Utilities R111
Shakes
Wood R702.6, R703.6, R905.8 Shingle Asphalt shingles R905.2 Metal R905.4
Slate R905.6
Frequently asked questions
What is the single easiest way to be code‑compliant for a roof diaphragm?
Use 3/8‑inch wood structural panel sheathing attached per the roof fastening schedule and provide blocking where the code triggers it (plan aspect ratio ≥ 3:1 and pitch ≥ 9:12). § R804.3.7.2.
If I block every sheathing edge, can I use much shorter diaphragm lengths?
For gypsum board ceilings the code permits multiplying the required diaphragm length by 0.35 when all panel edges are blocked; follow § R804.3.7.1 and the applicable table for the base length.
Is **flat blocking** a specific product or just any wood block?
The code calls out flat blocking as a C‑shaped or track section with minimum thickness 33 mils (0.84 mm) for the ceiling diaphragm details at gable endwalls — use the specified section/profile per § R804.3.7.1.
Does the roof diaphragm requirement change in high wind or seismic areas?
The baseline roof diaphragm sheathing thickness and blocking triggers are in § R804.3.7.2/§ R804.3.7.1, but wind/seismic conditions affect nail/screw spacing and may impose more restrictive requirements elsewhere in Chapter 8 or Chapter R6xx seismic sections; always check the applicable fastening tables and seismic provisions. file
Where do I find the sketch/detail for required blocking?
See Figure R804.3.7.2 for the roof blocking detail and Figures R804.3.7.1(1) and (2) for ceiling diaphragm to gable/sidewall details referenced in § R804.3.7.1 and § R804.3.7.2. file
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