What this code requires
CRC R302.13 requires floor assemblies that aren’t already required to be fire-resistance rated to have a membrane on the underside of the floor framing: 1/2-inch gypsum wallboard, 5/8-inch wood structural panel, or an approved equivalent. Penetrations for ducts, lights, piping and wiring are allowed. Five exceptions remove the membrane: floors over a sprinklered space, floors over a crawl space with no storage or heating appliances, unprotected areas up to 80 square feet per story with perimeter fireblocking, wood floors framed with 2x10 or larger dimension lumber or structural composite lumber, and floors under 600 square feet in detached accessory structures with no habitable space above.
How to fix it
Map every floor assembly and what sits below it
On the framing plans, identify each floor assembly in the project and the space directly below it: basement, crawl space, garage, storage, or another story. Note the framing type for each one (sawn joists and their size, I-joists, open-web trusses). R302.13 applies to floor assemblies that aren’t required elsewhere in the code to be fire-resistance rated, so this list tells you which floors need a membrane or an exception.
Check each floor against the five exceptions
No membrane is needed where the floor is (1) directly over a space protected by an automatic sprinkler system per R309 or NFPA 13D, (2) directly over a crawl space not intended for storage or for fuel-fired or electric-powered heating appliances, (3) a portion left unprotected that totals no more than 80 square feet per story, with fireblocking around it, (4) wood framed with dimension lumber or structural composite lumber 2x10 nominal or larger, or another approved assembly with equivalent fire performance, or (5) a wood floor under 600 square feet in a detached accessory structure with no habitable space above. Write the exception you rely on next to each floor on the plans.
Specify the membrane on the underside of the framing
For every floor that doesn’t meet an exception, call out the membrane in the building sections and floor framing notes: “1/2-inch gypsum wallboard (or 5/8-inch wood structural panel) on underside of floor framing per CRC R302.13.” R302.13 does not require Type X board or a rated assembly; 1/2-inch gypsum wallboard, 5/8-inch wood structural panel, or an approved equivalent satisfies it. Use the same wording in the general notes so the reviewer finds it on any sheet.
Note gypsum fastening per CRC Table R702.3.5
R302.13 sets no fastener schedule of its own; gypsum board is attached per CRC R702.3.5 and Table R702.3.5. For 1/2-inch board on a ceiling, the table allows framing at 16 inches on center in either direction, or 24 inches on center applied perpendicular, with fasteners at 7 inches maximum for nails or 12 inches for screws. Screws must be Type W or Type S per ASTM C1002 and penetrate the wood at least 5/8 inch. If the ceiling gets a water-based texture at 24-inch framing, the table calls for 5/8-inch board or 1/2-inch sag-resistant ceiling board. For a wood structural panel membrane, show the fastener size and spacing on the detail.
Show penetrations without over-detailing them
R302.13 permits penetrations and openings for ducts, vents, electrical outlets, lighting, devices, luminaires, wires, speakers, drainage and piping. You don’t need rated light fixtures or firestop collars to satisfy R302.13 itself. Do check whether the same ceiling is also doing another job, such as a garage separation or a rated assembly, because those sections carry their own rules.
If you leave part of a floor unprotected, show the limits
Exception 3 lets portions of a floor stay unprotected only when the aggregate unprotected area is no more than 80 square feet per story and fireblocking per R302.11.1 runs along the perimeter of the unprotected portion. Dimension the unprotected area on the plan, state the total square footage, and detail the fireblocking material, for example 2-inch nominal lumber, 23/32-inch wood structural panel with backed joints, or 1/2-inch gypsum board.
Treat floors over a garage under R302.6
Where habitable rooms sit above a garage, Table R302.6 requires not less than 5/8-inch Type X gypsum board or equivalent on the garage ceiling, and Table R702.3.5 fastens it at 6 inches on center (nails or screws) with framing at 24 inches maximum, applied perpendicular. Cite R302.6 for that ceiling instead of R302.13, and keep the garage detail separate from the R302.13 membrane note so each requirement is easy to verify.
Update general notes and cloud the revisions
Add R302.13 to the code analysis or general notes: list each floor, the exception relied on or the membrane provided, and the detail reference, e.g. “Floor over basement: 1/2-inch gypsum wallboard per CRC R302.13, see Detail 4/A5.” Cloud the changed sections, framing plans and notes, and describe the change in the revision block. A reviewer who can find every floor’s answer in one place closes the correction faster.
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Why you got flagged
R302.13 corrections show up on new homes, additions and ADUs whose floor framing uses I-joists, open-web floor trusses or lumber smaller than 2x10. Here’s why plan-checkers flag it.
The most common trigger is a floor framing plan with engineered joists or trusses over a basement, a storage area or a mechanical crawl space, and building sections that show nothing on the underside of the framing. I-joists and floor trusses aren’t dimension lumber or structural composite lumber, so the 2x10 exception doesn’t cover them unless the product has approved documentation showing equivalent fire performance. If no exception is claimed and no membrane is shown, the reviewer writes the correction.
The second pattern is a crawl space exception that doesn’t hold up. The exception only covers a crawl space not intended for storage or for fuel-fired or electric-powered heating appliances. When the mechanical plan puts a furnace or other heating appliance under the floor, or the plans label the crawl space as storage, the exception is gone and the floor needs the membrane.
Third, the sprinkler exception gets claimed without checking what’s below. It applies only where the space directly below the floor is protected by an automatic sprinkler system per R309 or NFPA 13D. Additions to existing homes without sprinklers usually don’t get sprinklers, and R309 lets some spaces go unsprinklered, so the reviewer will ask you to show coverage or show the membrane.
Finally, many plan sets cite R302.13 for the garage ceiling. The 5/8-inch Type X board under habitable rooms above a garage comes from Table R302.6, not R302.13. Mixing the two leaves the reviewer unsure which requirement your detail is meant to satisfy.
The official code
2025 California Residential Code R302.13, Fire Protection of Floors (UpCodes)
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