Sound & fire
Fire-rated drywall assemblies: what a rating means, where it is needed
A fire rating belongs to a whole wall, tested in a furnace, not to the board on it. Here is how ratings work and how to build to one.
What an hour means
A one-hour wall is one that, built exactly as tested, held back a standard fire in a laboratory furnace for an hour. The test, ASTM E119 (also published as UL 263), heats one side of a full-size wall along a standard time-temperature curve. The wall fails if flames or hot gases get through, if the unexposed side rises more than 250 °F on average or 325 °F at any point, or, for bearing walls, if it can no longer carry its load. Walls also face a hose-stream test afterwards.
An hour in the furnace is not an hour in a house fire. Real fires vary. The rating is a standard way of comparing assemblies and of setting code requirements, not a promise about any particular fire.
Ratings belong to assemblies
The rating is for the whole tested wall or floor-ceiling: the studs and their spacing, the board type, the number of layers, the screws and their spacing, the joint treatment, and often the insulation. Change one part and the rating no longer applies. Type X board does not make a wall one-hour; a wall built to a listing that uses type X does.
Listings are published by the Gypsum Association in GA-600, the Fire Resistance and Sound Control Design Manual, by UL in its directory, and by board makers. Each has a file number, such as GA WP 3660, and a drawing with every detail.
| Assembly | Board | Rating | Source |
|---|---|---|---|
| 2 × 6 studs at 16 in | 5/8 in type X, one layer each side, screws at 7 in | 1 hour | GA-600 WP 3660 |
| Same, with 5-1/2 in mineral fibre | One layer each side, screws at 12 in | 1 hour | GA-600 WP 3661 |
| Staggered studs | Two layers of 5/8 in type X each side | 2 hours | GA-600 WP 3910 |
| Double stud wall | Two layers of 5/8 in type X each side | 2 hours | GA-600 WP 3810 |
More are on the sound and fire assemblies table.
Where homes need them
Most of a single-family house needs no rated walls at all. The places that do:
- Attached garages: not a rated assembly, but a prescriptive separation of 1/2 in board, and 5/8 in type X under rooms above (IRC R302.6). The garage fire separation guide covers it.
- Two-family dwellings: walls and floors between the units are one-hour fire-resistance-rated assemblies tested to ASTM E119, with a reduction to half an hour where the building is sprinklered (IRC R302.3).
- Townhouses: walls between units are rated under IRC R302.2; the details depend on the code edition and sprinklers.
- Some furnace rooms and other spaces under local rules.
Multifamily and commercial buildings use rated assemblies throughout, specified by the architect.
Counting a listing
A 20 × 8 ft one-hour wall to GA WP 3660
- 160 sq ft a side: 6 sheets of 4 × 8 per side with waste, 12 sheets of 5/8 in type X
- Screws at 7 in on studs at 16 in: 7 studs × 7 = 49 per sheet, 588 screws of 2-1/4 in
- 845 lb of board, 112 ft of perimeter to seal with listed fire sealant
The same wall at two hours, two layers each side
- 24 sheets, about 1,690 lb
- Base layers at 24 in and face layers at 8 in, in this example: 840 screws, 1-5/8 in for the base layer and 2-1/2 in through both
- Every face-layer joint taped, every screw head covered
The spacing and screw lengths come from the listing you build to; the fire-rated assembly calculator takes them as inputs, and the fire-rated drywall calculator counts sheets for the common assembly families.
Building it right
- Follow the listing exactly: board, layers, orientation, fasteners and spacing.
- Full height: rated walls run from the floor to the structure above, not to a dropped ceiling.
- Joints: tape and cover the face-layer joints and fastener heads, at least to Level 1, more if painted. The levels of finish guide explains.
- Stagger layers: joints in the face layer offset from the base layer.
- Penetrations: pipes, cables and ducts through a rated wall need a listed firestop system, typically a fire-rated sealant or collar tested for that penetration.
- Openings: rated doors and frames, with closers where required.
- Inspection: many building departments inspect the screwing before taping.
Sound and fire together
Several of the most useful listings are rated for both: the staggered-stud wall in WP 3910 is two-hour and STC 50–54, the double stud wall in WP 3810 is two-hour and STC 55–59. If you need a quiet wall between units, the rated listing often gives you both. The guide to soundproofing a wall and the soundproofing assembly calculator cover the sound side.
Type X and type C
Type X board is 5/8 in gypsum board with glass fibres in the core, defined in ASTM C1396, which keeps it intact longer in a fire. Type C is an enhanced version with more fibre and other additives, used in some floor-ceiling and shaft listings. Both are named in the listings that use them; substituting one for the other needs the listing to allow it. The drywall thickness guide covers where type X goes in a house.
Common mistakes
- Assuming type X board makes a rated wall. Only the listing does.
- Stopping at the ceiling. Run to the structure.
- Adhesive where the listing does not allow it.
- Unsealed penetrations. Use a listed firestop.
- Mixing parts from two listings. No rating applies.