The concealed space between the stair stringers under the run has been left open at both ends, where it meets the wall cavities at the top and the floor framing at the bottom. What does the code require there?
- AGypsum board applied to the underside of the stair run and up the face of both stringers
Membrane protection under a stair is required only where the space beneath is enclosed and used, and it does nothing about the concealed cavity between the stringers. Fireblocking closes that cavity; a finished soffit does not.
- Fireblocking in the concealed space between the stringers at the top and the bottom of the run
- CDraftstopping installed across the concealed space at the midpoint of the stair run below
Draftstopping is a horizontal measure used in floor-ceiling assemblies and in attics with combustible concealed spaces. A stair cavity is blocked at its ends rather than divided somewhere along its length.
- DNothing, because the concealed space under a stair run falls outside the fireblocking provisions
The run of a stair is one of the specific locations the fireblocking section names. Leaving it open creates a chimney running from the lower level straight up into the floor system above.
Why B is correct
Stair stringers appear by name in the list of required fireblocking locations, with the blocking called for at the top and bottom of the run. Those are the two points where the diagonal cavity connects to other concealed spaces, so closing them isolates the stair from the rest of the structure.
What this question is testing
Whether the candidate can recall a fireblocking location that is neither a wall nor a floor, and knows the specific points along the stair where blocking is required rather than assuming continuous treatment. It also tests whether the candidate keeps the stringer blocking requirement separate from the protection of usable storage space under a stair.
On the job
Basement stairs get framed early, the stringers get open ends, and by final inspection the whole assembly is closed with drywall and trim. Inspectors who do not look during framing never see it again. On remodels the same cavity often connects an old open stringer space to a newly opened wall, which is worse because the fire path now runs between a finished basement and the main floor.
Memory technique
Block the stair where it starts and where it stops.
Exam tip
When a fireblocking question mentions stairs, the answer is top and bottom of the run, between the stringers.
Where to look it up
IRC Chapter 3, the fireblocking section, the item addressing concealed spaces between stair stringers.