The framer closed off the required draftstop in the floor-ceiling assembly with a sheet of 1/4-inch hardboard stapled to the truss webs. What is the correct finding on that draftstop?
- Reject it; sheet draftstops are 1/2-inch gypsum board or 3/8-inch wood structural panel at the thinnest, adequately supported
- BAccept it; a draftstop only has to slow air movement, so any continuous sheet fastened to the framing satisfies the requirement
The provision names specific materials and thicknesses precisely so that field judgment about what is thick enough does not enter into it. A sheet that flexes and pulls off its staples stops nothing.
- CReject it; a draftstop in a floor assembly has to be built as a tested one-hour rated assembly carried down both sides of the trusses
That is the standard for a fire-resistance-rated separation. Draftstopping is a lesser requirement aimed at limiting air movement inside a concealed space, and it is satisfied by a single listed material.
- DAccept it; hardboard qualifies once the joints are taped and sealed with mastic so that the panel is continuous from edge to edge
Sealing the joints does nothing about the thickness and support the provision requires, and taping a material that is not on the accepted list does not add it to the list.
Why A is correct
The code lists the materials that may be used as a draftstop and the minimum thickness of each, with 1/2-inch gypsum board and 3/8-inch wood structural panel among the thinnest sheet goods accepted. Quarter-inch hardboard is not a listed material, and stapling it to truss webs does not give the support the provision calls for.
What this question is testing
Whether the candidate knows draftstopping is a materials-and-thickness requirement with a published list, can separate it from fire-resistance-rated construction, and will not accept a substitute simply because it is continuous.
On the job
Truss floors over finished basements are where this comes up most, because the space between the trusses and the ceiling below runs the length of the house. Framers reach for whatever sheet stock is on the truck when the draftstop is the last item before the ceiling goes up. Inspectors carry a tape and check thickness, then push on the panel: if it moves, the support is not there. Getting it wrong is expensive, because the ceiling is usually closed within days.
Memory technique
Half gypsum, three-eighths panel: anything thinner is not a draftstop.
Exam tip
Draftstop materials are a short list with minimum thicknesses. Learn the two sheet goods and their numbers.
Where to look it up
IRC Chapter 3, fire-resistant construction, the draftstopping section that follows fireblocking; read the materials paragraph.