options for fireproofing underside of stairs

Trevor_Lambert
| Posted in General Questions on
Do the underside of stair treads need some kind of fire barrier?
If so, what are options besides drywall or cementitious board?
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Replies
Usually 5/8" drywall is used when fire rated assemblies are built. I don't know of any specific assemblies for stairs, but if you use 5/8" drywall, you should get similar performance to what you'd get using that drywall on a wall. A wall with 5/8" drywall on each side is a 1 hour wall assembly (assuming you build to one of the specified assemblies, but a 1 hour wall is pretty much a regular studwall using 5/8" type X drywall on both sides but no other unique differences). I would count a layer of 5/8" drywall as 30 minutes of resistance if this isn't something that needs a specific rating (which would require a listed assembly to be used).
Bill
Trevor,
If the stairs are not part of a rated assembly - that is a required fire separation between two occupancies - then all they need is fire-blocking at the floor above to stop flame spread. The stairs themselves usually act as that.
⅝" type X or C drywall is generally the requirement for a fire seperation, including under stairs when a fire seperation is required. (At least in my neck of the woods).
You asked for other options where I assume it's to satisfy a code, so this is pretty much the only option based on prescriptive building codes.
But if you're asking for personal performance, you can add mineral wool/rockwool under the stairs, a double later 5/8" drywall (both layers seams mudded) intumescent paint, and have no openings in the ceiling under the stairs (like downlights).
You'll get pretty robust fire resistant performance with that.
Jamie
I guess another way of phrasing it is that if a horizontal fire separation is required, the underside of a stair needs to be treated in the same way the other flat or sloped ceilings are.