I'm adding a staircase to a wood frame house, and since space is
tight, I'd like to minimize the size of the required opening in the
subfloor. I've determined where the staircase headroom will start to
intersect the 2x10 floor joists and framed an opening. However, the
opening in the subfloor could be smaller than this opening in the
joists, because of the height of the joists.
Now the typical subfloor won't span the 3' wide opening required for
the stairs. So what is the best way to support this subfloor
extension? I have a few ideas below, and I'm looking for guidance as
to which one(s) would be practical and strong enough.
(1) Add one or more 2x6, 2x4, or 2x3 headers across the opening, just
above the required headroom. The 2x6 or 2x4 could be attached
with joist hangers; not sure how to attach the 2x3.
(2) Cut a bevel on the edge of a 2x10 or 2x12 and install it as a
tilted header across the opening, following the angle of the
required headroom. The bevel would allow the subfloor to sit flat
on the top of this tilted header. For extra support, perhaps
install mitered blocking between this tilted header and the
(3) Add a subfloor material that can span 3' (such as 1 1/8" plywood
or a 2x) between the joists up against the existing subfloor.
This could be supported by stair angles such as Simpson TA.