On 2010-07-09, Borrall Wonnell wrote:

I've got 7 steps in my concrete-floor basement leading to a landing.
Now I'm adding a subfloor with a height of 1.25". Stair stringers
will now rest on top of the subfloor. Which is the best option?

1) Do nothing, (top step height reduced by 1.25")
2) Cut 1.25" off the bottom of the stringers (bottom step height
reduced by 1.25")
3) Cut 0.75" off the bottom of the stringers (0.75" reduction on top/
bottom step)
4) Build new stringers, adjusting each by 3/16"
5) ??

As others have mentioned, having one riser off by 1.25" is definitely
a safety hazard and not a reasonable solution. If you don't want to
rebuild the stringers, then I suggest you progressively shim the tops
of the existing stringers:

First triple check that after the finish floor and finish treads the
discrepancy will still be 1.25". Then cut 1.25" off the bottom of the
existing stringers, so all the error is accumulated at the bottom.

Now prepare a series of shims (rounding to the nearest 1/32 of an
inch): 3/16", 11/32", 17/32", 23/32", 29/32", 1 1/16", 1 1/4".
Depending on your skills and available tools, you could rip each of
these out of solid lumber. Or you can approximate each shim by a
couple thicknesses of plywood: e.g. nominal 1/4" (actual 7/32"), 3/8"
(11/32"), 1/4" + 3/8" (18/32"), 3/4" (23/32"), 3/4" + 1/4" (30/32"),
3/4" + 3/8" (34/32"), and 1/4" + 3/8" + 3/4" (41/32").

Then just apply the shims to the tops of the stringers progressing
down from the top.

Wayne