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I've got a interior stairwell with 20 steps of 7 1/2" rise on wood treads, except for the bottom step that's a concrete tread 6 1/2" up from a slab. The top of the stairwell features a pocket door with more or less a 3/8" sill (so technically that top step is 7 7/8"). Despite the somewhat uneven stringer cadence, it has been fine for many years.

But now we want to add a basement subfloor (dricore venting style) which adds at the very least 7/8". What can we do to even back up the stair treads? How bad is a short bottom step (I know it's better than a tall bottom step)? How thick a subfloor + carpet could be tolerated on the slab?

The current stair is carpeted, and we'll probably put back carpet. USA based California, home era 1920's.

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  • The only thing I know is to replace the stairwell with proper sizing. You will need to calculate your stairs and such .. here is a link: wikihow.com/Measure-for-Stairs – Ken Jan 22 '18 at 10:47
  • Replacing the interior stairwell on a 1920's house, over a matter of one inch, is out budget and scope. Something more creative is needed. – Bryce Jan 22 '18 at 18:02
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The key is to eliminate any variation over about 1/8" to prevent tripping. You can change the rise gradually and most folks won't notice it.

Sounds like you'll have about a 2" discrepancy to deal with. I'd remove 6-10 treads and start adding wood strips to the stringers to bring the bottom rise to say 7", then add 1/8" to each subsequent rise until you're at 7-1/2". Be sure to account for finished flooring.

If you want to better handle the tall top rise, remove all treads and work up as I described to its height in 1/8" increments.

Use construction adhesive and small nails or screws for all strips, and glue and 3" screws for the treads to prevent noise.

  • The existing treads are hard to remove, as there's a cut molding on both sides. Would it work to take 2" lumber and plane it down in 1/8" increments, making series of step toppers. Glue and screw those on top of the existing wooden treads, effectively making them thicker? – Bryce Jan 22 '18 at 18:05
  • You'd have oddly-sized nosings then. You'd have to cut off the existing nosings, which would present the same problem. – isherwood Jan 22 '18 at 20:00
  • Wood strips? And is there a modern quiet floor/gasket kind of product that would help with noise? – Bryce Jan 23 '18 at 6:35
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While dealing with a related situation, involving outside stairs on a mercantile occupancy, I found building code authorizing a short step at the bottom, where it meets the existing (possibly uneven) grade, provided the last tread, when descending, has safety-striped visual markings across its nose.

International Building Code (2009 ed): 1009.4.4 Exception

Where the bottom or top riser adjoins a sloping public walkway or driveway having an established grade and serving as a landing, the bottom or top riser is permitted to be reduced along the slope to less than 4 inches (102 mm) in height... The nosing ... at such nonuniform height risers shall have a distinctive marking stripe [1 to 2 inches wide], different from any other nosing marking provided on the stair flight...visible in descent of the stair and shall have a slip-resistant surface.

Absent specific code for interior equivalent, one could argue that an interior stair need only be as safe those exterior.

  • I cringe when I think of that last jolting step while carrying a box downstairs. It can actually be injurious to the spine. I'd want it fixed. – isherwood Jan 22 '18 at 20:01
  • The question is how much is too much. Code is very very strict, less than a lot of real world stairs nobody has a problem with. – Bryce Jan 23 '18 at 6:33

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