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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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Bryce
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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: https://www.wikihow.com/Measure-for-Stairs – Ken Jan 22 '18 at 10:47
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    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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Old question but surprised this suggestion isn't there. Without seeing the layout of the rest of the room, you can possibly add a landing. For example extend the second step and turn it into a landing. You can then split the difference of the height of the first riser. This would mean the entry to the stairs is now on one side. This should all be to code as the height variation code applies to flights, so you created a new flight.

Joe Schmo
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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.

isherwood
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  • 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
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    Wood strips? And is there a modern quiet floor/gasket kind of product that would help with noise? – Bryce Jan 23 '18 at 06:35
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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.

Upnorth
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  • 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
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    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 06:33