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It seems to me that many people put subfloors in their basements (and other floors), but what do they do when it comes to the existing stairs? I'm code says that you can't have more than 3/8" difference in the steps, and most subfloors are more than 3/8" thick (especially with flooring on top of it). What do most people do to fix their stairs?

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Honestly I have done the exact same thing more than a few times and have never had an inspector say anything about the "short" landing. Why don't you call the inspector and ask him what he wants done - if anything? –  DMoore Feb 19 '14 at 18:11

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Technically the floor below the last step is considered a landing, and stair risers may be measured from tread to tread, not landing to tread. The 3/8", depending on your exact local codes, may not apply to the tread to landing at the top and bottom of the stairs. It may have a different standard, or it may have the same standard. Of course if the height difference is large it's going to cause tripping and stumbling, but for minor changes a little more than the 3/8" code it may not make much of a difference or present a safety hazard.

However this can be fixed by putting a thin board on the bottom tread, then half that thickness on the next tread, then half that thickness on the next tread until the requirement is met. If your subfloor is under 3/4 inch higher than it used to be, then it should only take one 3/8 piece of wood on the bottom step to make the first two risers only 3/8 shorter than the rest of the risers, for instance.

You can, of course, replace the stairs or stringers entirely to meet the code requirements and account for the new floor height at the bottom landing. If the height difference is large enough that you'd have to modify more than half the stairs you'll probably have less work replacing the stringers than adding height to each tread.

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It seems like if you didn't adjust the stairs than that last step-to-landing would be really short. Let's say that I put in 1" of XPS insulation + 2x4 (1.5") sleepers + 3/4" plywood or OSB. That's 3 1/4" just for the sub-floor! If the existing risers are 7 3/4", that's cutting the stairs 40%. –  2 Left Thumbs Jan 19 '14 at 2:30
Adding to the stairs in that manner seems like it'd be a real pain. For one, you have to cut enough wood to cover all of the steps, plus what do you do with the existing nosing? Also, won't all of the stairs be of slightly different heights? That'd be weird. I was thinking that I could cut new stringers. Seems like a pain, but at least I could re-use the existing stairs and they'd all be the same height. –  2 Left Thumbs Jan 19 '14 at 2:32
@Adam Davis: I think your corrections are a little off. Say you have 6 risers, and put 1.5 inches on the bottom "landing". Then you need 0.25 inches on the first tread from the top, 0.50 inches on the next one down, and 0.75 on the next. Each new riser will be equal, and 0.25 inches less than before... In a real case, the numbers will never be that neat... –  User58220 Jan 19 '14 at 5:28
@2LeftThumbs Cutting new stringers is an obvious fix which I didn't mention, so I'll add that in. –  Adam Davis Jan 20 '14 at 13:55
@User58220 Your method is a linear change in riser height, which may be better simply from the standpoint of maintaining an even riser height top to bottom with no variation. My method will cause the bottom few treads to be a different height, but still within code, and require fewer modifications overall. If you're going to go to the trouble of adding height to every tread, it'd probably be easier simply to recut the stringers anyway. Still, 1.5" height adjustment for such a short stairway would suggest replacement of the stairs. –  Adam Davis Jan 20 '14 at 14:03

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