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I have set of stairs that leads from the main floor to the basement. A little over half way down there is a landing (typical L-shape). I'm looking to put in an insulated subfloor in my basement, which could raise the floor height quite a bit. If I did so, I might have to change the bottom set of stairs. I was thinking of cutting new stringers, which would allow my steps to all be of uniform height, but that would make the risers on the bottom set of stairs differ from the upper set. I have 6 steps, and if I were to put in a 3 1/4" sub-floor [1"XPS + 2x4 (1.5") sleepers + 3/4" plywood/OSB], that'd change the riser height by more than 3/8". Is this a problem?

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No it is separated by a landing so it doesnt matter. Just as long as the rise is between 4-7 3/4 inches.

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Is this your opinion, or can you cite an official source? Changing step height in the middle of a staircase is going to be strange, and could potentially be a tripping hazard. Your body will get used to the step height in the first part of the stairs, and will likely fumble on the second section if the step height changes by too much. I'm guessing some studies were involved to come to the 3/8" step variance, so if the step height change is less than or equal to 3/8" you'll probably be alright. Greater than that, and you risk a tripping hazard. –  Tester101 Jan 20 at 11:28

According to the International Residential Code (IRC), the 3/8" variance only applies to a "flight of stairs" not the entire "stairway".

International Residential Code 2009

Chapter 3 - Building Planning

Section 311 Means of Egress

R311.7 Stairways.

R311.7.4.1 Riser height. The maximum riser height shall be 73/4 inches (196 mm). The riser shall be measured vertically between leading edges of the adjacent treads. The greatest riser height within any flight of stairs shall not exceed the smallest by more than 3/8 inch (9.5 mm).

Looking at the definition of stairway and flight, you can see that the the 3/8" variance only applies to individual flights of stairs, and not the entire stairway.

Chapter 2 - Definitions

Section 202 Definitions

Stairway. One or more flights of stairs, either exterior or interior, with the necessary landings and platforms connecting them, to form a continuous and uninterrupted passage from one level to another.

Flight. A continuous run of rectangular treads or winders or combination thereof from one landing to another.

However, you may find that changing the riser height by too much within a stairway could lead to a tripping hazard. As you travel up/down the stairs, your body gets used to the step height. If midway through the stairway the step height changes, you may become temporarily disoriented and potentially trip or misstep.

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Yah, I'm slightly worried about that, but I don't know how much of a deal it'll be. They already allow 0.375" variance PER STEP, I'd think that 0.6" per step wouldn't be that big of a deal. If I didn't do it that way, I'd have to cover every single stair. I don't know how I'd do that as 1) the current treads are already rounded (what looks to be one solid piece) and 2) How would I cover the stairs in increments as small as 1/16"? –  2 Left Thumbs Jan 20 at 13:26
    
The other options would be to rebuild the entire stairway, or build up the floor to the height of the last step. I'm not sure what the answer is for you, I'm just telling you what the building industry follows. These values aren't simply plucked from the air, they are backed by science and experience. –  Tester101 Jan 20 at 13:42
    
It just boggles my mind that if you install any type of subfloor (just the plywood alone will put you over the 3/8" difference) that you'd have to completely rebuild a staircase. Yes, the last tread to landing doesn't "count", but if it's going to be too odd for more than 3/8" between steps, it'll be REALLY odd to have 1+" diff between last step and landing. You'd think that if the basement was unfinished that the last stair would have been extra high to accommodate a future floor. –  2 Left Thumbs Jan 20 at 14:46
    
@2LeftThumbs That sounds like an issue that should have been discussed with the original builder. There may have been an option to allow for future basement finishing, but the option was not offered to or accepted by the original owner. In my home, the builder left the original "construction grade" stairway in place. I'm assuming this was to save money as well as allow for future basement finishing (in which case a proper stairway would be built). Many homes are built with no thought of the basement ever becoming livable space. –  Tester101 Jan 20 at 14:52
    
@2LeftThumbs Contact a local contractor/builder, they may have a solution that is approved by the local inspectors. –  Tester101 Jan 20 at 14:54

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