I know that math is intimidating to some but this, I think, should be understood by most. I will cover only (1) how to establish the riser height and number of treads and (2) determine the “footprint” of the stairway.

You may have heard (or experienced) someone in the process of building their home and discovering there is not enough room for the stairway. I received a call from a friend last year regarding his stairway. His home was framed up and they were trying to determine were the first step of the staircase would go. To everyone’s disappointment the foyer was not long enough to get the staircase in. A few measurements was all we needed to decide that the footprint had to push into the living room by about two feet. Fortunately it turned out to be no so bad.

On another occasion prior to the one above, I was adding a second floor to a home years ago and was struggling to get the headroom I needed to satisfy the local building code requirements. In the end we had to get a “variance” for the stair project. We just couldn’t do it.

I said all this to say that it is really important to know how to establish the “footprint” of your staircase. And since riser height and tread depth are a part of the local building code is equally important to understand how to establish them as well.

The first measurement needed for both assessments is the total height from finished floor to finished floor. OK I know the floors are probably not installed yet so you’ll need to subtract the thickness of the bottom floor and add the thickness to the top floor to your overall height measurement.
You would then take this measurement and divide by 7 1/2″ (7 1/2″ is considered the ideal riser height)
So if your finished floor to finished floor is 125″ you would divide 125″ by 7 1/2″ (125 / 7 1/2 = 16.67). You would then round to the nearest whole number (in this case 17). There are seventeen risers in your staircase. To establish what your actual riser height divide 17 into 125 (125 / 17 = 7 13/16″). Your riser height is 7 13/16″. Here is our first problem. According to the International Stair Building code the riser height should not exceed 7 3/4″. Check with your local building code for their restrictions – it may still work in this case. If not then we would add a riser to reduce the riser height. You would divide 125″ by 18 to establish the riser height.

There is one less tread than the number of risers so if there are 18 risers the number of treads is 17. And if there are 17 treads then it is simple math to establish the length of the staircase. Seventeen X 11″, for example, would mean your staircase is 187″ long. The obvious question becomes: Do I have 187″ for my stairway plus enough space get on the first step.

The width of the staircase should be a minimum of 36″ from trim to trim. You should note as well that landings will not change the number of risers and treads. The landing becomes, in effect, a wide tread. Note also that a landing should be as deep as the stairway is wide.

There is obviously much more that can be said about stairway math but this should suffice to establish the stairway footprint and riser height considerations.

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