Stairways-geometry and measurements - Part 2

This is my second part on stairs and stair design. In this brief article I'd like to discuss some of the geometry of stair design and a few of the more important stair codes used throughout most of the US.

A friend of mine told me recently that his architect had not left enough space for his stairway in the new home he was constructing. He asked me come over and take a look. I went over with my tape measure and began measuring his foyer and the height between the first floor and the second floor. We determined that he, in fact, did not have enough room as planned and so he had to ask his carpentry crew to push the stairway into his living run by about 2 feet. This was one surprise he was not very excited about.

And so let me begin by explaining a simple way to determine the "footprint" of your stairway.

Floor-to-floor height: first measure the height of finish floor to finish floor. Secondly, divide this distance by 7 1/2". This will tell you how many risers you need. Risers are the vertical distance between the treads. You will need to round this number to the nearest whole number. For example, if your finished floor to finished floor height is 123 inches you will then need 16 risers (123 / 7.5" = 16). Sixteen risers will require 15 treads (you always need one less tread than the number of risers). If you're treads are 11 inches deep then your stairway is 165 inches long (15 X 11"=165"). So your stair "footprint" is 165" long and however wide your stairs need to be. Pretty simple. Of course it's possible you may need a landing either to make a turn or to breakup a long straight run. In either case, your number of risers and treads is the same. If you need a landing, just remember that it should be as deep as your stairway is wide. So if you're stairs are 42 inches wide a landing should be 42 inches deep as well.

We started off dividing the vertical distance by 71/2 inches because a 7 1/2 inch rise is considered to be the most comfortable. There used to be a rule of thumb that stated two rises and one tread should equal 25 inches. And so 7 1/2 times two equals 15 inches plus 10 equals 25inches. If you need 8 inches for a riser, this would reduce the size of your tread 8" X 2 = 16" + 9=25 which gives a 9" tread). And so, the theory is the higher the rise the smaller the tread, the lower the rise the deeper the tread.

However, according to the International Residential Code, riser height should not exceed 7 3/4". And the depth of the tread should not be less than 10 inches. And so in the old days, if you were short on space, it was not uncommon to eliminate a tread and a riser which resulted in a higher riser height. But today this is considered unsafe (above 7 3/4").

Notice in the drawing above the proper way to measure rise height and tread depth. Do not include the tread nosing when measuring the "run".

This is probably a good place to talk about the rail height or, in this case, the "rake" height. As you can tell by the picture on the right, the rake height is the vertical distance between the top of the hand rail and a tread. It is measured from the nosing on the stair tread.

According to the International Building Code, the rake height should be a minimum of 34 inches and maximum 38 inches.

The balcony rail height should be a minimum of 36".