Lesson 8 of 13
Stairs
A stair is the least forgiving thing you will lay out. Everything else on a job has some tolerance in it. A stair has almost none, because a person walks it in the dark, and their foot already knows what the next riser should be.
Step one
Read the material
Three ideas, and the first one contains the rounding rule that catches almost everybody once.
Concept one
Total rise, and how many risers
Total rise is finished floor to finished floor, not subfloor to subfloor. Measure it before anything else, and measure it after the flooring is decided, because a three quarter inch floor you did not know about changes every riser in the flight.
Now divide the total rise by the largest riser you are allowed. That almost never comes out even, and here is the rule: you always round up. Rounding down would put every riser over the maximum, which is the one thing you may not do. Rounding up adds a riser and makes each one a little shorter, which is always legal and usually more comfortable.
A hundred and fifteen inches of total rise, divided by a seven and three quarter inch maximum, is 14.8. Round up. Fifteen risers.
Concept two
Every riser is equal, and the arithmetic makes it so
Once you have the count, divide the total rise by it. That is your unit rise, and it will be some awkward number like 7.667 inches, which is 7 and 11/16 on your tape.
Do not round it and multiply. Divide, and lay the whole flight out from a single running measurement so the error never accumulates. If you round each riser to the nearest sixteenth and then stack fifteen of them, you can be most of an inch off by the top, and every one of those sixteenths lands in the last step.
That last step is why this matters more than anywhere else on the job. A person climbing a stair calibrates on the second riser and stops looking. If the top one is a quarter inch different, they catch a toe. Equal risers is not a code preference, it is the whole point.
Concept three
Total run, and the tread you always forget
Here is the one people get wrong. A flight has one fewer tread than it has risers, because the top riser lands you on the floor above and that floor is not a tread you build.
So total run is the tread depth times the number of risers minus one. Fifteen risers with a ten inch tread is fourteen treads, which is 140 inches, which is 11 feet 8 inches of floor eaten by the stair.
Check that against the opening before you cut a stringer. Most stairs that have to be rebuilt were not wrong in the risers. They were the right stair for a room that was a foot too short.
On code. The maximum riser and minimum tread come from the code your jurisdiction has adopted, and they are not the same everywhere or for every occupancy. This lesson uses 7 3/4 inch maximum riser and 10 inch minimum tread because they are common residential figures. Verify against your local code before you cut a stringer.
Step two
Work it with your hands on it
Set a floor to floor height and a tread depth. The flight draws itself and tells you whether it fits.
Hands on it, off the screen. Find a stair in your own house. Measure floor to floor, count the risers, and divide. Then measure one riser with a tape and see how close your arithmetic came.
Then measure the total run on the floor and check it against tread depth times risers minus one. If it does not agree, count the treads again — that is usually where it went.
Step three
Prove you understood it
One question per idea, new numbers every time.
All three solid.
You can set a riser count, divide out an equal unit rise, and know how much floor the flight will take. Lesson nine is the angles that are not ninety.
Trades Geometry
Lesson eight of thirteen. Written by our founder, Dr. Gene A Constant, and donated to the Foundation. Published by Global Sovereign University Press.
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