Building Stairs Formula
Building stairs doesn’t matter what type of staircase you are building, whether it’s a classic staircase, spiral staircase, straight, L shaped with a landing, curved, complete turn around 180 degree, or winders. You must determine the overall rise and run, then divide it into comfortable increments.
In house framing this distance is usually from the first floor to the second or from the basement floor to the first floor.
Determine Overall Rise and Run
The overall rise and run of the steps is determined by measuring the space where the staircase is to fit. Keep in mind what type of floor covering is to be used when determining rise between floors. If ceramic tile or similar material is to be used it must be figured into the rise. Measure horizontally where the steps are to land with the understanding that you need a minimum of three feet of clear space before any obstructions to the direction of travel.
Find Rise of Each Step
- Using a regular calculator, convert feet and inches to inches only.
- With an overall stair rise of 8′ 10 3/4″ you should come up with 106 3/4 or 106.75.
- Since 7 1/2″ is considered the perfect step, we divide 106.75 by 7.5 to find out how many steps are needed. 106.75 divided by 7.5 = 14.233 since we can’t have .233 of a step we need to drop it and divide the overall rise by 14. 106.75 divided by 14 = 7.625 or 7 5/8″.
- We now know that this set of steps requires 14 equal increments of 7 5/8″ to climb from one floor to the next. Write this number down, it is one of the keys to a step building success story.
Find Run of Each Step
- When building this type of stairs there is one less tread than riser because the upper floor is the final step.
- We need to find 13 equal increments to add up to our overall run.
- Using a regular calculator convert 11′ 2 7/8″ to 134.875.
- Divide 134.875 by 13 = 10.375 or 10 3/8″ run per stair tread. The actual run of the steps will be more due to the bottom step riser and the added bull nose of the tread. Make sure you will still have the three feet of required clear space at the bottom of the steps before laying out the stringers.
- Headroom minimum is almost always 6′ 8″ anywhere along the length of the stairway. This height is not only so people won’t bump their heads easily but also for moving furniture.
- Width should not be less than 3′ above the handrail and below the minimum required headroom.
- Handrails height should be between 34 and 38 inches measured vertically from the treads bull nose. A handrail should be provided on one side of every set with 4 risers or more. Handrails must not protrude into stairway path more than 4 1/2″
- Riser height shall not be more than 7 3/4″ and not vary more than 3/8″ from greatest to least. Be careful on this one, I’ve seen the riser height anywhere from 7 7/16″ to 8 1/8″ and not allowed to vary more than 1/8″.
- Tread width minimum is 10″ measured from bull nose to bull nose and should not vary more than 3/8″.
- Bull nose or tread depth overhang should be between 3/4″ and 1 1/4″ and not vary more than 3/8″