Stair Design Basics

Image Credits:

1. Ferguson and Shamamian Architects - www.fergusonshamamian.com

1.
Aug 11, 2017 9:10 AM ET

Edited Aug 11, 2017 9:16 AM ET.

How to compute exact riser height and tread depth
by Robert Opaluch

Agree that 7" should be the maximum riser height, and strive for closer to 6" for safety and ease of climbing stairs, especially for children and the elderly.

I’d add something like the following, for those who can’t view the Fine Homebuilding “subscribers only” article:
To compute the exact riser height and tread depth for your stairway:
1. Measure the vertical distance between the finish floors (height floor to floor). Divide by 7”. That tells you the approximate number of steps up the stairway, which is the number of risers. (That number might be about 15.4 for 8 foot ceilings plus floor framing.) Round the number up or down, preferably up, to the nearest integer (no fractions or decimals for the number of risers).
2. Divide the floor-to-floor height by the number of risers. That tells you the height of each riser. It should be about 7” (maximum 7 ¾” allowed). For a less steep stairway for the safety of children and ease of climbing stairs by the elderly, try to get riser height closer to 6”, by increasing the number of risers by 1.
3. Once you have the riser height, compute a comfortable and safe tread depth for that riser height:
• Tread depth equals 25" minus twice the riser height
For a riser height of 7”, that gives you a tread depth of 11”
For a riser height of 6.5”, that gives you a tread depth of 12”
4. The treads must stick out beyond the risers about 1” (3/4” minimum to 1 ¼” maximum allowed), called “nosing”. Therefore your tread stock must be tread depth plus nosing, or 11” + 1” = 12” depth (or 12” + 1” = 13” depth). If you design the risers to rest on top of the treads and reach the bottom of the tread above it, the tread stock also must be increased by the thickness of the riser boards, typically another ¾”.
5. Riser stock would be less than the rise described above, by subtracting the thickness of any tread stock above and below it. Useful to draw a diagram of the stair design.

2.
Aug 11, 2017 9:35 AM ET

by Reid Baldwin

Robert,

Item 3 above yields longer tread for a shorter rise. That intuitively seems backward to me. Is matching a typical stride more important than controlling the steepness?

3.
Aug 11, 2017 10:10 AM ET

Edited Aug 11, 2017 10:13 AM ET.

by Robert Opaluch

Reid,
Yes the formulas for computing tread width will make a comfortable and safe stride up and down stairs for adults. There are a few different formulas but they all yield similar results. Less steep stairs have both shorter rise and longer run (tread). Steep stairs have longer rise and shorter run. Like a ladder has a steep rise but very short run, and outdoor stairs have more like a 6" rise and 12" run, so you can't tumble down slippery outdoor stairs like you would tumble down a steep interior stairway or ladder.

4.
Aug 11, 2017 11:26 AM ET

by Malcolm Taylor

One of the reasons tread width is often shorter than might be desirable is that the stock they are commonly made from (both lumber and pre-made OSB) is typically 11" to 11 1/2".

5.
Aug 14, 2017 2:07 PM ET

Edited Aug 14, 2017 2:11 PM ET.

NAHB
by Gordon Franke

Just because the NAHB has "very deep pockets" doesn't mean they didn't have valid arguments against the 7-11 minimum. There is a lot more to such issues than "is it safer, if yes then make it mandatory."