Tread Depth - Stair Step Measurement

The tread depth is the horizontal measurement between treads. The perfect example of this would be the picture below. You can use a framing square to find the actual tread depth, but you could have a problem if you don't. The actual tread depth for the stairway below is only 10 inches.

The actual tread depth is the square measurement, not the actual measurement of the stair tread. For example, the stair treads below are 11 1/2 inches wide, but the actual stair tread depth is only going to be 10 inches.

Funny Story Involving Stair Tread Depth

You're going to find this hard to believe, but I was only 18 years old when my father sent me out to build my first stairs, for three model homes. I knew enough about stairs, but obviously didn't know enough about the stair tread depth.

The first set of stairs I laid out with an 11 inch tread depth, even though it was only supposed to be 10 inches. I cut my stringer and took it over to the home, to double check my measurements, to make sure that it was actually going to work.

To my surprise, and luckily no one was around, to see my mistake. The stairway extended past the hallway wall, about 14 inches. Since I didn't know what was wrong and we didn't have cell phones back then, I had to figure this one out on my own.

It took me a few minutes, but I finally found my mistake, made my corrections and finished installing my first stairs for three model homes, by myself. I learned a lot that day about the difference between a stair tread and the stair tread depth.

In case you're wondering where I came up with 11 inches, instead of 10 inches, it's because we usually add a 1 inch overhang or undercut to our treads. This can be confusing for novice stair builders.

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