Five stair mistakes that only show up after the concrete is poured
A stair is one of the few parts of a house where a quarter-inch error is not a cosmetic problem. It is a failed inspection, and sometimes a rebuild. What makes stairs unusual is that nearly every serious error is committed before any wood is cut, in the ten minutes spent measuring.
1. Measuring to the subfloor
Total rise is the distance between finished floor surfaces. If the upper level is getting three-quarter inch hardwood and the lower level is getting tile over backer board, both thicknesses belong in the measurement.
Get this wrong and the error does not distribute itself politely across the flight. It lands entirely on the top or bottom riser, producing exactly the uneven step that inspectors check for first.
2. Assuming the floor is level
Measure the total rise at both stringers, not just one. On a renovation, a half-inch difference across the width of a stair opening is unremarkable — old joists sag, and slabs are rarely flat.
If the two measurements differ, work from the larger one and pack the low stringer at the base. Splitting the difference produces a stair that is out of level along its whole length.
3. Forgetting the stringer drop
The bottom of a cut stringer sits on the floor, and the first tread sits on top of the stringer. If you do not subtract the tread thickness from the bottom riser cut, every riser in the flight is correct except the first, which is one tread thickness too tall.
This is the single most common cutting error, and it is invisible until the treads go on.
4. Treating tread depth and run as the same number
Tread depth is the horizontal distance from one riser face to the next. The visible tread board is usually an inch or so deeper because of the nosing that overhangs the riser below. Code measures depth between riser faces, not across the board.
Feeding board width into a calculator instead of the actual run inflates the total run and produces a stringer cut too shallow.
5. Designing to comfort instead of code
Blondel’s rule — twice the riser plus the tread landing between 24 and 25 inches — is a genuinely good predictor of whether a stair feels right underfoot. It has no legal standing.
The IRC caps residential risers at 7 3/4 inches and requires at least 10 inches of tread. A stair can satisfy every comfort formula ever written and still be torn out. Check the code limit first, then optimise for feel inside it.
The order that avoids all five
Measure finished floor to finished floor, at both sides. Divide by your target riser to get a whole riser count. Divide back to get the actual riser height. Confirm that height is under the code maximum before touching a saw. Then cut, remembering the stringer drop.