Eight stair mistakes that cost the most

In rough order of how often they turn up and how much they cost to put right.

1. Measuring subfloor to subfloor

Measure finished floor to finished floor. Different finishes top and bottom change the first and last riser.

Total rise is finished floor to finished floor. Lay out a stair on bare subfloor, then put ¾″ hardwood downstairs and ½″ tile upstairs, and the bottom riser is ¾″ taller than the rest while the top one is ½″ shorter. That is a ⅝″ spread in a code that allows ⅜″, and it is a trip hazard at the one step where people are least ready for it.

The stair calculator has fields for both finishes. Use them.

2. Forgetting the stringer drop

The stringer drop: cut the bottom of the stringer down by one tread thickness, or the first step is taller than every other one.

Cut a stringer as a row of identical triangles, set it on the floor, lay treads on it, and every riser is right except the bottom one — which is now taller by the thickness of one tread. The bottom cut has to be reduced by the tread thickness.

This catches people who have built stairs for twenty years. It is the single commonest cutting error there is.

3. Assuming treads equal risers

A normal stair has one fewer tread than risers, because the upper floor is the top step.

On a normal stair the upper floor is the top step, so there is one fewer tread than riser. Get this wrong and the total run is out by a full tread — enough to miss a wall, a door swing or the floor opening entirely.

4. Checking winder going at the centre of the tread

On a winder the going is measured along the walk line — 12 inches in from the narrow edge under the IRC — never at the middle of the tread.

On a tapered tread the going must be checked on the walk line — the path a person actually takes, 12″ from the narrow edge under the IRC. Measured at the tread centre a winder can look generous and still fail.

5. Framing the floor opening before settling the stair

Headroom is measured vertically from the pitch line. The opening has to be long enough for the stair to drop clear of it.

The opening length depends on the pitch, and the pitch is not known until the stair is designed. Framing first and designing after means discovering you need another eighteen inches of hole in a joist that is already carrying load. Work the opening out first.

6. Rounding the riser height

Round 7.2″ to 7¼″ and multiply by fifteen and you have gained ¾″ that has to go somewhere — usually into the last riser. Carry the exact figure through every step and round once, at the cut.

7. Setting out balusters from one end

Divide the leftover evenly. Setting balusters out from one end leaves an oversized gap where the sphere is tested.

Fix a spacing, start at the newel, work along — and whatever is left over becomes one oversized gap at the far end. That gap is exactly where the 4″ sphere goes. Divide the remainder evenly and mark them all before drilling.

8. Building to the code you remember

Editions are adopted years apart and amended locally. The stair your neighbour passed inspection with in 2014 may not pass now, and the figures in a ten-year-old handbook may not be the ones your inspector is holding. Check the edition in force.