Stairs· 3 min read
How to Calculate Stair Headroom and the Stringer for a Ceiling Opening
Loft stairs: how to check headroom at the floor opening, how long the opening must be for 2.1 or 2.0 m, and which board to buy for the stringer. Worked example.
On loft stairs it is usually the head that gets forgotten. The steps are comfortable, but halfway up you hit the edge of the floor above. Here is how to check it before anyone cuts the joists — and how the same calculation gives you the stringer.
The example
- Floor-to-floor height to the loft 2,600 mm, floor thickness above (upper finished floor to the ceiling underside) 300 mm.
- The joists allow an opening 3,000 mm long.
- Stair 800 mm wide, 40 mm treads with a 30 mm nosing, stringers from 300 × 50 mm board.
- 14 risers: 2,600 ÷ 14 = 185.7 mm, going 630 − 2 × 185.7 = 258.6 mm, pitch 35.7°, total run 3,361 mm.
What headroom is
Headroom is measured vertically from the line through the tread nosings (the pitch line) up to the ceiling above it. The critical point is the edge of the opening opposite the top of the stair. The US IRC asks for at least 6 ft 8 in measured from the sloped plane through the nosings; England's Approved Document K uses 2.0 m above the pitch line, with a relaxation for loft conversions; Czech practice designs to 2,100 mm. Check which value applies to your building.
Checking the example
The ceiling underside is at 2,600 − 300 = 2,300 mm. A 3,000 mm opening (measured back from the edge of the upper floor) ends near the foot of the stair, where the nosing line is 467 mm high. Headroom = 2,300 − 467 = 1,833 mm. That is 267 mm short of 2,100 mm — you would hit your head.
How long the opening must be
Work it backwards: at the edge of the opening the nosing line may be at most 2,300 − 2,100 = 200 mm high. But the first nosing is already 185.7 mm up, so the edge of the opening has to sit almost above the first step. The calculator gives a shortest opening of 3,372 mm — practically the whole 3,361 mm run.
The rule behind it: when the storey height minus the floor thickness is only a little more than the headroom you need, the opening must be almost as long as the stair.
| Stair | Headroom needed | Shortest opening | Headroom with a 3,000 mm opening |
|---|---|---|---|
| 14 risers, 185.7 × 258.6 mm, 35.7° | 2,100 mm | 3,372 mm | 1,833 mm |
| 14 risers, 185.7 × 258.6 mm, 35.7° | 2,000 mm | 3,232 mm | 1,833 mm |
| 13 risers, 200 × 230 mm, 41.0° | 2,100 mm | 2,790 mm | 2,283 mm |
| 15 risers, 173.3 × 283.3 mm, 31.5° | 2,100 mm | 3,953 mm | 1,517 mm |
A steeper stair needs a shorter opening: with 13 risers 2,790 mm is enough and the 3,000 mm opening leaves 2,283 mm. But the 230 mm going is below the default 240 mm limit and 41° is steep — check whether your rules allow it for a stair to a habitable loft. A gentler stair with 15 risers needs an opening of almost 4 m.
StairsCheck your headroomStringer: length, angles and board width
With 14 risers the stringer is 4,115 mm long (3,631 mm with 13). The plumb cut is 35.7° and the level cut 54.3° off square; on a framing square use 185.7 and 258.6 mm. The board width decides how much timber is left below the notches:
- 300 mm board — 149 mm left below the notches.
- 250 mm board — 99 mm left.
- 200 mm board — 49 mm left.
- 150 mm board — the notches do not fit.
How much timber must remain for a given span and load is structural design, which the calculator does not do. Buy boards at least 4,115 mm long plus an allowance for squaring the ends, and check the lengths your supplier stocks.
The calculator does geometry and quantities, not structural design. A floor opening means cutting joists or the slab; trimmers, stringer size and fixings must be designed by an engineer or to your local rules.