Checking a School Stair Against a Tight Stairwell
A school stair must connect floors 3.60 m apart. The available stairwell is 5.40 m long and 3.20 m wide, measured inside finishes. Two landings, each at least 1.20 m deep, are required. The stair width must be 1.50 m per flight, and a 0.20 m gap is needed between parallel flights. Local rules, stated in the brief, limit risers to 150 mm and require goings of at least 280 mm. The architect first proposes a straight flight using the full 5.40 m length. The contractor says it will not fit. Check the claim and then test a dog-leg arrangement. Also consider a 140 mm riser option if the dog-leg is too tight. Headroom is not the issue in this check.
1. For the straight stair, calculate risers, treads and required length; does it fit?
Read the worked answer
Total risers = 3,600 / 150 = 24 exactly. A stair with 24 risers has 23 goings between floor levels. Required going length = 23 × 280 = 6,440 mm = 6.44 m. Add two 1.20 m landings gives 6.44 + 2.40 = 8.84 m, far more than the 5.40 m available. Even without landings it exceeds 5.40 m. The contractor is right: a straight stair cannot fit.
2. Design a dog-leg: number of risers per flight, treads per flight, plan length, and slope angle. Check against the 5.40 m × 3.20 m stairwell.
Read the worked answer
Use a dog-leg with two flights of 12 risers each. Each flight has 11 goings because the upper landing is the 12th tread level. Going per flight = 11 × 0.28 = 3.08 m. In plan, the stairwell length is one flight plus one landing depth: 3.08 + 1.20 = 4.28 m, which fits 5.40 m with 1.12 m spare. Width = two 1.50 m flights + 0.20 m gap = 3.20 m, exactly the available width. Rise per flight = 12 × 0.15 = 1.80 m; slope angle = arctan(1.80 / 3.08) = 30.3°, acceptable for a school stair. Landing depth 1.20 m is met.
3. If the 140 mm option is used, recalculate risers and check whether it improves fit or comfort. State a judgement.
Read the worked answer
With 140 mm nominal risers: 3,600 / 140 = 25.7, so use 26 risers at 3,600 / 26 = 138.5 mm. Two flights of 13 risers have 12 goings each: 12 × 0.28 = 3.36 m, plus 1.20 m landing = 4.56 m, still within 5.40 m, and width is unchanged. The 138.5 mm riser is more comfortable and safer for children, but it adds two risers and lengthens the stair by 0.28 m. Judgement: if the extra length is available, the 140 mm option is better for a school; if not, the 150 mm dog-leg is code-compliant but at the maximum riser.