A staircase detail that cannot be built and a door dimension that disagrees with itself
The drawing set for a higher secondary school building has reached site. Sheet S-4, a 1:50 plan, shows the stair hall as 2.4 m wide and 5.0 m long. The stair detail shows a single straight flight of 18 risers at 175 mm and 17 treads at 300 mm, 1.1 m clear width and a 1.0 m landing at the head. The floor-to-floor height on the section is 3.20 m. On the same sheet the door schedule lists D1 as 900 mm × 2100 mm, but when the mason scales the plan with a 1:50 scale rule the opening measures 20 mm, that is 1000 mm. The site engineer says 'build what is drawn' and asks you to mark the treads on the wall tomorrow. The staircase will be the only escape route from the first floor and the school reopens in three weeks.
1. Check the stair against the drawing: does the flight reach the floor height, and does it fit the hall?
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Rise check: 18 risers × 175 mm = 3150 mm, but the floor-to-floor height is 3200 mm. The flight is 50 mm short, so the top riser would have to be 225 mm - far outside the 150–190 mm range normally permitted, and uncomfortable and unsafe for schoolchildren.
Going check: 17 treads × 300 mm = 5100 mm. Add the 1.0 m head landing and you need 6100 mm. The hall is only 5000 mm long. Even the flight alone (5100 mm) does not fit inside a 5000 mm hall.
Conclusion: the detail fails on both counts and is not buildable as drawn. Do not mark any treads on the wall.
A workable alternative: 3200 / 160 mm = 20 risers, comfortably inside limits. Use a dog-leg stair: two flights of 10 risers (9 treads each) at 280 mm tread, giving 2520 mm going per flight; with a 1200 mm landing the total length is 2520 + 1200 = 3720 mm, which fits inside 5000 mm. Width 2400 mm = two 1100 mm flights plus a 200 mm gap. Before marking, confirm headroom of at least 2100 mm under the landing and under the flight soffit, handrail height of about 900 mm, and that the escape route width is not reduced.
2. Which dimension governs the D1 opening, and what do you do before the frame is ordered?
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Rule: never scale a drawing to obtain a dimension. Written dimensions govern; a scaled measurement from a 1:50 print carries paper stretch, print distortion and the width of a pencil line, which easily explains a 20 mm reading that corresponds to 1000 mm.
But here two written sources conflict - the plan dimension reads 1000 mm and the schedule reads 900 mm. Where a schedule and a plan disagree, you do not pick one because it is convenient. The schedule is usually the more specific document for a component, but the conflict must be resolved by the designer.
Actions:
- Raise a written query (RFI) quoting both references: Sheet S-4 plan, and item D1 of the door schedule.
- Get a written revision with revision number and date, and enter it in the site instruction book.
- Do not order door frames, and do not build the opening, until the answer comes back.
Probable explanation to check rather than assume: a 900 mm door leaf commonly needs about a 1000 mm structural opening for the frame and clearance. If the schedule is quoting the leaf size and the plan the opening, both could be correct - but only the architect can confirm that, and the lintel bearing and shutter width depend on the answer.
3. The mason proposes reducing the tread to 260 mm so that the drawn single flight fits the hall. Evaluate this.
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It does not solve the problem, for three separate reasons.
The level is still wrong. The error in the riser count is independent of tread length. 18 risers of 175 mm give 3150 mm against a 3200 mm floor - the top riser would still be 225 mm unless a riser is added. The proposal fixes geometry in plan while ignoring the level.
The length still does not work. 17 treads at 260 mm = 4420 mm, plus a 1000 mm landing = 5420 mm, which is still longer than the 5000 mm hall. Only a tread of about 235 mm would fit with the landing - and that is too narrow for a school.
Proportion and safety. A 260 mm tread with a 175 mm riser gives 2R + T = 610 mm, which is at the edge of comfort and well below the 280–300 mm normally expected in a public building. A narrow tread encourages heel contact on the nosing and is a recognised fall risk on the only escape route.
Decision: reject the proposal. The correct action is a design revision - 20 risers at 160 mm in a dog-leg layout - with the revised detail issued by the architect, checked for headroom and escape width, and only then marked on the wall.