PreConstruct AIA stair is a handful of numbers that all have to agree: the height from floor to floor, how many risers divide it, how deep each tread is, and whether the whole thing fits in the opening with enough headroom. Ontario sets limits on each, and the limits changed on 1 January 2022. This is what they are, how a stair is laid out from them, and what comes off the drawings for it.
Rise and run: Table 9.8.4.1
The rise is the height of one step, nosing to nosing. The run is its horizontal depth, nosing to nosing. A private stair is one inside a dwelling unit, or outside serving a single dwelling unit or its garage. Everything else is public, apart from service stairs and stairs to an unused attic.
| Stair | Rise | Run |
|---|---|---|
| Private | 125 to 200 mm (about 5 to 7⅞ in) | 255 to 355 mm (about 10 to 14 in) |
| Public | 125 to 180 mm (about 5 to 7 in) | at least 280 mm (about 11 in), no maximum |
| Service stairs, stairs to an unoccupied attic | no maximum | no minimum |
A rectangular tread's depth, from its nosing to the back, is at least its run and no more than its run plus 25 mm. That extra 25 mm is the nosing overhang.
What changed in 2022
For permits completed after 31 December 2021, Ontario raised a private stair's minimum run from 210 mm to 255 mm, raised the handrail maximum from 965 mm to 1,070 mm, lengthened landings, and allowed spiral stairs in houses. Rise limits and tread depth didn't change. Houses built before 2022 can have 210 mm runs that were legal then. A stair rebuilt under a new permit is checked against the code in force for that permit, so find out which edition applies before you price it.
Uniformity: 9.8.4.4
Within one flight, risers can differ by at most 5 mm from the next one and 10 mmbetween the tallest and the shortest. Treads follow the same 5 mm and 10 mm rule. This is why a stair is laid out by dividing the total rise evenly, and why a finished floor added later (tile on a slab, a thicker hardwood) can push the top or bottom riser out of tolerance.
The rest of Section 9.8 that shapes a stair
| What | Private stair in a dwelling unit | Other stairs |
|---|---|---|
| Clear height over the stair (9.8.2.2) | 1,950 mm | 2,050 mm |
| Width (9.8.2.1) | 860 mm | 900 mm for residential exits and public stairs, more by occupant load |
| Tallest flight between landings (9.8.3.3) | 3.7 m | 3.7 m |
| Landing length (9.8.6.3) | the stair's width or 1,100 mm, whichever is less | the same |
| Handrail height (9.8.7) | 865 to 1,070 mm | 865 to 1,070 mm |
| Guards (9.8.8) | wherever the drop is more than 600 mm; 900 mm high in a dwelling unit | 1,070 mm high |
Headroom is measured straight up from a line along the nosings to the lowest thing above: the floor framing at the opening, a bulkhead or a duct. On a basement stair it's usually the header at the top of the opening that decides whether the stair fits.
Laying out a stair: a worked example
A basement stair in a new house, 2,800 mm finished floor to finished floor, straight flight, run of 260 mm.
- Fewest risers at the 200 mm maximum:
2,800 ÷ 200 = 14.0 → 14 risersof200.0 mm. That's right at the limit, and any site error goes over it. - One more riser gives room:
2,800 ÷ 15 = 186.7 mmeach, comfortably inside 125 to 200 mm. - Treads: one fewer than risers, because the top riser steps onto the floor above: 14 treads.
- Total run:
14 × 260 = 3,640 mm. The run is within 255 to 355 mm. - Stringer length:
√(2,800² + 3,640²) = 4,592 mm,15.07 ft, so 16 ft stock before any extra for the cut at the top or a kicker. - Flight height 2,800 mm is under 3.7 m, so no landing is needed part way.
- Check the headroom: 1,950 mm clear above the nosing line all the way up, usually tightest under the header at the top of the opening.
On the drawings this is usually already decided: the section prints the number of risers and the run. Your job is to check it adds up, and to count what it buys.
What comes off the drawings for a stair
- Framed stairs: stringers (how many is on the drawings, set by the width and the tread material), treads, risers, the header and trimmers at the floor opening, and the landing framing.
- Concrete stairs: the waist slab up the slope plus the triangle of each step, with soffit, riser and side forms, and rebar per the drawings.
- Finishes and metals: handrails by the length along the pitch, guards along open sides and around the opening, and nosings.
- Drywall: the stairwell walls are taller than a storey, and the sloped soffit under the stair is often missed.
Doing it on the drawings
In PreConstruct AI a framed stair is counted by the flight: stringers cut from the next even stock length up the slope, treads, risers and labour, from the rise and run you type. A concrete stair flight comes out as its waist and steps in cubic yards and metres, with forms and bars. The rise, run and number of risers are the ones on your drawings. It counts and measures, and never designs the stair.