Winder and spiral stairs in Alberta: angles, treads and limits

In Alberta, each winder tread must turn exactly 30° or exactly 45°, a set of winders can turn no more than 90°, and a stair between two floors can have only one set (NBC(AE) 2023, 9.8.4.6 and 9.8.3.1). Spiral stairs are allowed with at least 660 mm (2 ft 2 in) between the handrails and risers up to 240 mm (9.4 in), but they can never serve as an exit. Both rules are in the second printing of the National Building Code, 2023 Alberta Edition, in force since 17 August 2026 (Government of Alberta, 2026).

Which turning stairs does Alberta allow in a house?

Inside a home, or a house with a secondary suite and its shared spaces, the code lists five options: straight flights, curved flights, spirals, flights with winders, and flights that mix rectangular and tapered treads. In any other building, winders and mixed flights aren't on the list (NBC(AE) 2023, 9.8.3.1). So winders are a house-only option.

The code's note sorts treads into four kinds: rectangular treads in straight flights, tapered treads in curved flights, winders, and spiral stairs (NBC(AE) 2023, Note A-9.8.4.). Winders are tapered treads that meet at one centre point. That difference decides which rules apply.

Here is how each way of turning a stair is treated, as of October 2026:

Note: this covers configuration only. Rise, width and headroom rules still apply; the stair guide lists them.

Every tapered tread in one flight must turn the same way (NBC(AE) 2023, 9.8.3.1.(5)). A stair can't wind left at the bottom and right at the top.

What angles can winder stairs turn in Alberta?

Each winder tread turns exactly 30° or exactly 45°. The code allows no deviation above or below either angle, and each set stops at 90° (NBC(AE) 2023, 9.8.4.6). Only one set is allowed between floor levels (NBC(AE) 2023, 9.8.3.1.(3)), so a U-shaped stair can wind through one corner and needs a landing at the other.

The code's note works out what that leaves you, and explains the choice of angles:

Note: from Note A-9.8.4.6, which is guidance on the Article, not a separate requirement (NBC(AE) 2023, Note A-9.8.4.6).

The same note says a landing is the safest way to turn a stair. It accepts winders inside a home because people know their own stairs, and it calls 30° winders the best compromise because they change your stride the least. Winders still rise like any other step: up to 200 mm (7.9 in) each in a house (NBC(AE) 2023, Table 9.8.4.1).

Because winders converge on a point, they're exempt from the 150 mm narrow-end rule that applies to other tapered treads (NBC(AE) 2023, 9.8.4.3). They're also exempt from the rule that puts the handrail along the narrow end of a tapered flight (NBC(AE) 2023, 9.8.7.1.(5)).

The drawing shows a quarter turn with three 30° winders in an 860 mm (2 ft 10 in) stair, the minimum width for a stair between floors in a home (NBC(AE) 2023, 9.8.2.1). Each flight shows three runs of 255 mm, so the stair fills 1 625 mm (5 ft 4 in) each way. The circle shows the walking line, 300 mm (11.8 in) out from the corner.

What are the rules for curved and mixed-tread stairs?

A tapered tread that isn't a winder needs at least 150 mm (5.9 in) of run at its narrow end. Measured 300 mm (11.8 in) from the centre line of the handrail at the narrow end, its run must fall within the normal range, which for a house is 255 mm to 355 mm (10 in to 1 ft 2 in) (NBC(AE) 2023, 9.8.4.3 and Table 9.8.4.2). The tread can be up to 25 mm deeper than its run.

That 300 mm line is the walking line, and the code checks uniformity there too. Tapered treads in a flight must match within 5 mm of each other and 10 mm across the flight, measured at that line (NBC(AE) 2023, 9.8.4.4).

A flight that mixes straight and tapered treads must keep one run at the walking line, measured from the inside handrail. Tapered treads at the bottom of the flight are the exception: their run there may be longer (NBC(AE) 2023, 9.8.4.5). Inside a home, a curved or mixed flight also needs its handrail along the narrow end of the treads, where the steps are shortest (NBC(AE) 2023, 9.8.7.1.(5)).

What does the code require for a spiral staircase?

A spiral stair needs a handrail on each side, with the outer one at least 1 070 mm (3 ft 6 in) high and 660 mm (2 ft 2 in) clear between them. Risers can be up to 240 mm (9.4 in), treads at least 190 mm (7.5 in) deep at 300 mm from the narrower handrail, and you need 1 980 mm (6 ft 6 in) of clear height (NBC(AE) 2023, 9.8.4.7). The treads must keep one angle and one size and turn the same way.

A spiral stair trades width and depth for a taller step. The code's note defines it as any stair whose treads form part or all of a circle, with a narrower width and shallower treads than a curved stair allows and a taller riser than any other kind (NBC(AE) 2023, Note A-9.8.4.7). The 860 mm width and 1 950 mm (6 ft 5 in) house headroom rules both carve spirals out, and the spiral's own 1 980 mm clear height is 30 mm more than the house figure (NBC(AE) 2023, 9.8.2.1 and 9.8.2.2).

This is the steepest spiral the code allows, drawn with the 190 mm minimum tread depth as the run:

The limits on use matter more than the dimensions. A spiral can be the only means of egress only where it serves no more than 3 people, and it can never be an exit (NBC(AE) 2023, 9.8.4.7). That rules it out as the shared exit stair of a basement suite. Calgary's suite page asks for an exit stair at least 860 mm wide with a handrail and landings at least 860 mm long (The City of Calgary, secondary suites). A spiral up to a small loft or a reading platform is the typical use; the loft guide covers when that loft becomes a storey.

Should you turn the stair with a landing or with winders?

The footprint is the same, and the difference is three steps. A landing at a 90° turn must be at least as wide and as long as the stair, so an 860 mm stair turns on an 860 mm square (NBC(AE) 2023, 9.8.6.3). Three winders in that same square climb three risers. At the minimum 255 mm run, that's 765 mm (2 ft 6 in) of straight flight you don't have to fit elsewhere.

That 765 mm is often the gap between a stair that fits and a hallway that's too short. The hallway width guide shows what the hall at the top needs.

What does a walkthrough show for a turning stair?

The LED floor is flat, so you can't climb a stair on it, but you can see its footprint at full size. You can stand where the turn falls, see how much of the hall or the room below it takes, and look at a landing version and a winder version of the same stair one after the other, since changes are made on the spot. A spiral shows as the circle it is, so you can judge the space left around it.

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