Stair Width in BC: What 900 mm Means and What Eats Into It
The 860 mm and 900 mm minimum stair widths in BC Building Code Part 9, how handrails and stringers reduce clear width, and what to check before framing.
A stair drawn at exactly the minimum width is usually a stair that fails, because the number on the drawing is not the number an inspector measures.
This article is fabrication context, not engineering or permit advice. Code interpretation belongs with the authority having jurisdiction (AHJ) and the project’s licensed professionals. Confirm applicability with the local building department before finalizing design or permit submissions.
Stair width looks like the easiest dimension on the drawing. It causes trouble because the number written on the plan and the number an inspector measures are usually taken from different places.
Two minimums, and the difference is what the stair serves
BC Building Code 9.8.2.1.(1) requires required exit stairs and public stairs serving buildings of residential occupancy to have a width of not less than 900 mm. Sentence 9.8.2.1.(2) provides the exception: exit stairs serving a single dwelling unit are permitted a width of not less than 860 mm.
Forty millimetres does not sound like a design constraint. It becomes one on an infill lot where the stair sits between two framed walls and every millimetre was allocated during design.
The classification is what decides it, and classification is not a fabrication judgment. A stair inside one house is a different case from a stair serving two suites off a shared landing, and a former single-family lot rebuilt as a small multiplex changes category entirely. Confirm which sentence applies with the designer and the AHJ before the opening is framed, because the answer changes the framing, not just the steel.
The framed opening is not the stair width
This is where most of the confusion lives. A drawing that shows a 900 mm stair usually shows a 900 mm dimension between framing members. The walking surface is narrower, and sometimes by a lot.
Working from the framing inward, the things that consume width are the wall finish on each side, the stringer if the stair uses side stringers, the guard assembly where the stair is open on one or both sides, and the handrail where it projects into the run. None of those are optional additions. They are the stair.
A 12.7 mm drywall layer on each side removes about 25 mm before anything structural is installed. A steel side stringer made from plate or channel adds its own thickness on each side it appears. Two side stringers on a stair framed tight to 900 mm can leave a walking surface meaningfully under the minimum, on a stair whose drawing said 900 mm the whole time.
Handrail clearance is space that has to exist
The handrail cannot be pressed against the wall. BC Building Code 9.8.7.5.(1) requires the clearance between a handrail and the surface behind it to be not less than 50 mm, or not less than 60 mm where that surface is rough or abrasive.
That clearance, plus the handrail section itself, plus the bracket, is a real dimension projecting off the wall into the stair. Whether and how much of it counts against required width is a provision worth confirming with the AHJ for the specific case rather than assuming, but the physical space is not in question. It exists whether or not it is deducted on paper.
Position matters too. Under 9.8.7.1.(2), at least one handrail has to be located not more than 750 mm from the natural path of travel. On a stair wide enough that both edges sit beyond that distance from the middle, the layout needs more than one handrail, and each one occupies width. Handrail location, bracket type, and handrail continuity requirements are best resolved together rather than as three separate late decisions.
Stringer choice changes the arithmetic
Structure is the one input on this list a fabricator directly controls.
A stair with two side stringers loses width on both sides, because the stringers sit outboard of the treads. A stair with a single central beam does not. The treads on a mono stringer stair cantilever from a beam that runs under the middle of the walking surface, so the full tread width is available and nothing projects into it at guard height.
That is a genuine planning advantage in a narrow opening, and it is worth weighing against what a mono stringer asks for in return. The single beam carries everything, so the section is deeper, the connections at the floor and the landing work harder, and the tread brackets are engineered rather than assumed. A narrow stair is one of the cases where the structural choice and the code dimension are the same conversation.
Cantilevered treads anchored into a wall remove the stringer from the width calculation entirely, but they move the demand into the wall framing, which is a larger structural change than most renovations expect.
Width drives cost more than it looks like it should
A wider stair is not simply more square metres of the same thing.
Treads span between whatever supports them, and a wider tread spans further. On an open-riser stair, that can push the tread from a plate section that was adequate at one width to a thicker plate, a stiffened plate, or a folded section at another. The tread thickness and sizing decision follows from the span, and the span follows from the width.
Width also shows up on install day. A wider assembly is heavier, harder to turn through a doorway or a stairwell, and more likely to need mechanical lifting rather than hand carry. In our shop, the width that changes the fabrication approach is usually the width that also changes how the stair gets into the building, which is why access photos are worth sending early. Confirm cost implications with a current quote rather than assuming a per-metre rate.
Width and headroom compete for the same opening
Width is one of three dimensions fighting over the same hole in the floor, and the other two usually win because they are harder to change.
The opening has to be long enough for the total run, which follows from the rise and the number of risers. It also has to be long enough to give headroom under the edge of the floor above, measured along the walking line. Widening a stair does not change either of those directly, but it does change what the opening has to be framed as, and on most renovations the joists running perpendicular to the stair are the constraint.
Making an opening wider means cutting more joists and carrying them on a header, which sizes up the header and sometimes the posts under it. Making it longer for headroom does the same thing in the other direction. Where both are needed on a floor that was not designed for it, the structural work around the stair can cost more than the stair.
This is why the sequence matters. A fabricator asked to price a stair for a fixed opening will design to that opening. A fabricator brought in before the opening is framed can often tell the framer that 100 mm more width costs almost nothing now and a great deal later. The headroom and clearance requirements cover the third dimension in that trade-off.
Renovations inherit widths that no longer comply
Older Vancouver housing stock contains a great many stairs narrower than either current minimum. That is not automatically a problem, because existing buildings are handled differently from new construction.
What matters is scope. Replacing treads on an existing stair, replacing a stair in the same location and configuration, and building a new stair in a new opening are three different conversations with the building department, and only the last one starts from a clean application of the current minimums. Where a renovation touches a required exit, the review is usually stricter than where it does not.
The practical advice is to ask before designing rather than after. A stair replacement permit conversation held early tends to establish which width the project has to meet, and that single answer often determines whether the surrounding framing has to move.
What to confirm before the framing package is priced
Width is a framing decision disguised as a stair decision. Once joists are cut and a header is set, widening a stair means structural rework, and the cost of that lands on the framer and the engineer rather than on the steel.
The information that settles it is the applicable classification for the stair, meaning whether it serves a single dwelling unit or more than one, the clear dimension between finished wall faces rather than between studs, the intended structural type, whether the stair is open on one side or both, and the handrail arrangement including which side each rail sits on. The BC stair code overview for metal stairs covers how width sits alongside rise, run, headroom, and guards, which all compete for the same opening.
Sources
- BC Building Code, Section 9.8: Stairs, Ramps, Landings, Handrails and Guards
- BC Building Code 2018, Division B, Section 9.8 (official publication)
- BC Codes, Province of British Columbia
The stairs that end up feeling generous are rarely the ones drawn at the minimum. Where the opening allows it, adding 50 or 100 mm to the clear width during design costs very little and removes the risk that finishes, guards, and handrails quietly eat the dimension the drawing promised.
Related questions
What is the minimum stair width in BC?
BC Building Code 9.8.2.1.(1) requires required exit stairs and public stairs serving buildings of residential occupancy to have a width of not less than 900 mm. Sentence 9.8.2.1.(2) allows exit stairs serving a single dwelling unit to be not less than 860 mm. Which figure applies depends on what the stair serves, so confirm the classification with the authority having jurisdiction.
Is stair width measured between the walls or between the guards?
The dimension that matters is the clear width of the walking surface, not the rough opening in the framing. Guards, stringers, wall finishes, and in some cases handrails all sit inside the framed opening and reduce what is left. A 900 mm rough opening does not produce a 900 mm stair.
Do handrails count against the required stair width?
It depends on the projection and the applicable provision, which is why this is worth confirming with the authority having jurisdiction rather than assumed. What is not in doubt is that the handrail occupies space: BC Building Code 9.8.7.5.(1) requires a clearance of not less than 50 mm between the handrail and the surface behind it, or 60 mm where that surface is rough or abrasive.
How much width does a steel stringer take up?
It depends on the section. A side stringer made from steel plate or channel sits outside the tread on each side, so a two-stringer stair loses width on both sides. A mono stringer runs under the centre of the treads and takes nothing off the clear width, which is one practical reason it suits narrow openings.
Why does my architect's drawing say 900 mm but the stair feels narrow?
Usually because 900 mm was applied to the framed opening rather than to the finished walking surface. Drywall, a wall-mounted handrail, and the guard assembly each consume part of that dimension. In most of the projects we see, the useful conversation is about clear width between finished faces, not about the framing dimension.
Does a spiral or winder stair use the same width rule?
Spiral and winder stairs are covered by separate provisions in Part 9 and are measured differently, because the walking line is not straight. Do not apply the straight-flight width figures to a spiral without confirming the applicable article with the designer and the authority having jurisdiction.
Can a renovation keep an existing narrow stair?
Sometimes. Existing conditions are handled case by case by the authority having jurisdiction, and the answer often depends on the scope of the alteration and whether the stair is a required exit. Replacing a stair in kind and building a new one are treated differently, so raise it with the building department early.
Where should the handrail be located across the stair width?
BC Building Code 9.8.7.1.(2) requires at least one handrail to be located not more than 750 mm from the natural path of travel on the stair. On a wide stair that can mean a second handrail or an intermediate one, which is a layout decision worth resolving before the guard is fabricated.
Does a wider stair cost proportionally more?
Not proportionally, but it does cost more. Wider treads need more material and can need a heavier tread section or an additional support to control deflection across the span. Width also changes crane and access requirements on install. Confirm pricing with a current quote.
What width should a feature stair in a house be?
Above the minimum, width is a design decision rather than a code one. Many residential feature stairs we fabricate run wider than the 860 mm floor because the stair is meant to be seen and used as circulation, not squeezed into a corner. The constraint is usually the opening in the floor, not the code.
How does width interact with the stair opening in the floor?
The opening has to accommodate the stair plus the structure around it, and widening a stair after framing means changing joists or a beam. This is the main reason stair width belongs in the conversation before the framing package is priced rather than after.
Do exterior stairs use the same minimum widths?
The Part 9 width provisions apply to exterior stairs as well, and the classification question is the same: what the stair serves determines which figure applies. An exterior stair serving a single dwelling unit and one serving several units are not held to the same minimum.
