How Wide Do Back Deck Stairs Need to Be in Ontario? (And Can You Top-Mount the Railing?)
Standing on the job site with a tape measure, wondering if your back deck stairs are wide enough? Wondering if bolting the aluminum railing on top will fail inspection? Here’s the short answer.
A house needs stairs at least 860 mm wide (about 33 7/8 inches, or 2 feet 10 inches). A multi-unit building needs its main stairs at least 900 mm wide (about 35 7/16 inches). You measure this width between the guards, not the stringers. The handrail doesn’t count against it.
Can you top-mount the railing, or do you need to side-mount it? That depends on one thing: once the posts go on, is there still enough clear space between the guards?
One quick note on numbers. The code sets these minimums in millimetres. The inch numbers here are rounded so you can picture them easily. If your stair is tight, measure and build to the metric number.
The rest of this guide breaks down exactly why, so you can settle the argument on site.
The Two Width Numbers, and Which One Applies to You
Ontario Building Code section 9.8.2.1 sets stair width. It’s easy to grab the wrong number, because the section covers a few different situations.
People often quote 900 mm (about 35 7/16 inches) first, since it’s the first line in the code. But read closely. That number is for required exit and public stairs in residential buildings, meaning apartments, stacked townhouses, or any building with more than one home in it.
A single-family house is different. For a house, exterior stairs (including your back deck stairs) need to be at least 860 mm wide (about 33 7/8 inches, or 2 feet 10 inches).
That’s a separate line in the same section, and it’s the one that covers most residential deck jobs.
So before you argue about width, ask one question: is this a house, or a building with more than one unit? That answer changes your minimum by 40 mm (about 1 9/16 inches), and it tells you which line of the code applies.
Does “Required Exit” Change the Width? Usually Not for a House
Inspectors often ask whether a stair counts as a required exit. It’s a real and useful distinction, but it doesn’t usually control width for a house.
Here’s the idea. Every building needs a required way out in an emergency. If your front door already covers that job, your back deck stairs are just a second way down. They’re not carrying that legal weight.
What “required exit” status does change: whether the stair has to exist at all, and some added rules in Sections 9.9 and 9.10.
What it does not change, for a house: the 860 mm (33 7/8 inch) minimum width. A house stair needs 860 mm whether or not it’s a required exit. The required-exit question matters more for multi-unit buildings, where the number can jump to 900 mm (35 7/16 inches).
Bottom line: check the building type first. For most single-family back decks, your target is 860 mm, and the required-exit debate doesn’t change your number.
How to Measure Stair Width the Right Way
Here’s the easiest way to picture this. Think of your stairs like a hallway you walk through.
The stringers are the two long boards that hold the stairs up on each side, like the frame around a doorway. They sit at the very outside edge.
The guards are the railings that keep you from falling off the side. They sit a little bit inside the stringers, not right at the edge.
If you measure stringer to stringer, you’re measuring the full wooden frame, edge to edge. That’s the biggest number you’ll get, but it’s not the space you actually walk through, because the railing takes up some of that room too.
If you measure guard to guard, you’re measuring the real gap you walk through, the space between the railings on each side. That’s the number the building code cares about, because that’s the space a person actually uses.
So stringer to stringer is like measuring the outer walls of a hallway. Guard to guard is measuring the actual walkway between them. The code wants the second one.
One thing worth knowing: the 2012 code spelled out that you measure between wall faces or guards. The 2024 code kept the rule but dropped that explaining sentence. The measurement method didn’t change. The code just got shorter. If you’re reading the current code and it feels unclear on where to measure from, that’s why.
Now, the handrail. A handrail can stick into the stairway space. The code allows it to project up to 100 mm (about 4 inches) without reducing your required width (section 9.8.7.6). So a handrail mounted on the inside of your guard doesn’t eat into your clear-width number, as long as it stays inside that 100 mm. Measure to the guard, not to the handrail.
Top-Mount or Side-Mount? Here’s How to Decide
This is the real question behind most searches. Your stair is framed to a certain width, and you need to know if your aluminum railing system will eat into your clearance.
In simple terms, the difference is where the post attaches.
Top-mount (surface mount): The post base sits on top of the tread or landing and bolts straight down. Because the post sits inside the outer edge of the stair, the guard ends up set inward a bit on both sides. That inward shift eats into your clear walking width.
Side-mount (fascia mount): The post bolts to the outside face of the stringer instead of sitting on top. Since the post is mounted outside the walking surface, the guard stays out at the edge, and you keep almost the full framed width as clear width.
How to decide:
If your stair is framed close to the minimum (around 860 mm inside the stringers for a house), top-mounting will likely push your clear width under the line. Side-mount instead, or widen the stair.
If you have real extra room, say 100 mm (about 4 inches) or more per side beyond the minimum, top-mounting works fine and is usually simpler to install.
A worked example: Say your stair is framed at 1079 mm (42.5 inches) inside the stringers, and your house minimum is 860 mm (33 7/8 inches). That leaves 219 mm (about 8 5/8 inches) of total slack, or about 110 mm (4 5/16 inches) per side. A top-mounted post and guard rarely use up that much space, so you can top-mount and still clear 860 mm with room to spare.
Now try a stair framed at 900 mm (35 7/16 inches) inside the stringers. That leaves only about 20 mm (roughly 3/4 inch) of slack per side. A top-mounted system will almost certainly push you under the minimum. That stair needs a side-mount aluminum railing.
The move that saves you a callback: don’t guess. Set the posts, or just clamp the base plates in place without fastening them, and measure the actual clear width between the guards. Take a photo. Confirm you’re above your minimum before you drill a single hole.
Quick Checklist
☑ House or multi-unit? House needs 860 mm. Multi-unit required or public stair needs 900 mm.
☑ Is the back stair a required exit? If your front door is your main way out, probably not, which relaxes some egress rules but not the width, for a house.
☑ Measure between the guards, not the stringers. Handrails can project up to 100 mm without counting against your width.
☑ Framed close to the minimum? Side-mount to keep your width. Lots of extra room? Top-mount works fine.
☑ Always confirm on the actual stair with the posts in place before you fasten anything.
When to Call an Engineer or Your Building Official
Code minimums are the floor, not the finish line. Base-plate fastening, edge distance, and how the railing loads the stringer all matter. On anything you’re unsure about, an engineer’s stamp protects you if it’s ever questioned. A quick word from your building official on site helps too, but get anything important in writing. If your stair is tight to the minimum, a quick check now beats a rebuild later.
This article covers Ontario Building Code requirements as a general reference. It isn’t a substitute for a design professional or your local building official. Confirm details against the current OBC and your Authority Having Jurisdiction. Metric (mm) values govern; inch equivalents are rounded for reference.
A single-family house needs at least 860 mm (about 33 7/8 inches). A multi-unit building’s main stairs need at least 900 mm (about 35 7/16 inches).
From the guards. That’s the inside space people actually walk through. Stringer-to-stringer measurements don’t count.
No, not if it stays within 100 mm (about 4 inches) of projection. The code allows this as a permitted intrusion.
Only if you have enough clearance left after the posts go on. If your stair is close to the minimum width, side-mounting usually works better.
Top-mount sits on the tread and reduces clear width. Side-mount bolts to the outside of the stringer, keeping nearly the full framed width usable.
