OBC-Compliant Stair Dimensions for Basement Entrances: Riser, Tread, and Clearance Requirements

A below-grade separate entrance is usually the most expensive single item in a basement suite, and the stair is the part of it the inspector measures most closely. A riser a little too tall, a tread a little too short or headroom that dips under a beam can fail an inspection after the concrete has cured, and concrete does not adjust. This guide sets out the Ontario Building Code (OBC) stair dimensions for a basement entrance, explains how they translate into the size of the stairwell you dig, and flags the details that most often cause trouble on GTA projects. 

OBC Stair Dimensions at a Glance

The minimum run increased to 255 mm for permits applied for from January 1, 2022, replacing the older 210 mm figure. Plans drawn from older standard details can still show the shorter run, so check that the drawings reflect the current Code. 

Riser Height: 125 to 200 mm

Each riser must be between 125 mm and 200 mm. Just as important, risers must be uniform within a flight. The Code allows only a few millimetres of variation between adjacent risers and a slightly larger total variation across the flight. A person climbing in the dark adjusts their stride to the first few steps; one odd riser is a trip hazard, which is why inspectors measure every one. The top riser is where most GTA stairwells fail. It is set by the final grade, paving or landing slab, which is often installed after the stair is poured. If the walkway at the top ends up 40 mm higher than planned, the top riser is out of tolerance. Fix final grade elevations on the drawings and hold them. 

Run and Tread Depth: 255 to 355 mm

The run is the horizontal distance from one riser face to the next, and it must be between 255 mm and 355 mm. The tread can be slightly deeper than the run where it has a nosing, but by no more than 25 mm. Like risers, runs must be uniform within the flight. A comfortable stair balances the two. A long-standing rule of thumb is that two risers plus one run should total about 600 to 640 mm. A 185 mm rise with a 260 mm run gives 630 mm, which feels natural. A 200 mm rise with a 255 mm run is legal but steep, and less forgiving in winter. 

Width, Headroom and Landings

• Width: at least 860 mm clear between walls. Measure the finished width, after parging, waterproofing board or wall finishes. Many designers aim for 900 mm or more for moving furniture and future stair-lift space.
• Headroom: at least 1,950 mm, measured vertically from a sloped line through the tread nosings. On a walk-down entrance, the usual pinch point is the house wall above the door opening or an overhang at the top of the stairwell.
• Landings: required at the top and bottom of the stair, and wherever a door opens at the stair. A landing must be at least as wide as the stair, and in the direction of travel at least the lesser of the stair width or 1,100 mm. A door must not swing over the stair itself.
• Maximum flight height: a single flight cannot rise more than 3.7 m between floors or landings, which is rarely an issue at basement depth.

Handrails and Guards

Handrails go between 865 mm and 1,070 mm high, measured from the nosing line, with enough clearance behind them to grip. Stairs narrower than 1,100 mm generally need a handrail on at least one side; wider stairs need both. Fasten handrails into solid backing, not just anchors in the wall finish. 

Guards are required wherever there is a drop of more than 600 mm, which on a basement entrance means around the top of the stairwell at grade and along any open side of the stair. For a single dwelling, guards are typically at least 900 mm high, and 1,070 mm where the drop exceeds about 1.8 m. Openings must be small enough to stop a 100 mm sphere, and guards should not offer footholds that let a child climb them. 

Worked Example: Sizing a GTA Walk-Down Entrance

Take a typical GTA house where the basement floor sits about 2,400 mm (7 ft 10 in) below the finished grade at the entrance: 

Add the thickness of the stairwell walls and the working room needed to form and waterproof them, and the excavation is noticeably larger than the finished stair. This is why stair geometry has to be settled before anyone digs, and why a side-yard entrance can fail on width or setbacks before the Code dimensions are even reached. 

Details That Fail Inspections

• A non-uniform top or bottom riser, usually from a grade, slab or finish change made after the pour.
• Runs drawn to the old 210 mm minimum from outdated standard details.
• Width lost to finishes, where 860 mm framed becomes less than 860 mm finished.
• Headroom clipped by the house wall, a beam or an overhang at the top of the stair.
• No landing, or a door swinging over the stair at the bottom.
• Missing or short guards around the stairwell opening at grade.

Two related items belong on the same drawing. The bottom landing needs a drain, and under current Ontario rules it should not discharge storm water into the foundation weeping tile. And the entrance needs exterior lighting so the stair is usable after dark. Both are covered in our basement stairwell drainage guide. 

Planning a Basement Separate Entrance?

Maple Leaf Quality Renos designs and builds basement separate entrances across the GTA, with stair geometry, grade elevations, guards and drainage fixed on the permit drawings before excavation begins, so the stair passes inspection the first time. 

Contact us for a free site assessment. 

📞 +1 (647) 496-3360 

✉ contact@mapleleafqualityrenos.ca 

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