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Stairs, landings and handrails
How tall can a deck step be and how deep does the tread have to be?
A stair is a division problem with two limits and a tolerance. Set the deck height below and the demonstration divides it into risers that fit, tells you the run it will eat up in the yard, and flags the layout error that puts most stairs out of compliance.
Every riser in a deck stair has to be 125 mm (5 in) to 200 mm (7 13/16 in), and every tread 255 mm (10 in) to 355 mm (14 in) deep, with a nosing of no more than an extra 1 in. The City of Edmonton guide prints both sets of figures side by side, so a 7 7/8 in riser and a 10 in or 14 in tread meet the guide.
They all have to match. The rise may vary by no more than 5 mm (3/16 in) between adjacent steps, including the step onto the deck itself, and by no more than 10 mm (7/16 in) between the tallest and shortest riser in the flight.
The drawing is a scale sketch. The numbers beside it come from the stair rise and run limits, not from the picture.Turn JavaScript on for the controls. The drawing above is the opening state.
The arithmetic, done the way a stringer is cut
Take the total rise from the finished deck surface down to the finished ground. Divide it by the riser height you want, round to a whole number, then divide the total rise by that number again. That second division is the actual riser height, and it is the one that has to land inside 125 mm (5 in) to 200 mm (7 13/16 in).
A deck 4 ft off the ground at a 7 in target gives 6.86, which rounds to 7 risers at 6 7/8 in each. Seven risers means six treads on the flight and one step onto the ground, so the flight reaches about 5 ft 3 in into the yard at a 10 1/2 in run. People are routinely surprised by how much lawn a stair eats.
Uniform matters more than the range
The 5 mm (3/16 in) and 10 mm (7/16 in) tolerances exist because a stair is walked, not measured. People climb by rhythm, and a single riser that is out by 1/2 in is the one that catches a toe. That is why the deck surface itself counts as a landing for the purposes of the tolerance: the last step up onto the deck has to match the rest of the flight.
The bottom step, and the error everybody makes
When you lay out a stringer, the tread thickness has to come off the bottom riser. If you mark seven equal risers on the stringer and then lay 1 1/2 in treads on all of them, every tread sits 1 1/2 in higher than marked, so the top riser shrinks by 1 1/2 in and the bottom one grows by it. Both the Edmonton guide and every stair layout course call this out, and it is still the single most common reason a deck stair fails an inspection.
Switch it on in the Advanced panel of the demonstration and watch the tolerance blow out.
Nosing
A nosing may project no more than an extra 1 in beyond the tread. It is measured nosing to nosing, so a nosing does not buy you run, it just makes the tread feel deeper to the ball of the foot.
What carries it
Stair stringers are 1 1/2 in thick material. After the notches are cut, at least 90 mm (3 1/2 in) of effective depth has to be left behind the cut, and the stringer has to measure at least 235 mm (9 1/4 in) overall. A 2x10 meets the overall depth, but notched for a 7 in rise and a 10 1/2 in run it keeps less than 90 mm (3 1/2 in) behind the cut, which is why a 2x12 is the normal stringer.
Stringers may be up to 900 mm (35 7/16 in) apart, or up to 1.2 m (47 3/16 in) if riser boards support the front of the treads. The full detail is in the stair width and stringers answer.
- National Building Code of Canada, Part 9, Section 9.8.4, Steps, and 9.8.3, Dimensions of stairs. The 125 mm (5 in) to 200 mm (7 13/16 in) rise, the 255 mm (10 in) to 355 mm (14 in) run, the 5 mm (3/16 in) and 10 mm (7/16 in) uniformity tolerances and the nosing limit are from these.
- City of Edmonton, Uncovered Deck Design Guide, Section 4, Stairs, including the stringer depth rules and the warning about tread thickness at the bottom step.
Nothing on this page is an approval or a design. Code editions change and every authority having jurisdiction has the final word. Check your numbers with yours before you build.