Stairways – Strength Legislative Interpretations

Question

What are the strength requirements when building stairways?

Answer

As per paragraph 115(1)(a) of the General Regulation 91-191, stairways must be able to sustain a live load of 4.8 kPa. However, the National Building Code of Canada – 2020 (NBC) has two sets of design specifications for strength and rigidity under uniform loading:

• 1.9 kPa for stairs serving not more than one dwelling unit or a house with a secondary suite,
• 4.8 kPa for other stairs.

To align with NBC requirements, employers can design stairs that meet a specified load of 1.9 kPa, provided the stairs are intended for its corresponding application, as listed above.

Many conditions must be satisfied to ensure stairways are NBC-compliant. See illustrations below that visually demonstrate the specific NBC criteria for wooden stairs.

The red fracture lines shown are for illustrative purposes only and indicate potential locations where failure may occur.

Supported – The stringer must be structurally supported at both the top and bottom. The bottom of the stringer must sit on, and be fastened to, a floor, landing, or slab. At the top it should either sit on a ribbon board attached to the floor system or landing. Alternatively, a suitable engineered hanger would be acceptable.

Secured – The stringer must resist rotation (especially important for open riser stairs). Blocking between stringers may be used to prevent movement.

All measurements are nominal sizes.

Stair treads of lumber, plywood or OSB

• Not less than 25 mm (1”)
• If risers are open and stringers are >750 mm (30”) treads must be no less than 38 mm (1 ½”) in thickness.

Stair treads of plywood or OSB, not continuously supported by the riser:

• must have their face grain or direction of face orientation at right angles to the stringers.

Definitions from the NBC:

• Dwelling unit means a suite operated as a housekeeping unit, used or intended to be used by one or more persons and usually containing cooking, eating, living, sleeping and sanitary facilities.

• Live load means a variable load due to the intended use and occupancy that is to be assumed in the design of the structural members of a building.

• Secondary suite means a self-contained dwelling unit with a prescribed floor area located in a building or portion of a building of only residential occupancy that contains only one other dwelling unit and common spaces, and where both dwelling units constitute a single real estate entity.

Referenced Legislation 

115(1)(a) An employer shall ensure that a stairway is of sufficient strength to sustain a live load of 4.8 kPa.