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.

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.

Definition of words 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.

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.

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.