Stairway Design
Stairs may be site built but are most often factory manufactured. Stairways in houses can have a straight, continuous run without an intermediate landing, or consist of two or more runs with changes in direction, or may be curved, in which case special design criteria apply. The minimum allowable stairway headroom is 1.95 m (6 ft. 5 in.) (see Figure 141).
The width of any landing must not be less than the width of the stairs. Stairs in houses and houses with a secondary suite must be at least 860 mm (34 in.) wide measured between wall faces.
Figure 141
The length of a landing in a house cannot be less than 860 mm (337⁄8 in.). The vertical height of any flight of stairs cannot exceed 3.7 m (12 ft.). Each step in a flight of stairs should have the same height. A change in direction can be made with landings or winders. Figure 140 shows different types of stairway designs. If winders are used, they must form an angle of either 30°, (three treads for a maximum allowable 90° turn), or 45° (two treads for a maximum allowable 90° turn). Only one set of such winders is permitted between floor levels. Experience has shown that 30° winders are easier to negotiate than 45° winders.
Figure 140
Once the location and width of a stairway and any required landings have been determined, the next step is to figure out the rise and the run. To establish the rise, measure the exact distance between the finished floors of the two storeys under consideration and divide by 184 mm (71⁄4 in.), a comfortable riser height.
This calculation gives the number of risers needed over the total rise. Round up the number that results from this calculation to the next whole number. The run is determined by dividing the required number of treads into the total run of the stairs.
For example, if the total rise is 2 718 mm (107 in.) and if each riser is 184 mm (71⁄4 in.), then 14.8 risers are needed (that is, 2,718/184 = 14.8). Rounding up, 15 risers will be required with each one having a rise of 181 mm (7.13 in.) (that is, 2,718/15 = 181.2). Alternatively 14 risers at 192 mm (7.64 in.) rise would be appropriate when space is limited.
A sloped or bevelled edge on leading edges (nosings) makes treads more visible. The size and shape of the slope or bevel is limited by the building code to safeguard against tripping or slipping (see Figure 139).
Figure 139
Source : Canada Mortgage and Housing Corporation (CMHC)
