Treads and risers are supported on stringers that must be solidly supported, firmly fixed and properly positioned. The stringers may be either cut (see Figures 142 and 143) or ploughed to fit the outline of the treads and risers.

Figure 142

Stair components

Wood stringers must not be less than 25 mm (1 in.) thick when they are supported along their length or 38 mm (11⁄2 in.) when supported only at the top and bottom. The overall depth must be at least 235 mm (91⁄4 in.), and when the stringer is cut out to fit the treads and risers, the portion remaining (the “effective depth”) must not be less than 90 mm (31⁄2 in.) deep. A third stringer is required when the width of the stairs is more than 900 mm (35 in.). The width may be increased to 1200 mm (48 in.) where risers support the front of the treads.

Figure 143

Basement stairs

Treads must be at least 38 mm (11⁄2 in.) thick when used with open risers. This thickness can be reduced to 25 mm (1 in.) where the stringers are not more than 750 mm (29 in.) apart or where the tread is supported by a closed riser attached to the treads. The wall stringer may be ploughed out to the profile of the tread and riser with sufficient space at the back for wedges (see Figure 142). The treads and risers are fitted into the space ploughed into the wall stringer and wedged and glued in place. The wall stringer extends above the line of the leading edges and serves as trim that can mate with baseboards at the top and bottom of the stairs. The top of the riser may be connected to the bottom of the tread by angle-blocks glued to the concealed (back) side of both surfaces with screws added to reinforce the joint. The bottom of the riser is attached to the back of the tread with screws (see Figure 142) or the top of the riser is tongued into the front of the tread and the back of the tread is tongued into the bottom of the next riser (see Figure 142). The wall stringer is screwed behind the treads and risers to the wall.

If the outside stringer is an open stringer, it may be cut out to fit the risers and treads. The edges of the risers are mitred with the corresponding edges of the stringer, and the leading edge of the tread may be returned on its outside edge along the face of the stringer (see Figure 142).

Basement Stairs

Closed risers are safer but open risers may be used for basement stairs (see Figure 143).

Source : Canada Mortgage and Housing Corporation (CMHC)