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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. 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.
142 Stair components Risers and treads tongue and grooved together tread leading edge (nosing)
Risers and treads connected with angle blockes
riser
glued angle block
space for wedges angle block
nail or screw nail or screw
grain direction for lumber, plywood and OSB treads Housed stringer stringer ploughed out for stairs and wedges
Cut-out (open) stringer
tread return moulding balusters placed in tread cut stringer