Stairway Design

Stairways may have a straight, continuous run without an intermediate landing, or they may consist of two or more runs with changes in direction. In the best and safest practice, a landing is introduced at any change in direction, but the turn may be made with radiating treads called winders. The length or width of any landing must not be less than the width of the stairs. Stairs must be at least 34 in. (860 mm) wide measured face to face of wall.

The diagrams in Figure 99 show different types of stairway designs. If winders are necessary because of cramped space, they must form an angle of 30°, so that three treads would be necessary for the maximum 90° turn permitted. Only one set of such winders is permitted between floor levels.

Basement stairs: (A) ploughed stringers; (B) cut-out stringers; (C) cut- out stringers with finish member nailed to the outside of the stringer.

® stair tread nosing slightly chamfered ploughed stringer i" (25 mm) thick finished member on outside of stringer stair tread (supported stringer)

cut-out stringer

Basement stairs: (A) ploughed stringers; (B) cut-out stringers; (C) cut- out stringers with finish member nailed to the outside of the stringer.

i" (25 mm) thick finished member on outside of stringer stair tread (supported stringer)

cut-out stringer

stair tread (supported on cut-out)

cut-out stringer stair tread (supported on cut-out)

cut-out stringer

Once the location, the width of the stairway and any landings, if required, have been determined, the next step is to fix the tread rise and the tread run. To fix the tread rise, the exact distance between the finish floors of the two storeys under consideration is divided by 7 1/4 in. (184 mm), 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 tread width is determined by dividing the required number of treads into the total run of the stairs.

For example, if the total rise is 8 ft. 11 in. or 107 in. (2718 mm) and if each riser is set at 7 1/4 in. (184 mm), then 14.8 risers are needed (i.e., 107/7.25 = 14.8). Rounding up, 15 risers will be required each at 7.13 in. (181 mm) (i.e.,

It is important to remember that the minimum headroom for a stairway must be 6 ft. 6 in. (1.95 m) (Fig. 97).

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