Building codes strictly govern all aspects of stair construction, including a stair's rise and run, the height and shape of guardrails and handrails, and the headroom above the steps.
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Building codes strictly govern all aspects of stair construction. These include the stair’s rise and run, the height and shape of handrails and guardrails, and the headroom above the steps.
Local codes may vary, but most are consistent with the guidelines established by the FHA. The preferred angles shown in the figure below are the safest and most comfortable.
Figure: Safe Angle for Stairs
The minimum distance from the tip of any nosing (the nosing line) to the ceiling directly above should be 6 ft. 8 in. (below). For basement or utility stairs, the minimum is typically 6 ft. 4 in.
Figure: Stair Layout
Most residential codes require stairs to be 36 in. wide. However, they allow railings to project 31/2 to 41/2 in. into the width on either side. They also allow trim, such as finished stringers, up to 11/2 in. thick.
Tread and Riser Dimensions
It is critical for safety that the rise and run of every step in a stairway is consistent. Codes allow up to 3/8 in. variation between the smallest and the largest riser and tread, but even this will be noticeable by clients and will reflect poorly on workmanship.
Risers. The maximum riser height allowed ranges from 73/4 in. (IRC) to 8 in. (UBC). The optimum for comfort is considered 7 to 71/2 in.
Treads. The minimum tread width allowed ranges from 9 in. (UBC) to 10 in. (IRC). The optimum for comfort is 10 to 11 in.
To find the right combination of rise and run, use one of the following rules of thumb (local codes may have other restrictions):
The sum of two risers and one tread equals 24 to 25 in.
The sum of one riser and one tread equals 17 to 18 in.
Riser height multiplied by tread width should equal 70 to 75 in.
Common Stair Flaws
Most stair problems can be traced to a mistake in the framing layout. Typical mistakes include the following:
Wrong total rise. Layout calculations must be based on the distance from finished floor to finished floor, not subfloor to subfloor. Otherwise, if finish flooring thicknesses differ, the top or the bottom step will be a “trip step” that’s shorter or taller than all the rest.
Failure to drop the stringer. To equalize the heights at the top and bottom risers, the bottom of the stringer must be cut to lower it by the thickness of the tread material minus the thickness of the finished flooring on the lower level.
Not enough headroom. The solution is to recut the stringer or it may require reframing the stair opening.
Improperly located landing. If the rough landing is too high or too low, the stairs above the landing will not match the pitch of the lower flight, or there may be trip stairs at the landing or at the top or bottom of the floors.