Stairs

A stair is the one part of a building sized almost entirely by the body. Every step lifts the whole weight of a person and then catches it again, so a riser a quarter of an inch taller than the rest, or a tread too short for the foot, is felt at once and is a common cause of falls. That is why stairs are among the most closely regulated elements in any building code, and why their proportions follow a rule older than any code.

On level ground an adult’s pace is about 24 to 25 inches. Climbing, each inch of rise costs roughly as much effort as two inches of going, so a comfortable step trades rise for run at about two to one. Designers write it as a sum:

2R + T = 24 to 25 inR is the riser height, T the tread run, both in inches

A second check uses the product, R × T = 72 to 75. The two rules agree in the middle of the range and part company at the extremes, which is a useful warning in itself: a 5 in riser on a 15 in tread is legal and feels like a ramp with bumps; a 9 in riser on a 7 in tread is a ladder. Outdoors, where rain and ice make footing uncertain, the sum is often let out to 26 for a shallower, safer pitch. The figure uses 2R + T = 25 for every stair it draws, so the tread is always fixed by the riser.

The floor-to-floor height is fixed by the building; the stair has to divide it evenly. Divide the height by a riser you would like, round to a whole number of risers, and divide again to find the actual riser. The tread follows from the stride rule. There is always one tread fewer than there are risers, because the last step up lands on the floor itself. For a 9 ft 0 in (2745) floor-to-floor:

Each extra riser makes the climb easier and the stair two feet longer. That is the real trade in stair design: comfort against floor area. Set the figure to 15 risers and switch from residential to commercial, and both the riser and the tread fall outside the limits; one more riser puts them right.

Codes separate private stairs inside a dwelling from stairs that serve the public, which must also work as a way out in an emergency. The figure compares two teaching presets using the following limits:

Accessible stairs add a few more: risers closed, nosings no more than 1½ in (38) deep with a small radius, and a handrail grip 1¼ to 2 in (32 to 51) round, held 1½ in (38) clear of the wall. A flight should have at least three risers; a single step is easy to miss.

Headroom is measured straight up from the nosing line, the sloping line that touches the front edge of every tread, to the lowest thing overhead. On a straight run that lowest thing is the header at the near end of the opening in the floor above. So the pitch of the stair sets the length of the hole: a steep stair climbs clear of the header quickly, a shallow one needs a longer opening.

The figure sizes the opening for you. At 9 ft with 15 risers the floor above has to open 11′ 3⅛″; at 16 risers 12′ 11¼″; at 17 risers 14′ 8⁷⁄₁₆″. Light Headroom in the legend to see the clearance line pass just under the header’s corner. The opening is framed with doubled joists: trimmers along its sides and headers across its ends, which carry the cut-off tail joists and the top of the stringers.

A handrail is something to hold; a guard is something to stop a fall. Along an open side the same balustrade usually does both, with its rail at handrail height and balusters close enough that a 4 in (100) sphere cannot pass between them, a size chosen so a small child’s head will not fit. Where the stair reaches a floor, or the floor above looks down into the opening, the guard rises to its full height. On commercial stairs the wall rail runs on 12 in past the top riser and a tread plus 12 in past the bottom one, so a hand is already on the rail before the first step and still on it after the last.

Stringers

The inclined beams that carry the steps from floor to floor, here three 2×12s cut to the saw-tooth of the treads and risers. A stair as wide as this one gets a third, middle stringer so the treads do not bow underfoot.

The drop

The foot of each stringer is cut one tread thickness short, so that once the treads go on, the first step is exactly as high as all the others.

Treads

5/4 boards, each projecting 1 in (25) past the riser below as a nosing, with a return nosing on the open side.

Risers

¾ in boards closing the back of each step, set on the tread below and under the tread above.

Newels

The heavy posts that anchor the balustrade at its ends, here through the first tread and at the landing.

Headers

Doubled joists across the ends of the opening; the stringers hang from the far one on metal hangers.

The figure solves a straight run, and the rules for risers, treads and headroom are the same whatever shape the stair takes. What changes is the space it takes and the opening it needs. Turning the stair at a landing shortens the opening and gives a fall somewhere to stop; winders turn it in less space at the cost of treads that taper; a spiral packs it into a circle and is allowed only steeper, narrower steps in return.