The most critical characteristic of stairs, even more important than the size of any of the parts, is that every step be the same. In fact, building codes enforce this rule. Fire and building codes devote a lot of space to stairs; falls on stairs kill 4,000 people a year in the United States and another 2 million are seriously injured. If you are a homeowner planning to build or repair a stair, please read this advice first.
Many different terms have been used in describing stairs; we will use those defined in the illustration below. The ratio of unit rise to unit run determines the angle of the stairs.
The 1996 Council of American Building Officials (CABO) and the 2000 International Code Council recommendations call for unit runs to be not less than 10 inches and unit rises not more than 7¾ inches.
Staircases which consist only of treads are said to have “open risers.” Under the 1996 CABO model code, open risers are no longer permitted because they are a danger to children. They are also a danger to the elderly, who tend to catch their toe on the tread and trip.
Nosing projections are also a danger to the elderly.
Again, limits are probably specified in the local building code. Typically the minimum width permitted in residences is around 2 feet 8 inches. Three feet is better, and 3′6″ is the standard for normal occupancy. If a stair is more than 44 inches wide, a handrail is required on both sides.
Most fire codes do not allow stairs to rise more than 12 feet without providing a landing. The length of the landing should be at least equal to the width of the stair tread.
According to the 1996 CABO code, the openings between balusters are to be no greater than 4 inches. This is half of what was allowed a few decades ago; smaller holes reduce injuries to young children. The balustrade is topped by a handrail 30 to 38 inches above the top of the stringer; the handrail's grip size is between 1¼ to 2 inches. If the handrail is mounted on a wall, a space of at least 1½ inches must be left between the edge of the handrail and the wall.
Rules of thumb for determining satisfactory rise/run ratios have existed at least since Classical times. In De Architectura, Vitruvius suggests a unit rise of between 9 and 10 inches, and a unit run between 18 and 24 inches. To modern tastes, this proportion would create a very stately stair. Sir Henry Wooten (1568–1639) suggested that the unit rise not exceed 6 inches and that the unit run be between 1 and 1½ feet. Both authors, however, are describing public buildings or grand houses.
Jacques-Francois Blondel (1705–1774) argued in his Cours d'Architecture that the rise/run ratio should be based on the length of the human pace, which he took to be 25.5 inches. Since in one step on a staircase a foot travels by two risers and a tread, Blondel arrived at the formula two times the unit rise, plus the unit run = 25.5", or unit rise = (25.5" - unit run) over 2. This formula works well only for moderate values for unit rise (or unit run). It was, nonetheless, enshrined in the National Fire Code.
Among American architects, an old rule-of-thumb is that the sum of the unit rise and the unit run should be about 17½″. Common practice has been to make the unit rise about 7½ inches, the unit run 9″ for interiors and 11″ for exteriors.
In modern times, stair researchers have gone beyond observing which existing stairs cause the most accidents. Using tools like endless mechanically driven staircases with variable unit rise and unit run, they have been able to experiment with many combinations of unit rise and unit run, and to capture in stroboscopic photography how missteps occur. The results largely confirm the rules of thumb, but some interesting results emerge, such as that the optimal rise/run ratio for descent is not the same as the one for ascent. Several researchers feel that for descent the unit run should be at least 11 inches.
But requiring an 11″ unit run is controversial. Increasing the unit run even an inch or two can greatly increase the size of the staircase. With a rise of 12 feet and a unit rise of 7.2 inches, increasing the unit run from 9″ to 11″ makes the staircase two feet longer–probably 6 square feet of floor area carved out of the living room. When a “7-11” standard was adopted by a building code in the Northeast, the National Association of Home Builders got it reversed, arguing that it increased costs without any proof that it was safer.
Imagine climbing a staircase in which alternate halves of the treads have been removed. A foot moving to the next empty tread does not need to clear the tread on which the other foot stands. Such staircases exist. Their great advantage is that they can be very steep ( up to 70°, compared to up to 50° for normal stairs) and still be safe and comfortable. The great disadvantage is that such a staircase can accommodate only one person at a time. Such staircases are most commonly found on shipboard. In the United States, Lapeyre Stair in New Orleans specialized in such staircases.
The Stairway Manufacturers Association offers an excellent Visual Interpretation of the International Residential Code 2006 Stair Building Code, very well illustrated with photography. Highly recommended. Available on the web as a pdf file (requires the free Adobe Reader): www.stairways.org/pdf/2006 Stair IRC SCREEN.pdf
A useful link list of building code sites is maintained at www.codesourcepc.com/Pages/Links.html
An inexpensive ($13), excellent Excel spreadsheet for calculating, and more impressive visualizing, the dimensions in a stringer. Design your own staircase. About the only assumption is that the top deck will be the top step. See http:\\shalla.net
Cleo Baldon and Ib Melchior.
Steps and Stairways.
J. M. Fitch et al.
The Dimensions of Stairs.
Scientific American, October 1974.
The Staircase. (2 vols)
Cambridge: the MIT Press, 1992.
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Last revised: 20 December 2002.