Stairs are one of the few finish carpentry items where precision is required by code. Building inspectors don’t care if casing reveals vary or if the kitchen counter is 1/4 inch too high, but they do care if the stair treads aren’t all the same width. Here are the steps to follow for a successful end result.
1. Find total rise. Measure the distance from subfloor to subfloor. If the finish flooring materials differ in thickness at the top and bottom of the stairs, use the following formula:
Total rise equals subfloor-to-subfloor measurement plus thickness of second floor finish minus thickness of first floor finish.
2. Find total run. Measure the horizontal distance from the upper end of the stair opening to the point where the bottom riser will meet the floor.
3. Find unit rise and run. Unit rise is the vertical distance between treads; unit run is the horizontal distance between risers. My local code requires a maximum rise of 8 1/4 inches and a minimum run of 9 inches. In my mind, though, the ideal stair rise-run is 7 1/4 by 10 inches. If you have to deviate from this, use the following rules of thumb:
- Unit rise times unit run (in inches) should equal 72 to 75.
- Two rises plus one run should equal 24 to 25 inches.
To calculate unit rise, divide the total rise by the number of risers you want to use, making sure the result falls within code requirements. A house with an 8-foot ceiling, for example, will have a total rise of, say 108 inches; using 13 risers, the unit rise will be just over 8 1/4 inches, not acceptable for code. Using 15 risers, however, gives a unit rise close to my ideal of 7 1/4 inches
To calculate unit run, divide the total run in inches by the number of treads, which will be one less tread than the number of risers. (A stair landing counts as a tread, but should be treated as an intervening floor; the rise and run above and below the landing must match.) For instance, a stair with 15 risers and a total run of 147 inches will have a unit run of 101/2 inches.
4. Adjust top and bottom risers. If floor finishes are different thicknesses at the top and bottom of the stair, you’ll need to adjust the top and bottom risers. Use the following formulas: (see illustration, above).
Bottom riser: The distance between the first-floor subfloor and the first level cut in the carriage equals unit rise minus tread thickness plus thickness of the first-floor finish.
Top riser: The distance between the top of the second-floor subfloor and the last cut on the carriage equals unit rise plus tread thickness minus thickness of second floor finish.
Be aware that errors in layout and construction are additive. Under my local code, the rise and run of adjacent treads may vary no more than 3/16 inch and the maximum variation between any two treads over an entire set of stairs is limited to 3/8 inch. So if the first 16 risers on a 17-riser stair are each 1/32-inch short, the last riser will be off by 1/2 inch — a code violation. I check my layout by marking the total rise on a board, setting a compass to the unit rise, and walking off the correct number of risers on the board. If the math is correct, the point of the compass will start on one mark and end on the other.
Some other code requirements I have to watch for: a minimum headroom of 6 feet 8 inches, and a maximum tread nosing overhang of 1 1/2 inches. Make sure you know the provisions of your own local code before you lay out stairs.