How to Calculate Stair Stringers
Stair stringers are the boards that lie at an angle to support the treads of a stairway. They're usually 2-inch by 12-inch boards, cut on one side in a saw-tooth pattern that's spaced according to the height and width of the steps. Calculating a comfortable width for the treads (the "run") and a natural height between them (the "rise") will make the stairs feel easy and safe to use. Poorly spaced stairs will make people feel as if they must shorten or lengthen their stride. Does this Spark an idea?
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Instructions
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1
Measure the vertical distance that the stairs will need to cover in inches, from the lower floor surface to the upper floor surface. Measure straight up, not at an angle, using a tape measure.
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2
Divide that number by 7.5. If the result is a whole number, use 7.5 in Step 3. If the result isn't a whole number, round the result to the next closest whole number and divide the vertical distance by that whole number. Use that result instead of 7.5 in Step 3. Whichever number you use is a comfortable rise, in inches, for the height of your stairway. For example, 110 inches divided by 7.5 is 14.67, which rounds to 15. In the next step, you would use 110 divided by 15, or 7.33, and your stair rise would be 7.33 inches.
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3
Multiply the number from the previous step by two and subtract the result from 25. For example, 7.33 times 2 is 14.66. Subtract 14.66 from 25 and the result is 10.34. The result is a comfortable run or tread width in inches, based on the rise. In the example, the rise would be 7.33 inches and the run would be 10.34 inches.
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Divide the vertical distance by the number from Step 2 and multiply it by the result from Step 3. The answer is the horizontal space that the stairway will take up, expressed in inches, assuming the stringer's top step will be level with the upper floor. Mark this distance out from the upper floor where your stairs will be and make sure there's enough space and headroom.
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5
Decide whether you want the stringer's top step to be level with the upper floor or one step below it. Multiply the height of each rise in inches by the number of rises, minus one if you want the stringer to end one step below the upper floor. Square the total. Multiply the length of each run in inches by the total number of runs and square that total. Add the two results together and take the square root. The result is the length of the lumber you'll need to purchase for the stringer, in inches.
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Tips & Warnings
If you need to adjust the length of the rise or run to fit the stairway in the horizontal space, try to keep the rise less than 8 inches and the run between 9 and 11 inches. As the rise gets shorter, make the run longer, using the following proportions for natural-feeling steps: two times the rise plus the run should equal 25.
References
- Photo Credit stairs image by cherie from Fotolia.com
