Designing Stairs

The ascent should be gradual for outdoor stairs -- short risers and deep treads are the rule.

At first glance, designing stairs for your deck may seem mysterious, but a simple rule makes it easy twice the riser height (the height of each step to the top of the tread) plus its run (the front-to-back depth of the tread) should equal between 24 and 27 inches.

Start by measuring the total rise and run for your stairs. You can plan a specific location for the stairway landing pad and cut stringers (the diagonal supports) to meet it. Or you can decide how many steps and what rise and run you want and position the pad according to the stair calculations. Even better is to start with a prospective pad location and estimate whether the resulting rise and run will be comfortable and aesthetically pleasing.

Stringers are the key stair components. Choose straight, clear 2x12s, preferably boards with vertical grain. Don't settle for second-best boards for stringers; buy the best lumber you can find.

Prestart Checklist

Time
About seven hours to measure and cut the stringers and install the treads, not including installation of concrete pad

Tools
Tape measure, framing square, stair gauges, circular saw, jigsaw or handsaw, cordless drill, socket wrench

Skills
Using simple mathematical formulas, measuring, cutting, fastening

Prep
Install decking and landing pad

Materials
2x12s, fasteners

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Measuring the Total Rise and Run

To measure the total rise of your stairs, hook the tape over the decking and measure to the ground or post footing. On level ground, that's the total rise of your stairs.

On sloped ground mark the prospective end of the stairs with a stake and extend a 2x4 to the stake. Prop the 2x4 on scrap until it's level, then measure the space between the bottom of the board and the ground (assuming the pad will be level with the grade). Add this measurement to the first one. This is the total rise.

To measure the total run of the stairs, extend a level 2x4 (or keep the one from the previous measurement) and measure from the edge of the deck to the stake. This is the total run of the stairs. Compute the rise and run for each step from these measurements, and adjust them so they give the look you want and meet the guidelines for comfort.

Marking the stringers: Step 1

Calculate the rise and run for each step and place stair gauges at these measurements on your framing square. Set the gauge for the rise on the tongue of the square and for the run on the blade. Starting at what will be the top of the stringer, set the framing square on the stringer so the tongue (the short side) intersects the corner. Mark the rise and run lines. Extend the line for the rise to the opposite edge of the stringer (you will cut at this line later).

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Figuring rise and run

For easier climbs, deeper stair treads should have a shorter rise. The standard formula is: Twice the riser height plus the run should equal between 24 and 27 inches. (If rise is 7 inches and run is 11 inches, for example, 2 times 7 equals 14; adding 11 equals 25.)

To calculate rise and run, measure the total rise of the stairway (see box, opposite) and divide by the amount of rise you want for each step. Round to the nearest whole number; this is the number of rises. Then divide the total rise by the number of rises to find the exact rise for each step.

If you want each rise to be about 7 inches, for example, and the total rise is 58 inches, divide 58 by 7. The result is 8.28; round that down to 8 rises. Divide 58 by 8 rises to find the unit rise, 7.25 (7-1/4) inches. To find the total run, or length, of the stairway, multiply the run you want, say 11 inches, by the number of rises minus one (remember, the last rise is up to the deck surface). In this case 7x11 = 77. So the stairway will end 77 inches away from the deck.

Marking the stringers: Step 2

Slide the framing square along the edge of the stringer until the rise marker lines up with the end of the previous run line. Mark the rise and run lines for the second and each subsequent step, keeping the stair gauges against the edge of the board.

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Marking the stringers: Step 3

When you have marked the rise and run for the bottom step, extend the last rise line to the edge of the board. Then set the framing square on the opposite edge of the stringer so the run length lines up with the extended line.

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Marking the stringers: Step 4

Move the framing square up the board by the thickness of the tread, and mark a line parallel to the bottom run line. You will actually cut on this second line because the stringer needs to be shortened by the thickness of the tread. Otherwise the bottom step will be too high.

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What If... You want closed stringers? Step 1

If your design requires only two closed stringers, mark them as you would an open stringer but cut only the top extended rise and the bottom edge and rise. If your design calls for more than two stringers, those in the middle will be open stringers. Mark and cut one stringer and use it as a template to mark the others. Do not cut the outside stringers.

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What If... You want closed stringers? Step 2

Mark the tread locations as a guide for positioning the stair brackets. Then cut the bottom edge and rise and the extended rise at the top of the stringer.

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