How to Build Deck Stairs

Calculating the step dimensions, laying out stringers and building a sturdy set of deck stairs.

How to estimate the landing zone

Whether you're replacing an old, rickety set of deck stairs or building a set for your new deck, deck stairs are among the most challenging projects for the average do-it-yourselfer to tackle.

One little mistake in calculations or layout and you'll wind up wasting lots of expensive wood, or worse, you'll build a downright dangerous set of stairs. But building a strong, safe set of stairs is doable if you meticulously follow the layout and cutting rules outlined in this story.

You almost always have to design site-built stairs yourself because the number and height of the steps will vary with the landscape. Begin by drawing a side view of your site and adding dimensions (Fig. A). That usually means going through the calculations a few times to determine where the stairs will fall and to figure out how long your skirt and stringer material needs to be. This sounds complex, but if you work through it a few times and rely on your sketch, it'll become clear.

Here's what to do
  1. First determine the approximate height “X” (Fig. A). Start by estimating where you think the last stair will fall by using a 40-degree slope (Photo 1). Rest a straight board on the deck and level over to that spot and measure down to the ground. That'll be the approximate height of the stairs, “X.”
  2. Now find the approximate number of steps. Divide “X” by 7 in. (an approximate step height) and round off the remainder, up if it's .5 or more, or down if it's less than .5. That'll give you an approximate number of risers (Fig. A). The actual recommended riser height is 6-1/2 to 8 in., but you'll determine that later. If the riser height is too short, re-divide “X” by 8 and start again. On uneven ground, find the number of treads so you can find the exact stair landing point. Simply subtract 1 from the number of risers. (There's always one fewer tread than risers, as you can see in Fig. B.) Then, multiply by 10.25 in., the ideal tread width for two 2x6s, to get the total run. Measure out that distance from the deck to find the exact landing point. From this point, you can measure the exact stair height and determine the stringer and skirt length.
  3. Measure the exact total rise (Photo 1). Divide the height (X) by your estimated number of risers to find the exact riser height. The figure will usually fall between 6-1/2 and 8 in., the ideal range. Use this figure for your stringer layout (Fig. B). If the riser height isn't in this zone, add or subtract a riser and divide again. This will change the number of treads and shift the landing point, so re-measure the exact height and divide again.
  4. Draw a sketch (Fig. B) to confirm the plan in your mind and lay out the first stringer (Photos 2 and 3) using the exact riser and tread dimensions and your framing square. Plan to establish a solid base at the landing point. The base can be a small concrete slab, a small deck or even a treated 2x12 leveled in over a 6-in. gravel base. After you cut the stringers, use them as guides to position your landing. Cut and mount the stringers by following our photos.
In your layout (Fig. B), note that:
  • The top tread is 3/4 in. shorter than the other treads.
  • The bottom riser is 1-1/2 in. shorter than the other risers. Be sure to test-fit the first stringer (Photo 4) before you cut the others. If you made a mistake, you'll at least be able to save the other two 2x10s.

Buying the Materials

Measure from the deck rim to the landing spot and add 2 ft. Buy three treated 2x10s, two 2x12 skirts and two 2x4s sized to the next larger length and you'll have plenty of material to work with (the worst mistake is buying material that's too short!). Get a 6-ft. 2x6 for securing the stairs to the deck (Photo 8). You'll also need two 2x6s for each tread and a 1x8 for each riser. Use 3-in. deck screws to fasten the skirts and treads to the stringers and the skirts to the deck. Fasten the risers to the stringers with 8d galvanized nails.

For extra-strong stairs, reinforce the middle 2x10 stringer with 2x4s nailed to both sides (Photo 7). There are a million ways to fasten the stringers solidly to the deck. Photo 8 shows a simple, foolproof, extra-strong method that works especially well even for open-sided stairs built without skirts.

There you go—a pretty, rock solid set of stairs ready for balusters and railings.

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Solid 2x12 skirts for solid stairs and rails

These stairs call for 2x10 treated material for the rot-resistant notched stair stringers (also known as jacks or carriages, Photo 1) that won't be seen. This design also uses 2x12 skirt boards that attach to the sides of the outside stringers. The skirts serve several purposes:

  • Cosmetically, they hide the unsightly notched, treated stringers to make your stairs look polished.
  • They make it easy to attach the stringers.
  • Structurally, they make for rock-solid stairs by reinforcing the stringers, which have been weakened by notching.
  • And when it comes time to attach guardrails and handrails to the stairs, you'll have a solid board to fasten pickets or posts to for a wobble-free rail. (If you'd rather not use the 2x12 skirt boards, be sure to use 2x12s for the notched stringers for adequate strength.)
For the parts that show—the skirts, treads and risers (lead photo) —choose material that matches the deck. In our case, that was cedar.

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Designing safe, comfortable stairs

Building codes contain specific requirements for safe stair design. If you follow the directions in this story, your stairs will be legal and safe. In a nutshell, treads should be more than 9 in. deep and risers 6-1/2 to 8 in. high. Riser heights can vary no more than 3/8 in. from one step to another to reduce trip hazards. However, even a 1/4-in. variation can cause tripping.

If you use 2x6s for tread material like we show, you can build stairs up to 48 in. wide with only three stringers because 2x6s can span up to 2 ft. But if you use the common and thinner 5/4-in. bull-nosed decking for your treads, you'll have to keep stringers no more than 16 in. apart and you'll be limited to 32-in. wide stairs with three stringers. For wider stairs, add one or more evenly spaced stringers depending on the width of your stairs and the tread material you choose.

And remember, you need one right and one left skirt assembly, not two lefts or rights.

A Carpenter's Square and a Set of Stair Gauges are Crucial

You'll need a 4-ft. level, tape measure, calculator, circular saw and a handsaw. If you don't already have a carpenter's square, now's the time to buy one ($10; Photo 2). To do the job right, pick up a set of stair gauges ($5), too. Stair gauges are little clamps that you tighten onto the square at the proper rise (vertical stair height) and run (horizontal tread depth) for exactly duplicating each step as you draw it onto the stringers (Photo 2). The gauges save time and ensure that all the steps are consistent.

Converting Decimals to Fractions

Not many calculators are set up to give you fractions, and a readout like 7.65 isn't much help for setting the carpenter's square and stair gauges. Use this chart to help you convert the readout to fractions or for converting fractions to decimals for calculator entries. Choose whichever fraction is closest to the decimal reading for setting your gauges when you lay out your stringers. .125 = 1/8 in. .25 = 1/4 in. .375 = 3/8 in. .5 = 1/2 in. .625 = 5/8 in. .75 = 3/4 in. .87 = 7/8 in.

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