How to Build Deck Stairs

How to estimate the landing zone

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Photo 1: Measure the total rise to the deck landing

Picture a stair slope in your mind to estimate about a 40-degree slope and guess at a landing point. Then measure the total rise to the landing spot with a straight 2x4 and a 4-ft. level.

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Photo 2: Layout the stairs

Clamp the stair gauges to the carpenter's square. Use the narrow part of the square for clamping the riser gauge and the wider part for the 10-1/4 in. tread. Lay out the stairs by drawing on the outside of the square, sliding the square along until it meets the last mark.

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Photo 3: Mark the top and bottom of the stringer to remove extra material

Mark the top of the stringer to remove 3/4 in. of material to allow for the missing top riser. Mark the bottom of the stringer to remove the tread thickness.

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.

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.

• 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.

Solid 2x12 skirts for solid stairs and rails

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Photo 4: Test fit the stringer

Cut only the top and bottom of the stringer with a circular saw. Test fit the stringer by placing it against the deck, and check the tread level with a small level.

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Photo 5: Cut the notches out of the stringers

Cut the notches with a circular saw. Stop the cuts when you reach the corner of the notch and finish the cuts with a handsaw to prevent weakening the stringers. Use the stringer as a pattern to mark and cut the two other notched stringers.

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Photo 6: Outline the pattern onto the skirt boards

Outline the pattern onto one of the skirt boards. Redraw the top and bottom lines with the carpenter's square and gauges at the original settings. Cut the top off so it will be even with the bottom of overhanging deck boards (see Photo 7) and cut off the end of the bottom so it's about 5 in. high. Fasten stringers to the skirts with 3-in. deck screws spaced about every 8 in., alternating from the front and from the back. Nail 2x4 supports to both sides of the middle stringer flush with the bottom for extra support (Photo 7).

• 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.)

Designing safe, comfortable stairs

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Photo 7: Attach the skirt boards to the rim joist

Use a level to draw two plumb lines to mark the left and right positions for the skirts and horizontal lines to mark the top tread location. Tack the skirts to the rim with 3-in. deck screws. Then screw through the back of the deck rim into the skirts with three more deck screws (Photo 8). Center the middle stringer and screw it into the rim with two deck screws.

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Photo 8: Attach 2x6 supports

Screw a 2x6 the width of the stringers to the backside of the stringers with two deck screws into each board. Screw two upright treated 2x6s to the backside of the rim and into the horizontal 2x6 with four deck screws into the rim and four more into the 2x6.

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Photo 9: Check the stair assembly to make sure it's square

Nail on the bottom riser with three 8d galvanized nails into each stringer and square the stairs by “cross-taping” the assembly and shifting it back and forth until the measurements are the same. (The bottom riser will probably need to be ripped to height.)

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Photo 10: Add the risers and threads

Nail on the second riser board and then screw on the two 2x6 bottom treads, leaving a 1/4-in. gap between the boards. Nail on the next riser, then the next tread and so on to work your way to the top of the 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.

Additional Information

• Fig. A. Rise and run sketch
• Fig. B. Stair layout sketch