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How to Build Deck Stairs

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

how to build stairs stair stringer calculatorFamily Handyman
Sure, building deck stairs can be tricky. But in this story, we'll make it easy by showing you how to estimate step dimensions, layout and cut stair stringers, and assemble the stair parts. And you won't have to do any hard math to figure it all out (but your calculations will have to be accurate!). These DIY steps will work for replacing an old set of stairs and for building stairs on a brand new deck. So grab your tools and let's start building!

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One day
$100 – $500

Video: How to Build Stairs for your Deck

How to estimate the landing zone

Measure the total rise to the deck landing

Photo 1: Measure the total rise to the deck landing

Here’s how to build stairs for your deck. 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 2×4 and a 4-ft. level as you start to figure out how to build stairs.

stair layout diagram

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 to learn how to build steps. This will create a stair stringer calculator.

Mark the top and bottom of the stringer to remove extra material diagram

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.

How to Build Stairs for Your Deck

Whether you’re replacing an old, rickety set of deck stairs or building a set for your new deck, deck stairs or porch 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 deck steps. But building a strong, safe set of deck steps is doable if you meticulously follow the layout and cutting rules outlined in this how to build stairs story.

Good luck finding a pre cut stair stringer that will work for your deck. You almost always have to design site-built stairs yourself because the number and height of the steps will vary with the landscape. Begin this how to build stairs project 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. The following with walk you through the stair formula.

Start with accurate stair measurements using the stair formula

  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 2×12 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 this how to build stairs project.

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

Accurate stair measurements are incredibly important. Measure from the deck rim to the landing spot and add 2 ft. Buy three treated 2x10s, two 2×12 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. 2×6 for securing the stairs to the deck (Photo 8). You’ll also need two 2x6s for each tread and a 1×8 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 2×10 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.

Solid 2×12 skirts for solid stairs and rails

Test fit the stringer to make sure your stair rise and run are on point

Photo 4: Test fit the stringer to make sure your stair rise and run are on point

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

Cut the notches out of the stringers

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 stair stringer as a pattern to mark and cut the two other notched stringers.

Outline the pattern onto the skirt boards

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 2×4 supports to both sides of the middle stair stringer flush with the bottom for extra support (Photo 7).

These stairs call for 2×10 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 2×12 skirt boards that attach to the sides of the outside stair 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 2×12 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.

Designing safe, comfortable stairs

Attach the skirt boards to the rim joist

Photo 7: Attach the skirt boards to the rim joist

When attaching stair stringers, 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.

Attach 2x6 supports

Photo 8: Attach 2×6 supports

Screw a 2×6 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 2×6 with four deck screws into the rim and four more into the 2×6.

Check the stair assembly to make sure it's square

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.)

Add the risers and DIY stair treads

Photo 10: Add the risers and DIY stair treads

Nail on the second riser board and then screw on the two 2×6 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 how to build stairs story, your stairs will be legal and safe. In a nutshell, DIY stair 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.

Additional Information

Required Tools for this How to Build a Staircase Project

Have the necessary tools for this how to build stairs DIY project lined up before you start—you’ll save time and frustration.

  • Chalk line
  • Circular saw
  • Clamps with a reach of at least 18 in.
  • Corded drill
  • Framing square
  • Hammer
  • Handsaw
  • Level
  • Safety glasses
  • Sawhorses
  • Stair gauge

Required Materials for this Building Stairs Project

Avoid last-minute shopping trips by having all your materials ready ahead of time for this how to build stairs project. Here’s a list.

  • 2x12x12 ft (3; you may need longer 2 x 12s for your stairs)
  • 2x6x12 ft. (1)
  • 3-in. deck screws
  • Decking for risers and treads