Deck Stairs: How to Build Them, With Every Code Number Tied to DCA6

Deck stairs look simple until the inspector arrives with a tape measure. One riser that is a half inch too tall, a stringer that spans too far, or a bottom step resting on loose gravel is enough to fail the stair. This guide gives you the deck stairs code in one table, then walks through how to build them: the math, the stringers, the footing at the bottom, landings for tall decks, and handrails and guards. Every number is tied to a row of the free AWC¹ DCA6 deck guide or to the residential stair rules it follows.

We are not your building department. Where a number is our own design choice, we say so, and where we could not read the source in full, we say that too.

Deck Stairs Code in One Table

Most of the stair rules come from the DCA6¹ guide (pages 20 to 22) and from IRC section R311.7, the residential stair section. Here are the values side by side.

Check the edition your town has adopted, because local amendments change these numbers more often than people expect.

Riser and tread

The riser is the vertical height of one step and the tread is the horizontal depth you stand on. The code limits the riser from above and the tread from below, so a steeper stair fails on the riser and a cramped one fails on the tread. Keep every riser in a flight the same height. Inspectors measure them, and an uneven step is the one people trip on.

Width, stringers and landing

A stair must be at least 36 in wide. The guide then asks for 2x12 stringers, at least three cut stringers on a standard stair, no more than 18 in apart on center. A flight cannot climb more than 12 ft of total rise before it needs a landing, and the section on landings below covers what that means for a tall deck.

Handrails, guards and light

Once a stair has four or more risers it needs a handrail between 34 and 38 in high. The guard rules and the light at the top are covered further down, because they are the part builders tend to forget.

How to Build Deck Stairs, Step by Step

This is the order that keeps the math honest. The arithmetic belongs to the stair stringer calculator, so use it for your exact numbers and treat this section as the method.

Measure the total rise

Total rise is the vertical distance from the finished ground (or the finished landing surface) to the top of the deck boards. Measure it at the spot where the stair will land, not at the house. If the ground slopes, measure at the bottom step location and plan to level that spot.

Count the risers

Divide the total rise by 7-3/4 in and round the answer up to a whole number. That is your riser count. Then divide the total rise by the riser count to get the actual riser height, which will be a little under the limit. For example, a 30 in total rise divided by 7-3/4 in is just under 3.9, so you need four risers of 7-1/2 in each.

Work out the run

A stair has one less tread than risers, because the deck surface is the top step. With four risers that is three treads, and at the 10 in minimum the stair covers at least 30 in of ground. Our designer builds a 10 in tread from two 5-1/2 in boards with a 1-1/8 in nosing. That build-up is our choice, not a code number, and any decking pattern that gives 10 in or more works.

Lay out and cut the stringers

Mark the rise and run on a 2x12 with a framing square, and leave the notch width the same on each step. Cut one stringer, test it in place, and use it as the pattern for the rest. Take the thickness of a tread off the bottom of each stringer so the first step down is the same height as the others. This is the cut that most often goes wrong, and the calculator draws it for you.

Set the stringers at the top

Fasten the stringers to the deck frame so the top tread lines up with the deck surface. The DCA6¹ commentary counts the screws that hold the bearing block at the top of each stringer: 7 #8 screws per stringer for Southern Pine and 11 for the northern species. Treat that as a floor for what you use, and ask your building department which top connection it accepts.

Add treads and risers

Lay the treads with a small gap and the same overhang on every step. Our designer closes the risers with boards when the stair is higher than 30 in, so a foot cannot slip through the back of a step. That is a design choice of ours and not a DCA6¹ line, but it is worth copying. Our deck drawings for permit guide shows where the stair sheet fits in a permit set.

Deck Stair Stringers: Count, Spacing and Span

The stringers do the carrying. Get their size, number and span right and the treads on top matter much less.

How many stringers

The code guide asks for at least three cut stringers, no more than 18 in apart. For a 36 in wide stair that works out exactly: one at each edge and one in the middle, 18 in apart. For a wider stair, add stringers until no gap is wider than 18 in. A 48 in stair, for example, takes four. A stair that is narrower than 36 in does not meet the width limit in the first place.

Stringer size

The guide sets 2x12 as the minimum stringer. A cut stringer loses a lot of its depth at each notch, which is why the guide starts at the deepest common board. Do not drop to a 2x10 to save money, because the tables were not written for it.

How far a stringer can span

A cut stringer is limited in how far it can run horizontally between supports. The figure we use is 6 ft, and we need to be straight about it: it comes from a figure in the DCA6¹ guide that we could not read as text in our copy, and we confirmed it in secondary copies of the guide, including a reproduction from Everett, Washington, and a Wisconsin code page. Treat the 6 ft as well supported but not primary. The commentary gives 13 ft 3 in for a solid stringer, one that is not cut into a sawtooth. If your run is longer than 6 ft, plan a support under the middle of the stair, or ask for a different design.

Treads on stringers 18 in apart

The decking rows in the 2021 IRC limit 5/4 boards to 16 in between supports and 2x6 boards to 24 in. Those rows are written for deck surfaces, so applying them to stair treads is our reading and not a stair rule. It points toward 2x6 treads when the stringers sit 18 in apart.

The Bottom of Deck Stairs: Footing, Pad or Post

The bottom of the stair is where most mistakes happen, because the top is bolted to a framed deck and the bottom is just sitting on the ground.

Footing under a stair post

The DCA6¹ guide shows a 4x4 post that shortens the stringer span, standing on a footing 12 in round or 10 in square and 6 in thick. Stair footings go below the frost line. The general deck footing rule is 12 in below grade or below the frost line, whichever is deeper, and it applies here too. Our deck footings guide covers how to find your frost depth.

Pads and slabs

A concrete pad at the bottom of the stair is a common and sensible finish, but the guide says stringers must never rest on a slab that is not founded below frost. A shallow slab can heave in winter and lift or tilt the stair, which then no longer meets the deck. Either pour the pad on a footing that reaches frost depth, or leave a gap you can adjust. The deck footing calculator sizes the concrete.

Gravel and pavers

We found no source that accepts loose gravel or pavers as the bearing surface for a stair stringer. They may suit the walking area below the bottom step, but the stringer needs a footing. If your building department allows something else, follow it.

Deck Stair Landing for Tall Decks

A landing is the flat platform that breaks one long flight into two. The DCA6¹ stair rule is that a landing is needed when the total rise is more than 12 ft.

When a landing is required

Under 12 ft of rise, one flight is allowed. Above it, you need an intermediate landing. Most backyard decks are far below that height, so a landing is mostly a question for second-story decks. Your building department may also want a landing where the stair meets a door or turns a corner, and we have not read those rules, so ask.

What we cannot tell you

The size and framing of a landing are not in the rows we hold, so we will not give you a depth or a post spacing. A landing is a small deck with its own footings and its own guard. Treat it as part of the deck design and have it drawn.

What our designer does

The deck designer flags a stair over 12 ft of total rise and does not draw a landing. If your deck is that tall, draw the deck itself in the designer and ask for the landing detail from your building department or an engineer.

Guards and Handrails on Deck Stairs

Stairs and guards are checked together, and the handrail is the item new builders skip.

Handrail height and when it starts

A handrail is required once a stair has four or more risers, at 34 to 38 in high. Our 30 in example above has four risers, so it needs one.

Guards beside the stair

A guard is required where the deck floor is more than 30 in above the ground below, measured at any point within 36 in of the edge. Guards must be at least 36 in high on the deck, and the guard along a stair can be 34 in. Openings in the guard must stop a 4 in sphere. The residential code gives the stair two exceptions: the triangle under the rail may pass a 6 in sphere, and the gap between open treads 4-3/8 in. The top rail must withstand a 200 lb load applied in any direction. The DCA6¹ guide also asks for guard posts of 4x4 or larger, on joists or rims of 2x8 or deeper.

Light at the top

The DCA6¹ stair page asks for a light at the top of the stair. It is cheap to wire during the build and awkward to add later.