How To Build An Interior Staircase

Stairs are an essential part of many construction projects, from decks to interiors. It can seem daunting to think of making your own, but they’re actually made of just three main parts: stringers, treads, and risers. Stringers are diagonal 2 in × 12 in (5.1 cm × 30.5 cm) boards that support people’s weight as they walk up the stairs. Treads are the top baseboards onto which you step, and risers are placed perpendicularly under each tread. Measure and cut the stringers accurately, and the other parts will mostly fall into place.

There are several ways we could have attached the top of each flight to the platform or the floor framing. For example, stringers can be notched to hang on a 2× cleat or can fit into a metal stair bracket. We chose to use a ½-in.-thick plywood hanger instead. The hanger is as wide as the stairs and tall enough to capture the plumb cuts at the tops of the stringers. This method has a couple of advantages. Nailing through the front side of the plywood secures the hanger to the house framing. After that it’s easy to nail through the back side of the plywood and into the ends of the stringers. Using a plywood hanger also enables you to install one stringer at a time. Measure for the hanger when the test stringer is in place to be sure it will be wide enough to catch the ends of the stringers.

Once all the stringers for the top flight are complete, set them aside and build the landing platform. Because the landing is much smaller than the main floors, it can be framed with 2×10s. The perimeter framing was nailed directly to the studs, and the 2×10 that carried the ends of the stairs extended beyond the platform into the wall framing where it was supported by a 2× post.

Install the top flight

The next part is easy: nailing the stringers into place. Space the stringers evenly across the width of the stairway. These stairs were 40 in. wide, and the stringer stock along with the spacers totaled 9 in. Four stringers created three spaces, so divide the remaining 31 in. by three. The spacing just has to be close, not exact, so we made the two outside spaces 10¼ in. wide and the middle space 10½ in. wide.

Toenail the top of the stringer through the plywood and into the LVL beam that frames the opening. Where the stringer hangs down below the LVL, drive nails through the back of the plywood hanger and into the ends of the stringers. Nail the bottom of each stringer through the notch and into the platform.

Install the temporary treads

A lot of carpenters put the temporary treads in haphazardly. But if you’re careful and make them flush with the outside and front edges of the stringer, the stairs will be easier to use for the rest of the construction process, and there won’t be any overhanging treads to catch hoses and extension cords.

Install the bottom flight

The bottom flight of stairs uses the same rise and run dimensions as the top flight. The overall layout is the same, too, except that the bottom of the stringer lands fully on the first-floor deck, which is actually preferable to being notched. Make a test stringer as before and check its fit. Use that stringer as a pattern, and then cut the rest of the bottom-flight stringers.

Nail the top of each stringer to a plywood hanger and toenail the other end directly into the first-floor deck. The final test (and my favorite part) comes after the last tread is in place. It’s a great feeling to walk up and down a set of stairs for the first time.

As with the top flight, set one bottom stringer in place to make sure it fits properly. Then use that stringer as a pattern for the rest of the bottom stringers. The bottom edges of these stringers land on top of the first-floor deck sheathing.

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