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How Many Stair Stringers Do I Need? Everything You Need to Know!

brown wooden stairs indoor

Stair stringers are the foundation of any staircase. When you’re building a staircase, three stringers are typically enough unless the tread is wider than 36 inches. If it is, you’ll want to use four stringers to provide extra stability and durability. Some small stairways may only need two stringers, but we always recommend using at least three. Three is also the minimum if you’re using cut stringers.

Composite stringers change the game a bit. If you’re working with composite stringers, you need to position them closer than with closed or cut stringers. Ideally, you want to aim for 8–12 inches on the center for composite stringers.

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About Stair Stringers

Stringers are critical in stair construction because walking down stairs transmits a lot of force. The downward motion of your foot climbing downstairs is transmitted sideways through the treads to the stringers. Ideally, you want a stringer every 16 inches on center, but you can go as wide as 18 inches. Never space stringers any wider or you risk your stairs becoming unstable.

This rule also goes for stairways wider than 36 inches, in which you’ll want to use a mix of solid and cut stringers. Closed or solid stringers can’t be any longer than 13 feet, while cut stringers are weaker and should only be used for a maximum length of 6 feet. If you need a stringer to go further than these lengths, you’ll have to install a support like a knee board to reduce the stringer’s length.

Though it’s really a good idea to install supports like knee or kicker boards at the base of your stringers anyway. These will attach the stringers more securely to the surrounding structure, like your walls and floor. Without ample support, the stringers may creak when you walk on the stairway.

One popular method of securing stringers is to treat the floor above the stairway as the first step and install the stringer flush with this landing against the wall. If you really want extra support here, you can add a ledger board, but you need to make sure that’s attached securely, or the structure could fail later.

Before you attach stringers to other wood, you should liberally apply wood glue to both the stringer and your other piece of wood, like the tread. Wood glue will provide a lot of sturdiness to the stairway combined with the wood screws.

The importance of stringers can’t be understated. If your stringers are compromised by improper installation or shoddy materials, the entire stairway could collapse at a moment’s notice. Many localities have building codes relating to stair construction, including stringer specifications and placement. Before building stairs in or outside of your home, you should check with your local building authority to see if they have regulations like this in place.

white wooden stairs outdoor
Image Credit: Damon Lam, Unsplash

Stringers and Treads

How many stringers you use will also be determined by how thick your treads or steps are. For thicker boards like 4x12s, you can probably even get away with just two stringers for a stairway less than 36 inches in width. However, thinner 2x10s would really need three stringers for maximum support. Also, remember that thinner boards will have shorter spans, so they’re best used for smaller stairways.

multi-storey staircase
Image Credit: Jason Briscoe, Unsplash

How Long Should My Stringers Be?

Stringer length is calculated using the Pythagorean theorem: square the height of your stairs and square the length of the stairs. Next, add those numbers together and find the square root of the result. Round up to the nearest number, and you have the total length of your stair stringers.

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Stair stringers provide most of the strength of any stairway, so you should be absolutely sure they’re installed correctly. Using sturdy wood screws, wood glue, and careful planning, you can build a stairway to last your lifetime or longer. If you fail, though, your stairway could collapse and hurt someone.

Featured Image Credit: Jean-Philippe Delberghe, Unsplash


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