Adding Center Stringers to Composite Deck Stairs: Support Requirements for Wider Stairs
Why Wider Composite Stairs Need More Stringers
The assumption is correct: wider stairs do require more stringers. But the real driver isn’t just the width—it’s the material. When you’re switching to composite treads like Trex or Azek, the structural requirements change significantly compared to traditional pressure-treated wood stairs.
A composite tread is roughly 1 inch thick, while a wood tread is about 1.5 inches. That thickness difference alone can make reusing old stringers problematic, because riser heights would no longer be consistent. Code requires uniform riser heights, and stepping up and down uneven stairs is a tripping hazard. Most builders simply construct new stringers rather than trying to adapt existing ones.
Stringer Spacing Requirements for Composite Materials
Wood stairs traditionally space stringers 16 inches on center. Composite decking is less stiff than wood, so it needs more support. Trex, one of the leading composite brands, specifies 9-inch on-center stringer spacing for most of its products, while some manufacturers allow 12-inch spacing depending on the specific board profile and thickness.
To calculate how many stringers you’ll need, divide your stair width by the required spacing. For a 4-foot-wide (48-inch) staircase using 9-inch spacing, you’d need at least five stringers. A narrower staircase might use four stringers with slightly wider spacing, but never exceed the manufacturer’s maximum spacing recommendation.
Center Support: Posts, Headers, and Connectors
Beyond just adding more stringers, many wider staircases benefit from center support to eliminate bounce and movement underfoot. A common approach is a single post positioned mid-span under the staircase with a header beam connecting to it. This single midpoint support is often more practical than multiple posts, as long as your deck design allows for the post foundation below.
If a post isn’t feasible, heavy-duty structural fasteners like Simpson Strong-Tie connectors provide strong mechanical connections between stringers and the deck rim joist. These metal connectors prevent vertical movement and ensure the stringers stay firmly attached under load.
Building New Stairs: Materials and Fastening
Use pressure-treated 2×12 lumber for cut stringers, which can span up to about 6 feet horizontally. When installing composite treads, pre-drill 1 inch from the board edges and use color-matched composite fasteners. Many manufacturers recommend sealing all cut stringer areas with protective tape (such as Trex Protect tape) to extend the lifespan of your pressure-treated lumber.
Mechanical connections between stringers and the header should use appropriate fasteners. Simpson Strong-Tie metal straps are commonly specified in installation guides to prevent vertical separation under the dynamic loads that stairs experience.
When in Doubt, Check the Manufacturer
Every composite product has slightly different structural properties. Trex, Azek, Timber Tech, and other brands each publish installation guidelines that specify stringer spacing for their products. Before you build, download the manufacturer’s stair installation guide for the exact material you’re using. The small difference between 9-inch and 12-inch spacing, or between different tread thicknesses, can affect whether your stairs are code-compliant and safe.