Composite treads
Stair Stringer Spacing for Composite Decking Treads
Stringer spacing for composite decking stair treads: why composite needs tighter spacing than wood, checking span ratings, and adding stringers safely.
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Composite vs wood tread spacing
Composite stair treads usually allow a shorter span than wood, pushing stringer spacing tighter and sometimes adding a stringer.
Composite Treads Change The Spacing Rules
If you are using composite decking for stair treads, the stringer spacing that works for wood may be unsafe. Composite boards generally allow a shorter unsupported span than solid wood, especially on stairs where the load is concentrated. That usually means tighter stringer spacing and sometimes an extra stringer compared with a wood-tread stair of the same width.
Check The Manufacturer Span Rating
Composite decking comes with a manufacturer span rating, and stair applications often have a stricter rating than flat deck surfaces. This is the number that governs your spacing, not a generic rule. Find the stair tread span allowed for your specific product and use it as the maximum clear distance between stringers.
Why Stairs Are Stricter Than Decks
On a flat deck, load is spread across many boards. On a stair, a person's full weight lands on a single tread at the edge, the most demanding case. That is why composite makers commonly specify a tighter span on stair treads than on the deck field. Treating a stair tread like a deck board is a common and risky mistake.
Add A Stringer If Needed
If your stair width divided by the allowed composite span calls for more support than three stringers provide, add a fourth. The extra stringer keeps each tread span within the rating and prevents the bouncy, weak feel composite treads get when overspanned. The cost of one more stringer is small next to the safety it buys.
Fastening Composite Treads
Composite treads also have their own fastening requirements, often hidden fasteners or specific screws, and the spacing of those fasteners assumes proper support beneath. Correct stringer spacing and correct fastening work together; neither alone makes a safe composite stair. Follow the product's instructions for both.
Plan The Stringers To Match
Decide the composite product and its stair span rating first, then plan the number of stringers and their spacing to suit. A stair calculator gives you the stringer layout for any count, so once you know you need three or four stringers, the rise, run, and cut marks are the same calculation, just repeated for each stringer.
Compare
Wood vs composite tread support
| Factor | Wood treads | Composite treads | Effect |
|---|---|---|---|
| Allowed span | Longer | Shorter on stairs | Tighter spacing |
| Stringers (36 in) | Often 3 | Sometimes 4 | Add support |
| Span source | General practice | Manufacturer rating | Follow the product |
| Fastening | Standard screws | Often hidden/specific | Per instructions |
Field Checklist
- Use the composite stair tread span, not the deck span.
- Find the manufacturer's stair rating for your product.
- Keep each tread span within that rating.
- Add a stringer if the width demands it.
- Follow the product's fastening requirements.
FAQ
Common questions
Does composite decking need tighter stringer spacing?
Usually yes. Composite stair treads generally allow a shorter unsupported span than wood, so stringers are spaced closer and sometimes an extra one is added.
Where do I find the composite span rating?
From the decking manufacturer. Stair applications often have a stricter span rating than the flat deck surface, and that stair number governs spacing.
Why are stairs stricter than decks for composite?
Because a person's full weight lands on a single tread edge, the most demanding load, where a flat deck spreads load across many boards.
How many stringers for a composite stair?
Often four where wood would use three, depending on width and the product's stair span rating. Add a stringer if the span exceeds the rating.
Do composite treads have special fasteners?
Often, such as hidden fasteners or specific screws. Correct fastening assumes correct stringer support beneath, so follow the product instructions.
How do I lay out the extra stringer?
It is the same rise, run, and cut marks as the others. A stair calculator gives the stringer layout, which you simply repeat for each stringer.
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