What's different about estimating composite
The board count works the same way as wood — rows across the deck, then lengths for each row — but a few composite details change the order:
- Longer lengths. Composite comes in 12, 16 and 20 ft, so many decks can be done without butt seams at all. On a 24 ft run, the planner splits rows across joists — for a 24 × 12 deck it recommends 37 × 20-ft boards.
- Doubled joists at seams. TimberTech's guide says board ends must land on a joist and joists should be doubled at butt seams. The calculator counts how many joists need doubling.
- Specific gaps. Side gaps are set by the manufacturer, and end gaps depend on temperature.
- Hidden fasteners. Clips usually set the side gap for you; count one per board per joist.
Manufacturer gap and spacing specs
From current installation guides — always check the guide for your exact product:
| Brand | Side gap | End gap (butt joints) | Joist spacing |
|---|---|---|---|
| Trex | 3/16" (5 mm) | 3 mm above 4°C (about 40°F), 5 mm below | 16" o.c. residential; 12" for diagonal |
| TimberTech | 1/8"–3/16" | 3/16" at ≤32°F, 1/8" at 33–74°F, 1/32" at ≥75°F | 16" o.c. max; 12" for 45° layouts |
| Fiberon | 3/16" (5 mm) | 1/4" at 30°F down to 1/16" at 90°F | 16" o.c. residential; reduce by 4" for diagonal |
Brand names are used only to reference their published guides. This site isn't affiliated with any manufacturer.
Tips for ordering composite
- Keep the offcuts. The cut plan shows which offcuts are long enough for stair treads or repairs.
- Match the allowance to the layout. 5–10% is a common allowance for straight layouts; diagonal and picture-frame layouts usually need 15% or more.
- Price it properly. Use the deck cost calculator with per-board prices to see whether 20-ft boards are worth the premium.
Frequently asked questions
How many composite boards do I need for a 16×12 deck?
With 5-1/2" boards and a 3/16" gap running along the 16 ft side, you need 26 rows of 16-ft boards. With a 10% allowance, buy 29 × 16-ft boards. Change the inputs above for your own deck and board.
Are composite boards really 6 inches wide?
No. “6-inch” composite boards are about 5-1/2" wide, and some grooved lines are 5.36". Thickness is usually just under 1". Check the spec sheet for your product line and enter the actual width.
How many hidden clips do I need?
Plan on roughly one clip per board at every joist. The fasteners panel under the calculator counts this from your layout. Box coverage varies by brand — a Trex Hideaway box of 90 clips is rated for 50 sq ft at 16" on center, and a TimberTech CONCEALoc box covers 100 sq ft — so compare your count with the packaging.
Can I use this as a Trex or TimberTech deck board calculator?
Yes. Open the installation guide for your product line and enter its actual board width, side gap and the lengths you can buy — most lines are about 5-1/2" wide and sold in 12, 16 and 20 ft. The board count will then match that line. This site is independent and not affiliated with any decking brand.
Can I mix different composite board lines on one deck?
Check with the manufacturer first. Lines can differ in thickness — for example, Trex Select is thinner than Transcend and Enhance, and dealers note they shouldn’t be mixed on the same surface because of the height difference.