Deck Board Spacing Guide
Proper spacing between deck boards is essential for drainage, expansion/contraction, and long-term durability. Too little gap traps debris and causes rot; too much gap is uncomfortable to walk on and looks unprofessional.
Standard Gap Sizes by Material
| Material | Gap (inches) | Gap (decimal) | Reason |
|---|---|---|---|
| Kiln-dried cedar/redwood | 1/8" | 0.125 | Minimal shrinkage; stable dimensions |
| Pressure-treated (new/wet) | 1/4" | 0.25 | Shrinks significantly as it dries |
| Pressure-treated (aged/dry) | 1/8" | 0.125 | Already shrunk; stable |
| Composite (Trex, TimberTech) | 1/8" to 1/4" | 0.125-0.25 | Check manufacturer spec — varies by brand |
| IPE / tropical hardwood | 1/8" to 3/16" | 0.125-0.1875 | Very dense; minimal movement |
| Aluminum decking | Per manufacturer | — | Clip system determines spacing |
Why Pressure-Treated Needs a Bigger Gap
New pressure-treated lumber is saturated with water from the treatment process. As it dries over the first 3-6 months, it shrinks — sometimes by 1/8" or more across the board width. If you install PT boards with a 1/8" gap, they'll shrink to a 1/4" gap, which is fine. But if you start with a tight (1/16") gap, they'll open to 3/16", which can look uneven.
How to Maintain Consistent Spacing
- Use spacers: 16d nails work as 1/8" spacers. Buy composite spacers ($10-15) for consistent gaps.
- Check every 3-4 boards: Cumulative error can drift the spacing. Measure from the wall periodically.
- Straighten boards as you go: Use a board bender or pry bar to straighten warped boards before fastening.
Butt Joints (When Boards Don't Span the Full Deck)
When two boards meet end-to-end on the same joist, leave a 1/8" gap at the butt joint for expansion. Stagger butt joints so they don't all fall on the same joist — this looks better and distributes structural load.
Use the Calculator
The Deck Board Calculator lets you enter your specific gap size. The gap affects the row count — a larger gap means fewer rows needed.