How to Calculate Deck Boards and Lumber
I built this deck calculator to help you figure out exactly how many boards you need before heading to the lumber yard. Ordering the right amount of decking material upfront avoids costly return trips mid-build and ensures all your boards come from the same production run — which matters for consistent colour and grain, especially with pressure-treated and composite decking.
The core calculation divides your total deck surface area by the coverage area of a single board. Standard deck boards are typically 5/4" × 6" (actual width 5.5") or 2" × 6" (actual width 5.5"), but the gap between boards eats into coverage. With a standard 1/4" gap, a nominal 6-inch board covers about 5.75 inches of deck width. Always use the actual width plus your intended gap spacing — not the nominal size — for an accurate board count.
Waste Factors and Layout Considerations
Waste adds up faster on decks than most people expect. Here are the main factors to account for:
- Straight runs on rectangular decks: 10% waste is a good baseline.
- Diagonal or herringbone patterns: allow 15–20% waste because more end cuts are required.
- Picture-frame borders: add the perimeter boards separately to your count.
- Composite decking: boards often have hidden fastener clips that set the gap automatically — check the manufacturer's coverage rate.
Choosing the Right Decking Material
Pressure-treated pine is the most affordable option and holds up well with proper sealing and annual maintenance. Cedar and redwood are naturally rot-resistant, take stain beautifully, and are lighter to work with, but cost more per board foot. Composite decking (PVC or wood-composite blends) costs the most upfront but requires almost no maintenance and holds colour for 20–25 years — making it cheaper over a long time horizon.
For structural framing (joists, beams, ledger boards), always use pressure-treated lumber rated for ground contact (UC4B or UC4C) in areas where the wood may be close to soil or hold moisture. Regular framing lumber will rot within a few years in those conditions.
Frequently Asked Questions
What spacing should I use between deck boards?
For wood decking, a 1/4" gap when installing is standard — the boards will shrink slightly as they dry, opening to roughly 3/8". For kiln-dried hardwoods, use a 1/8" gap at installation. Composite boards typically specify a 3/16" gap for drainage. Too tight and water pools; too wide and heels catch in the gaps.
How far apart should deck joists be spaced?
For 5/4" decking boards, joist spacing should be no more than 16" on centre. For 2" thick boards, you can go up to 24" on centre. Diagonal decking installations require tighter spacing — drop to 12" on centre for 5/4" boards laid diagonally. Always check the decking manufacturer's span rating and your local building code, which may require specific joist spacing for permit approval.
Do I need a permit to build a deck?
In most jurisdictions, any deck attached to the house or over a certain height (often 30") requires a building permit. The permit process typically requires structural drawings and sets minimum standards for footings, beam sizing, and railing height. Skipping the permit can cause problems when you sell the home — unpermitted decks often need to be removed or retroactively inspected before closing.