How many deck boards for a 20×20 deck?
Quick Answer
A 20×20 foot deck (400 sq ft) requires approximately 85–90 boards of 5/4×6 decking at 20-foot lengths, or about 57 boards at 16-foot lengths (with a butt joint mid-span). With 10% waste, order 95–100 boards at 20-foot length. This covers decking boards only.
Quick Reference Table
| Board size | Deck boards for 20×20 deck |
|---|---|
| 5/4×6×20 (no seam) | 43 boards + 10% = 47 |
| 5/4×6×16 (with seam) | 57 boards + 10% = 63 |
| 5/4×6×12 | 76 boards + 10% = 84 |
| 2×6×20 | 43 boards + 10% = 47 |
How to Calculate It Yourself
- 1
Area: 20 × 20 = 400 sq ft.
- 2
Effective board width (5/4×6 with 1/8" gap): 5.625".
- 3
For 20 ft span: 240" ÷ 5.625" = 42.7 → 43 boards.
- 4
Add 10%: 43 × 1.10 = 47 boards (running the full 20-ft length).
- 5
If using shorter boards: double the count and add 15% for seams.
Pro Tip
For a 20×20 deck, use 20-foot boards to avoid mid-span butt joints. Joints weaken the deck surface and are a tripping hazard. If 20-ft boards are too expensive, stagger joints so they never align on adjacent boards.
Assumptions to Check
Before ordering materials, confirm the dimensions, product coverage, waste factor, and local installation requirements for your project. Manufacturer coverage tables and local code rules can change the final quantity, especially for structural work, exterior projects, and irregular layouts.
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