How Many Deck Boards for a 16×20 Deck?
Quick Answer
A 16×20 foot deck with boards running the 20-foot length needs approximately 38 deck boards (5.5-inch wide) after a 10% waste factor, plus 16 joists, beams, posts, and fasteners.
- Decking boards (5.5-in wide, 20 ft long): ~38 boards
- Joists (2×10, 16-in OC): 16 joists at 16 ft
- Beams: doubled 2×10s spanning 20 ft
- Posts (6×6): 6–9 depending on height
Formula
Divide the deck width in inches by the board coverage (5.5-in board + 0.25-in gap = 5.75 in). This gives the number of rows of boards running the length.
Step-by-Step Calculation
- 1
Deck width in inches
16 ft × 12 = 192 inches
- 2
Board coverage
5.5-in board + 0.25-in gap = 5.75 in
- 3
Number of boards
192 ÷ 5.75 = 34 boards × 20 ft
- 4
Add 10% waste
34 × 1.10 = 37.4 → 38 boards
16×20 Deck — Full Materials List
| Material | Quantity | Treated Cost* | Composite Cost* |
|---|---|---|---|
| Deck Boards (20 ft) | 38 | $530–$1,060 | $1,330–$2,660 |
| Joists — 2×10×16 ft | 16 | $240–$400 | $240–$400 |
| Beams — 2×10×20 ft (doubled) | 4 | $160–$280 | $160–$280 |
| Posts — 6×6×8 ft | 9 | $135–$225 | $135–$225 |
| Joist Hangers | 16 | $50–$80 | $50–$80 |
| Deck Screws (5 lb box) | 5 | $75–$125 | $150–$250 |
| Concrete Footings | 9 bags | $45–$75 | $45–$75 |
*Pressure-treated at $14–$28 per 20-ft board. Composite at $35–$70 per 20-ft board.
Frequently Asked Questions
How much does a 16×20 deck cost to build?
DIY with pressure-treated lumber: $2,500–$4,500 in materials. DIY with composite: $4,500–$8,000. Professional installation: $6,400–$12,800 (treated) or $10,000–$19,000 (composite) including labor.
What size joists for a 16-foot span?
A 16-foot span needs 2×10 joists at 16 inches on center as a minimum, and 2×12 for heavier loads. Add a mid-span beam to shorten the joist span if you want to use smaller lumber.
Do I need a permit for a 16×20 deck?
Yes. A 16×20 deck is 320 square feet, well above the permit-exempt threshold everywhere. Attached decks always require a permit and inspection. Budget $150–$500 for permits.