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.
Pro Tips
- At 320 square feet, this deck typically requires a building permit. Check your local setback requirements, maximum height above grade, and whether an engineer-stamped plan is needed — many jurisdictions require plans for decks over 200 sq ft or 30 inches above grade.
- For a 16×20 deck with boards running the 20-foot length, you will need to special-order 20-foot boards or plan a staggered butt joint at the midpoint. If using butt joints, always land them on a double joist and stagger adjacent rows by at least 4 feet.
- Use 16-foot joists spanning the 16-foot width with a beam at the midpoint (8 feet). This beam eliminates bounce — a 16-foot joist span with standard 2×10 joists at 16-inch centers will feel springy underfoot.
- Pre-stain or seal all four sides of deck boards before installation. Once installed, you cannot reach the bottom surface, and moisture wicking from underneath causes cupping and premature failure.
Key Cost Factors
- Pressure-treated decking for a 16×20 deck costs $2,000–$3,200 for boards alone (38 boards × 20 ft × $2.50–$4.00/ft). Composite decking: $3,800–$6,400. Substructure (joists, beams, posts, footings, hardware): $1,200–$2,000. Railing (64 linear feet): $600–$1,800 depending on material. Total materials: $3,800–$7,000 PT or $5,600–$10,200 composite.
- A contractor charges $30–$55 per sq ft installed ($9,600–$17,600 for 320 sq ft) for a complete deck with railing and stairs. DIY saves $5,800–$10,600 in labor, making this one of the highest-value DIY projects — but budget 3–5 weekends for a deck this size.
Common Mistakes to Avoid
- Estimating by area only. Board count depends on board width plus the gap between boards. Dividing 320 sq ft by board area without the spacing gap undercounts the boards.
- Forgetting the border and stairs. A 16×20 deck usually has a picture-frame border, fascia, and often stairs — all extra material beyond the field decking.
- Wrong joist spacing for the material. Composite and diagonal layouts need 12-inch joist spacing, not 16. Getting this wrong means a bouncy deck or a failed inspection.
- Ignoring board-length waste. Running 16-foot boards across a 20-foot span leaves awkward seams. Plan lengths so butt joints land on joists and waste is minimized.
Related Project Considerations
- Pick the decking material first. Cost, span, spacing, and fasteners all depend on whether you choose pressure-treated, cedar, or composite.
- Size the substructure for the span. A 16×20 deck needs properly sized beams, joists, and footings rated for your snow and live loads.
- Plan stairs and railings. Local code dictates railing height and stair dimensions on a deck this size; budget the extra lumber and hardware early.
Frequently Asked Questions
How much does a 16×20 deck cost to build?
DIY with pressure-treated lumber: $2,500–$4,500 in materials for framing, decking, fasteners, and footings. DIY with composite: $4,500–$8,000, as composite boards run 2–3× the cost of treated lumber. Professional installation: $6,400–$12,800 (treated) or $10,000–$19,000 (composite) including labor, permits, and hardware. Add $1,000–$3,000 for railings and stairs if the deck is elevated.
What size joists for a 16-foot span?
A 16-foot span needs 2×10 joists at 16 inches on center as a minimum for residential deck loads (40 psf live + 10 psf dead). 2×12 joists are better for heavier loads, wider spacing, or a stiffer feel underfoot. Alternatively, add a mid-span beam to break the 16-foot run into two 8-foot spans, which lets you use smaller 2×8 joists and reduces lumber cost significantly.
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 in virtually every jurisdiction. Attached decks almost always require a permit regardless of size because the ledger board connection to the house is a structural element. Budget $150–$500 for permits and expect inspections at the footing, framing, and railing stages. An unpermitted deck can delay or derail a future home sale.