Your Result Explained: Exact Takeoff Breakdown
A 16 ft by 20 ft composite deck (320 square feet) provides enough room for both an outdoor dining table and a separate lounge seating zone. When installing grooved composite decking (such as Trex, TimberTech, or Fiberon) with universal hidden fastener clips, the clip spacer tabs automatically set a uniform 3/16-inch (0.1875") side-to-side drainage gap between every 5.5-inch board.
With a 5.5-inch board plus a 0.1875-inch (3/16") hidden-clip gap, each row covers 5.6875 inches (0.4740 feet). Across 320 square feet, that equals 675.16 net linear feet—or 742.7 linear feet when adding a 10% allowance for picture-frame miter cuts, breaker boards, and starter courses. Divided by 16-foot stock boards, you need 47 composite deck boards. Framing the 20-foot span at 16 inches on-center requires 16 floor joists (15 sixteen-inch bays + 1 end joist) plus 2 twenty-foot rim/ledger boards for a total of 18 structural members. Fastening 742.7 linear feet takes 1,115 fasteners (or 4 boxes of 350 hidden clips/screws), and laying a 3-inch under-deck #57 gravel drainage pad requires 88.00 cubic feet (3.26 cubic yards / 4.40 tons).
At $38 to $55 per 16-foot capped composite board, your 47 boards cost $1,785 to $2,585. Because 16-foot composite boards weigh 36 to 44 lbs each (~1,900 lbs for 47 boards) and flex if unsupported, schedule flatbed boom delivery.
Step-by-Step Math for a 20 ft × 16 ft Composite Deck (320 Sq Ft)
Computing 16-foot composite boards (5.5" width, 3/16" clip gap), 16" OC joists, fasteners, and 3-inch under-deck gravel for a 20 ft × 16 ft deck with 10% waste.
- Net deck area: 20 ft × 16 ft = 320 sq ft (29.73 sq m).
- Effective row width with 3/16" (0.1875") clip gap: (5.5 + 0.1875) ÷ 12 = 0.47396 ft.
- Net linear feet of composite decking: 320 sq ft ÷ 0.47396 ft = 675.16 linear ft.
- Adjusted linear feet (+10% waste): 675.16 × 1.10 = 742.68 → 742.7 linear ft.
- 16-ft composite board count: ⌈742.68 ÷ 16 ft⌉ = 47 boards.
- Framing joists (16" OC along 20 ft span): ⌈(20 × 12) ÷ 16⌉ + 1 + 2 rim boards = 18 joists.
- Fasteners/clips: ⌈742.68 × (12 ÷ 16) × 2⌉ = 1,115 fasteners (4 boxes of 350).
- 3-inch under-deck #57 gravel: (320 × (3 ÷ 12) × 1.10) ÷ 27 = 3.26 cubic yards (4.40 tons).
Bottom Line: Order 47 sixteen-foot composite deck boards (742.7 linear ft), 18 framing joists/rims, 1,115 fasteners (4 boxes of 350), and 3.26 cubic yards of #57 under-deck gravel.
How This Scenario Is Calculated
We multiply 20 ft by 16 ft to get 320 sq ft, divide by the 0.47396 ft effective row width (5.5" composite board + 0.1875" hidden-clip gap) to get 675.16 net linear feet, and multiply by 1.10 for 10% waste to reach 742.7 linear feet—which divides by 16 feet per board to equal 47 boards.
Dividing the 240-inch (20 ft) span by 16" on-center spacing yields 15 bays (16 joists) plus 2 rim/ledger boards = 18 framing boards. Multiplying 742.68 linear feet by 1.5 fastener points per foot gives 1,115 fasteners, and 320 sq ft at 3 inches deep (+10% waste) equals 88.00 cubic feet (3.26 cubic yards) of #57 stone.
Row Width (ft) = (Board Width In + Gap In) ÷ 12 | Linear Ft = (Area ÷ Row Width) × (1 + Waste%) Deck Boards = ⌈Linear Ft ÷ Board Length Ft⌉ | Joists = ⌈(Span Ft × 12) ÷ Joist Spacing In⌉ + 3 Screws = ⌈Linear Ft × (12 ÷ Joist Spacing In) × 2 Screws/Crossing⌉ Project-Specific Tips & Common Mistakes
Run 16-Foot Boards Along the 16-Foot Dimension or Use a Breaker Board
Because composite decking expands and contracts thermally along its length, manufacturers prohibit butt-joining two composite boards end-to-end on a single 1.5-inch joist. Either orient your 16-foot boards across the 16-foot deck dimension, buy 20-foot boards, or frame a double joist down the 10-foot center line for a perpendicular breaker board.
Order Square-Edge Boards for the Perimeter Picture Frame & Stairs
Grooved composite boards are designed for interior field rows using hidden clips; their slotted edges should never be left exposed on the outer deck overhang. Out of your 47 boards, order 4 to 6 solid square-edge boards for the outer perimeter picture-frame border and stair treads.
Plane High Joist Crowns Before Laying Composite Decking
Composite boards are more flexible than 2×6 wood lumber and will telegraph any 1/4-inch hump or dip in your pressure-treated joists. Stretch a string across the joists before taping them and power-plane any high crowns flat.
Mistake Butting composite board ends tightly against each other or against the house
Recommended Fix: Dark-colored composite boards expand in hot afternoon sun. Always leave a 1/8-inch to 3/16-inch end-to-end gap at breaker boards and a 1/4-inch gap against the house wall per manufacturer installation guides.
Mistake Skipping extra sister joists under picture-frame borders
Recommended Fix: Adding a perimeter picture-frame border requires installing short perpendicular ladder blocking between the first two joists on each side so the perimeter board has solid framing underneath every 12 to 16 inches.
Reference Coverage Table (#57 Crushed Stone (3/4" Clean Angular Limestone/Granite))
Compare how coverage changes across common depths for #57 Crushed Stone (3/4" Clean Angular Limestone/Granite) (default planning density 2700 lbs/yd³):
| Depth | 1 Cubic Yard Covers | 1 Ton Covers | Yards / 100 sq ft | Tons / 100 sq ft | Bags (0.5 ft³) / 100 sq ft |
|---|---|---|---|---|---|
| 2 in | 162 sq ft | 120 sq ft | 0.62 yd³ | 0.84 tons | 34 bags |
| 3 in Active | 108 sq ft | 80 sq ft | 0.93 yd³ | 1.26 tons | 50 bags |
| 4 in | 81 sq ft | 60 sq ft | 1.23 yd³ | 1.66 tons | 67 bags |
Reference Source: FHWA Aggregates Reference & ASTM C29 bulk density standard (95–105 lbs/ft³).
Frequently Asked Questions
How many 16-foot composite deck boards do I need for a 16×20 deck?
With a 3/16-inch (0.1875") hidden-fastener gap and 10% cutting waste, a 320 sq ft (16×20) deck requires 742.7 linear feet of decking, which equals 47 sixteen-foot composite boards (or 38 twenty-foot boards).
How many hidden fasteners do I need for 320 sq ft of composite decking?
At 16-inch on-center joist spacing, 320 square feet requires approximately 560 hidden clips (1 clip per joist crossing, containing 1 screw each) plus starter clips and cortex plug screws for perimeter boards—equivalent to 1,115 standard screw points.
How many joists are needed for a 16×20 deck at 16" OC?
Spacing joists 16 inches on-center along the 20-foot length requires 16 sixteen-foot joists (15 bays + 1 end joist) plus 2 twenty-foot rim/ledger boards, for a total of 18 framing boards (plus extra blocking if adding a picture-frame border).
Should I frame a 16×20 composite deck at 12" OC or 16" OC?
While 16" OC meets code for perpendicular 1×6 composite boards, many professional builders upgrade composite sub-frames to 12" OC (which adds 5 extra joists on a 20-foot span) for a rock-solid, zero-flex feel underfoot.
How much #57 gravel do I need under a 16×20 deck?
Covering 320 sq ft to a depth of 3 inches (with 10% waste) requires 88.00 cubic feet—equal to 3.26 cubic yards (4.40 tons) of bulk #57 crushed stone.