Everything a 16x20 deck needs —
320 square feet, itemised, with the reasoning
behind each quantity.
Ledger, rim and beam Joists at 16″ on centre Deck boards Railing Post on footing
16 joists spaced 16 inches on centre along the 20 foot length, spanning the 16 foot width. The house side is carried on a ledger; the outboard edge is carried on a beam. 4 posts on footings 12 inches across. Boards run the length of the deck, parallel to the house. Stairs drop 36 inches to grade in 5 risers.
320sq ft deck
20′ × 16′footprint
15line items
Read before you order
Your deck is 20 ft long but the boards are 16 ft, so every row needs a butt seam. Seams must land on a joist, and most builders double the joist under a seam line.
Ledger attachment and flashing is the most common point of deck failure. The fastener schedule and flashing detail must come from AWC DCA6 or your local building department, not from a calculator.
Span limits are not yet verified against AWC DCA6 and are withheld. Size your joists and beams with your local building department.
Footings must reach below your local frost line. 42 in is a placeholder, not a recommendation — frost depth ranges from zero in the far south to 100+ in in the far north.
Post spacing is assumed at 6 ft because the beam span table is not yet verified. Confirm beam size and post spacing before ordering.
The stair needs 45.0 in (3.8 ft) of clear run at the bottom. Check it fits before you build.
Decking
Item
Qty
Unit
5/4x6 decking, 16 ft lengths
34 rows across 16 ft of width. Each board covers 5.75 in (5.50 in face + 0.25 in gap), so 16 ft of width needs 34 rows. 34 rows x 20 ft = 680 linear ft, plus 10% cutting waste for a straight layout = 748 linear ft, bought as 16 ft boards.
47
each
Framing
Item
Qty
Unit
2x8 joists, 16 ft
Joists run the 16 ft width, spaced 16 in on centre along the 20 ft length: floor(240 in / 16 in) + 1 = 16. The +1 is the joist that closes the far end.
16
each
2x8 rim joist, 20 ft
One rim joist closes the outboard edge, running the full length of the deck.
1
each
2x8 ledger board, 20 ft
The ledger carries the house-side end of every joist and runs the full length.
1
each
2x8 blocking, cut to fit between joists
A 16 ft joist span gets 2 row(s) of blocking (one per 8 ft of span). Each row needs a piece in each of the 15 bays: 2 x 15 = 30.
30
each
Structural posts
One post per footing: 4.
4
each
Hardware
Item
Qty
Unit
Joist hangers
One hanger per joist where it meets the ledger: 16 joists = 16 hangers.
16
each
Foundation
Item
Qty
Unit
12 in diameter footings, 42 in deep
One footing per post. Posts assumed at 6 ft on centre along the 20 ft beam: floor(20 / 6) + 1 = 4.
4
each
Concrete mix, 80 lb bags
Each footing is a cylinder: pi x (0.50 ft radius)^2 x 3.50 ft deep = 2.75 cu ft. 4 footings = 11.00 cu ft total. One 80 lb bag yields 0.60 cu ft, so 11.00 / 0.60 = 19 bags.
19
bags
Fasteners
Item
Qty
Unit
Deck screws (face fastening)
2 screws at every board-to-joist crossing: 34 rows x 16 joists x 2 = 1088 screws, plus 5% spares = 1143.
1,143
each
Railing
Item
Qty
Unit
Guard posts
Posts at no more than 6 ft apart on each open run (16 ft, 20 ft, 13 ft), plus one to close each run, less 2 shared corner posts = 11.
11
each
Top and bottom rails
49.0 ft of guard, with a top and a bottom rail: 49.0 x 2.
98
linear ft
2x2 balusters
A 4 in sphere must not pass through the guard (IRC R312.1.3). With 1.50 in balusters and a 3.75 in design gap, they sit 5.25 in on centre: 588 in of guard / 5.25 = 112.
112
each
Stairs
Item
Qty
Unit
2x12 stringers
Stringers no more than 16 in apart across a 36 in wide stair: ceil(36 / 16) + 1 = 4.
4
each
Tread stock (2 boards per tread)
4 treads x 2 boards x 3.0 ft wide = 24.0 linear ft.
24
linear ft
What this assumed
Each 5/4x6 board covers 5.75 in including a 0.25 in gap.
10% cutting waste (Straight (boards parallel to the house)).
Joists at 16 in on centre.
Blocking added once per 8 ft of joist span.
Posts assumed at 6 ft on centre.
80 lb concrete bag yields 0.60 cu ft.
Face screwed, 2 screws per crossing.
Guards assumed at least 36 in tall (IRC R312.1.2).
Balusters at 5.25 in on centre.
5 risers at 7.20 in and 4 treads at 11.25 in deep. Total run 45.0 in.
20 ft x 16 ft = 320 sq ft of deck.
Change any of it
This list uses our standard assumptions: 5/4x6 decking in 16 ft lengths with a
1/4" gap, a straight layout, 2x8 joists at 16" on centre, attached to the
house, with railing and stairs. Every one of those is an input, not a fixed rule.
Two numbers on this page are placeholders until you check them locally. Footing depth
must reach below your frost line, which varies enormously by region. And joist and beam
spans are structural decisions we deliberately do not publish until our tables are
verified — see the methodology for why.