A deck has two material lists running in parallel — the decking you see and the frame you don't — and under-ordering either one stalls the build. This calculator estimates the surface boards from your deck size and board width, then adds the joist count and fastener quantity so you can frame and finish from a single trip.
How deck materials are calculated
Boards = rows × 1.05 (waste), boards cut to deck length
Joists = (deck width × 12 ÷ spacing) + 1
Screws ≈ deck area × 2.5
The board width plus the gap between boards is what each row actually occupies, so a 5.5 in board with a 3/16 in gap covers about 5.69 in of width. That small gap, multiplied across a wide deck, changes the row count.
Planning the layout
- Run boards across the joists, so decking laid the long way of a 16×12 ft deck spans the 16 ft length and the joists run the 12 ft direction.
- Match board length to deck length where possible to avoid butt joints; where you must join, stagger the seams over joists.
- Decide the gap before you buy: dry boards need the full 3/16 in; green pressure-treated boards will shrink, so they can be butted tighter.
The frame beneath the boards
| Component | Role |
|---|---|
| Ledger board | Attaches the deck to the house |
| Joists | Carry the decking, spaced 12–16 in OC |
| Beams & posts | Support the joists down to footings |
| Joist hangers | One per joist end |
A worked example
A 16×12 ft deck with 5.5 in boards, 3/16 in gaps, joists at 16 in OC:
- Row width = 5.5 + 0.1875 = 5.69 in = 0.474 ft
- Rows = 12 ÷ 0.474 = 25.3 → 26 rows
- Boards = 26 × 1.05 = about 27 boards, each 16 ft
- Joists = (12 × 12 ÷ 16) + 1 = 10 joists
- Screws ≈ 192 × 2.5 = 480
Fasteners and hidden costs
Use exterior-rated screws — coated or stainless — sized for your decking; composite often uses hidden clip systems instead of face screws, which changes the fastener count. Beyond boards and joists you will need joist hangers, post bases, structural connectors and possibly concrete for footings, which the related concrete calculator can size. Budget for these structural parts up front; they are easy to overlook and essential to a safe deck.
Finishing and protecting the deck
Timber decks need sealing or staining, ideally within weeks of the build once the wood has dried enough to accept finish. The related deck stain calculator works out how much stain the surface needs. Plan to reseal every couple of years to keep the deck weather-tight and looking new.
Decking material changes everything
The board count is only the start; the material dictates spacing, fasteners and lifespan. Pressure-treated pine is the budget standard but moves as it dries and needs regular sealing. Cedar and redwood resist rot naturally and look beautiful but are softer and pricier. Composite and PVC boards never rot or need staining, but they flex more than wood, so they require tighter joist spacing — often 12 inches on centre — and many use hidden fastener clips rather than face screws, which changes the fastener count entirely. Decide the material before finalising the joist plan, because it can change how many joists and what fasteners you need.
The structure you do not see
A deck is only as safe as its frame and its connection to the house. The ledger board, which attaches the deck to the building, is the single most failure-prone component — it must be properly flashed to keep water out and bolted (not just nailed) to the house structure. Joists hang from the ledger and a beam on hangers and connectors rated for the load. Posts carry the beam down to concrete footings sized below the frost line. None of this shows once the decking is on, but all of it is what keeps the deck standing, so it deserves more care than the visible boards.
Gapping, fastening and board layout
Leave a consistent gap between boards — about 3/16 inch for dry lumber — so water drains and the deck breathes; wet, green pressure-treated boards can be laid tighter because they shrink as they dry. Stagger any end joints over joists rather than lining them up, both for strength and appearance. Use exterior-rated screws or the manufacturer's hidden clips, never interior fasteners, which corrode and stain. Plan the board direction before you buy: running boards across the joists is standard, but a diagonal or picture-frame layout looks striking and changes the board count and waste, usually upward.
Permits, codes and finishing
Most jurisdictions require a permit for a deck above a certain height or attached to the house, and inspections of the footings and framing. Build to the local code for joist spans, railing height and stair geometry — these are safety requirements, not suggestions. Once built, a timber deck needs sealing or staining after the wood has dried enough to accept finish, then re-coating every couple of years; the related deck stain calculator sizes that. An accurate material list, a code-compliant structure and a regular finishing routine together turn a pile of boards into a deck that lasts decades.
Estimating cost and the full build
A deck's material cost spans a wide range driven mostly by the decking choice: pressure-treated pine is the budget option, cedar and redwood cost more, and composite or PVC boards cost most up front but eliminate staining for life. Beyond the boards and joists, budget for the ledger and its flashing, beams, posts, joist hangers and structural connectors, concrete for footings, exterior fasteners or hidden clips, and railing materials — the hidden structure often costs as much as the visible surface. Add a permit fee where required. The calculator's board, joist and fastener counts get the surface and frame sized; the related concrete and footing calculators handle the foundations, and the deck stain calculator the finish. Pricing the whole assembly up front, structure included, prevents the common shock of discovering the framing and hardware nearly double a budget that only counted the pretty boards on top.
Frequently asked questions
How many deck boards do I need?
Divide the deck width by the board width plus the gap to get the number of rows, then add about 5% for waste and cutting. A 16×12 ft deck with 5.5 in boards needs roughly 26 boards 16 ft long.
What spacing should deck joists be?
Standard is 16 inches on center for most decking. Use 12 in for composite boards or diagonal patterns, and 24 in only for light-duty structures where the decking span rating allows it.
How many screws do I need for a deck?
Plan for about 2.5 screws per square foot of decking — two screws at each joist crossing. A 192 sq ft deck needs roughly 480 deck screws plus extras for the frame.
Should I leave a gap between deck boards?
Yes. A gap of about 3/16 inch lets water drain and timber expand. Wet pressure-treated boards shrink as they dry, so a slightly smaller gap is fine for green lumber; composite needs the manufacturer’s specified gap.