What Is a Deck Board? Types, Sizes and How They're Installed

What is a deck board? It’s one of the long, flat boards that make up the walking surface of a deck. Deck boards are laid side by side with a small gap between them and fastened across the joists, the framing underneath. Most are about 1” thick and 5-1/2” wide, made from pressure-treated pine, cedar, redwood, tropical hardwood, composite or PVC, and sold in lengths from 8 to 20 ft.

That’s the short answer. Below: materials, what the size names mean, gaps, fasteners and a worked board count.

What is a deck board, and what is framing?

A deck has two layers, and it helps to keep them apart in your head:

• Framing is the structure: footings, posts, beams, joists, the ledger against the house and the rim joist around the edge. It carries the load and is almost always pressure-treated lumber.
• Decking is the surface you walk on: the deck boards, plus any fascia, stair treads and trim made from the same material.

The two meet at the joists. Every deck board crosses several joists and is screwed or clipped to each one. That’s why the joist spacing and the board you pick are linked. A thin board needs joists closer together than a thick one.

Deck board materials

Six materials cover nearly every residential deck. The decking materials comparison has costs and upkeep in detail; here is the quick version.

Pressure-treated pine is the cheapest. It’s often still wet from treatment when you buy it, so it shrinks across its width as it dries.

Cedar and redwood look better than treated pine and are lighter, but they’re soft and dent more easily. For decay resistance, buy heartwood grades.

Tropical hardwoods are very hard and heavy. They’re usually sold dry and often come with the supplier’s own gap and fastening instructions.

Composite boards don’t need staining and carry long warranties. They move with temperature, mostly along their length, so their gaps are set by the manufacturer.

PVC is the lightest synthetic and handles moisture best, which makes it popular around pools. It also moves the most with temperature.

Deck board sizes: nominal vs actual

Wood deck boards are sold by a nominal size, a trade name that’s bigger than the real board. ProWood explains that nominal dimensions are the rough-sawn size before the wood is kiln-dried and planed, while actual dimensions are what reaches the lumberyard. The two common decking sizes are:

Actual sizes are from ProWood’s lumber dimension chart. Wet treated boards can measure a little wider until they dry.

Composite and PVC boards are sold by their actual size, which varies by brand and product line. Trex, for example, lists its standard boards as 1 × 6 (Transcend and Enhance) or 7/8 × 6 (Select) in its installation guide. Other lines run slightly narrower than 5-1/2”. The deck board sizes guide lists widths by brand and shows how a small difference in width can add a row.

Why it matters: width sets how many rows you need, and thickness sets how far a board can span between joists.

Common lengths

Wood decking is generally stocked in 2 ft steps, usually 8 to 16 ft, with longer boards at some yards. The major composite brands sell mainly 12, 16 and 20 ft boards; Trex lists all three lengths for its standard decking in the guide above. Check what your local store actually carries before you plan the layout. The best length is usually the shortest one that covers each row in one piece, so there are no seams.

Gaps between deck boards

The gap between boards lets rain drain through, lets air dry the boards and the joists, and gives the boards room to move.

• Composite: most brands specify about 3/16” side to side. TimberTech allows 1/8” to 3/16” in its installation guide. End gaps depend on the temperature on the day you install.
• Wet pressure-treated wood: often laid tight or nearly tight, because it shrinks as it dries and opens its own gap.
• Dry wood (kiln-dried, cedar, redwood): about 1/8”.

The deck board spacing guide has each brand’s numbers and explains how the gap changes your row count.

How deck boards are fastened

There are three common ways to hold deck boards down. The hidden fasteners vs deck screws guide compares them on cost and labor.

1. Face screws. Driven straight down through the top of the board into each joist. For wood decking, IRC Section R507.7 calls for at least two 8d threaded nails or two No. 8 wood screws at every supporting joist, as reproduced in Anne Arundel County’s 2021 IRC deck guide. TimberTech’s guide also calls for two screws per joist on its square-edge boards.
2. Hidden clips. Grooved composite and PVC boards have a slot along each edge. A clip sits in the gap, grips both neighboring boards and is screwed to the joist, so you need about one clip per joist crossing instead of two screws. TimberTech’s instruction is to install one fastener and screw at each joist. Clips also set the gap for you.
3. Edge screws. A guide tool drives screws at an angle through the edge of a square-edge board, hiding the heads in the gap. Still two per joist.

Use the fasteners your board manufacturer lists; warranties often depend on it.

How far a deck board can span

Each deck board has to bridge the gap from one joist to the next. Thinner boards need closer joists.

For wood decking, the IRC sets the limit in Table R507.7. As reproduced in the Anne Arundel County deck guide:

“Multiple span” means the board crosses at least three joists. So 5/4 decking laid straight is fine at 16” on center, but short pieces resting on only two joists need 12”.

Composite and PVC follow the manufacturer instead:

• TimberTech: joist spacing “should never exceed 16” on center,” and all angled installations need 12” on center maximum (TimberTech guide).
• Trex: its span chart allows 16” for 1” and 7/8” boards and 24” for its 2 × 6 boards on residential decks (Trex installation guide).

If you’re putting new composite on an old frame built at 24” for 2× wood, you’ll probably need to add joists. The joist spacing guide covers the framing side.

Worked example: deck boards for a 12 × 12 deck

Here is what the deck board calculator gives for a 12 × 12 ft deck. Boards run the full 12 ft, so every row is one 12-ft board with no seams. Joists are 16” on center (10 joists), boards are 5-1/2” wide, and we add 10% for defects and mistakes.

Three things to notice:

• The material doesn’t change the count much. Wood and composite need the same 29 boards here. The wider composite gap only shrinks the last row, which becomes a thin strip you may want to avoid by adjusting the gap or overhang.
• Width changes it a lot. Narrow 2 × 4 decking needs 40 rows instead of 26, and 44 boards instead of 29.
• Fasteners follow the rows. With 26 rows crossing 10 joists there are 260 crossings: 520 face screws or 260 hidden clips before spares. The 2 × 4 deck has 400 crossings, so 800 screws.

These numbers match our 12x12 deck material list, which adds the framing, footings and hardware. For composite builds, the composite deck calculator uses brand-specific board widths, and the deck screw calculator turns the fastener count into boxes.

How deck boards are installed, step by step

A typical install on a finished frame goes like this:

1. Check the frame. Joists should be level and spaced for your board. Each butt seam needs support, usually a doubled joist or blocking.
2. Set the first board against a snapped chalk line so the rows start straight.
3. Work across the deck. Lay each board with spacers (or clips) to hold the gap, and fasten it at every joist.
4. Stagger seams if the run is longer than one board, so joints don’t line up row after row.
5. Plan the last row a few boards early. If it would be a sliver, adjust the gap slightly or rip the first and last boards equally.
6. Trim the ends along a chalk line, then add fascia if you’re using it.

The calculator handles rectangular and L-shaped decks and lets you run boards along or across the deck. Diagonal and herringbone patterns need their own planning and tighter joist spacing.

Frequently asked questions

What is a deck board made of?

Most deck boards are pressure-treated pine, cedar, redwood, tropical hardwood, composite (wood fiber and plastic with a protective cap) or PVC. Treated pine is the cheapest. Composite and PVC cost more but don’t need staining.

What size is a standard deck board?

The standard wood deck board is a 5/4 × 6, which actually measures about 1” × 5-1/2”. A 2 × 6 measures 1-1/2” × 5-1/2”. Most composite boards are a little under or about 1” thick and 5-1/4” to 5-1/2” wide, so use the exact width from the product’s spec sheet.

What is the difference between a deck board and a joist?

Joists are part of the frame: the 2 × 8, 2 × 10 or similar boards set on edge that hold the deck up. Deck boards lie flat on top of the joists and form the walking surface. Every deck board is fastened to each joist it crosses.

How many deck boards do I need?

Count rows, not square feet. Rows equal the deck width across the boards plus one gap, divided by the board width plus the gap, rounded up. Then pick lengths to cover each row and add about 10%. A 12 × 12 ft deck with 5-1/2” boards takes 26 rows, or 29 boards in 12-ft lengths.

To count boards, seams and cuts for your own deck, try the deck board calculator.