A “5/4x6” wood deck board actually measures about 1” × 5-1/2”, and a “2x6” measures 1-1/2” × 5-1/2”. Composite boards are sold by their real dimensions, which are usually a little under 1” thick and between about 5-1/4” and 5-1/2” wide. The difference matters because width sets how many rows you need, and thickness sets how far the board can span between joists.
Why lumber sizes don’t match their names
Wood framing and decking are named by nominal size, a trade name that’s larger than the finished board. As ProWood explains, nominal dimensions refer to the rough-sawn size before the wood is kiln-dried and planed at the mill. The actual dimensions are what reaches the lumberyard. A board sawn roughly 2” × 6” ends up about 1-1/2” × 5-1/2” once it’s dried and surfaced.
The deck industry adds a second naming system on top of that: “5/4” (said “five-quarter”). It refers to a nominal thickness of 1-1/4”, and it’s the standard decking-specific profile, usually with rounded edges.
Wood decking: nominal vs actual
| Nominal | Actual size | Typical use |
|---|---|---|
| 5/4 × 6 | 1” × 5-1/2” | Standard wood decking |
| 2 × 4 | 1-1/2” × 3-1/2” | Narrow decking, railing parts |
| 2 × 6 | 1-1/2” × 5-1/2” | Heavier decking, stair treads |
| 2 × 8 | 1-1/2” × 7-1/4” | Joists, fascia backing |
| 2 × 10 | 1-1/2” × 9-1/4” | Joists, beams |
| 2 × 12 | 1-1/2” × 11-1/4” | Joists, beams, stair stringers |
Dimensions are from ProWood’s lumber dimension chart. Some sources note that 5/4 decking can run from 1” to about 1-1/16” thick depending on the mill.
Wet lumber measures bigger
Pressure-treated lumber is often sold wet from treatment, and wet wood is wider than dry wood. The Southern Forest Products Association notes that the width of treated Southern Pine decking varies with moisture content, and it advises adjusting your spacing to suit. A wet 5/4x6 can measure noticeably more than 5-1/2” on delivery and then shrink as it dries. Measure a few boards from your actual delivery before you finalize a layout, and see the deck board spacing guide for how this affects gaps.
Composite and PVC board sizes
Composite boards are listed by actual size, and those sizes vary from brand to brand and even within a brand’s product lines. Some published examples:
| Product | Thickness | Width | Lengths | Source |
|---|---|---|---|---|
| Trex Transcend (square edge) | 0.94” | 5.5” | 12, 16, 20 ft | Trex installation guide |
| Trex Transcend 2x6 | 1.3” | 5.5” | 12, 16, 20 ft | Trex installation guide |
| Trex Select | 0.82” | 5.5” | 12, 16, 20 ft | Platinum Decking (dealer) |
| TimberTech Pro Legacy | 0.94” | 5.36” | 12, 16, 20 ft (grooved) | TimberTech |
| Deckorators Vault | about 7/8” | 5-1/2” | 12, 16 ft | DeckStore |
| Cali Bamboo BamDeck | see spec sheet | 5-1/4” | 16 ft | FlooringClarity |
Always confirm against the current spec sheet for the exact product and color you’re buying, because product lines change.
Why the small differences matter
A difference of 1/8” in width sounds trivial, but it adds up across a deck. One warning from the Trex dealer source above is worth repeating: Trex says never to mix Select boards with Transcend or Enhance on the same deck surface, because the thickness difference creates a trip hazard. The same logic applies to any repair or extension. Matching the brand name isn’t enough; match the actual profile.
How board width changes your row count
Rows are counted from the board width plus the gap, sometimes called the pitch. The formula is:
- Rows = (deck width across the boards + gap) ÷ (board width + gap), rounded up.
Example: a deck 12 ft (144”) across the boards.
| Board | Width | Gap | Rows |
|---|---|---|---|
| Typical 5/4x6 or composite | 5-1/2” | 3/16” | 26 |
| TimberTech-style | 5.36” | 3/16” | 26 (25.99, so very close) |
| BamDeck-style | 5-1/4” | 3/16” | 27 |
| 2x4 decking | 3-1/2” | 1/8” | 40 |
On that deck, if the run is 16 ft, the 5-1/4” board needs one more 16 ft board than the 5-1/2” board. Narrow 2x4 decking needs about 50% more rows than 2x6-width boards, which means more boards, more fasteners and more labor, though some people like the look.
The same idea in linear feet per 100 sq ft of deck:
- 5-1/2” board, 3/16” gap: about 211 linear ft
- 5.36” board, 3/16” gap: about 216 linear ft
- 5-1/4” board, 3/16” gap: about 221 linear ft
- 3-1/2” board, 1/8” gap: about 331 linear ft
These are before waste. The composite deck calculator uses brand-specific widths, and the main deck board calculator lets you enter any width and gap.
How thickness affects joist spacing
Thicker boards span farther. For wood decking, the IRC sets maximum joist spacing by thickness in Table R507.7. As reproduced in Anne Arundel County’s 2021 IRC deck guide:
| Wood decking | Perpendicular, single span | Perpendicular, multiple span | Diagonal, single span | Diagonal, multiple span |
|---|---|---|---|---|
| 1-1/4” (5/4) | 12” | 16” | 8” | 12” |
| 2” (2x) | 24” | 24” | 18” | 24” |
“Single span” means the board rests on only two joists, and “multiple span” means three or more. So a 5/4 board running across joists at 16” on center is fine as long as it spans at least three joists. Short pieces resting on only two joists need 12” spacing. The same guide says decking must be fastened to each supporting member with at least two 8d threaded nails or two No. 8 wood screws.
Composite and PVC aren’t covered by that wood table. They follow the manufacturer’s rated spans, typically 16” on center for perpendicular installs and 12” for diagonals according to guides like TimberTech’s. Local codes vary and some jurisdictions amend the IRC, so confirm with your building department. The deck joist spacing guide goes into the framing side.
Stairs need closer support
Stair treads often span farther than deck boards between supports, and thinner or hollow profiles need more support. Fiberon’s stringer chart, for example, ranges from 24” down to 7” depending on the product line, with the 24” figure tied to a solid 2x6 underlayment. That’s why stringer count can change when you switch board brands. The deck stair calculator helps you lay out the stringers.
Metric and other markets
Outside the US, decking is sold in metric sizes. In the UK, composite boards around 140 mm wide are the most common, and Trex sells its 5.5” boards in metric lengths of 365, 487 and 609 cm, which are its 12, 16 and 20 ft lengths. If you’re working from a non-US product, enter its actual millimeter width and gap rather than converting from a nominal size.
Choosing a size
- 5/4 × 6 wood is the default for wood decks. It’s comfortable underfoot, widely stocked and works at 16” on center when boards span three or more joists.
- 2 × 6 wood is heavier and stiffer and can span wider joist spacing under the IRC table. It’s useful for stair treads or where you want a more rugged look.
- Composite comes mostly in the 5/4-equivalent thickness. Choose by product line, then use that line’s exact width, gap and span. The trade-offs between materials are covered in decking materials compared.
Frequently asked questions
What is the actual size of a 5/4x6 deck board?
About 1” thick and 5-1/2” wide. Some mills run 5/4 decking slightly thicker, up to about 1-1/16”. Wet treated boards can also measure wider until they dry.
Is composite decking the same size as wood decking?
Close, but not identical. Many composite boards are about 0.94” thick and 5-1/2” wide, while some lines are 5.36” or 5-1/4” wide. Use the exact width from the product spec sheet when you count rows.
Why is a 2x6 not 2 inches by 6 inches?
The nominal size describes the rough-sawn board before drying and planing at the mill. After processing, a 2x6 measures about 1-1/2” × 5-1/2”. The name stuck as a trade convention.
Can I mix board thicknesses on one deck?
Avoid it on the same walking surface. Even small thickness differences create a lip that can trip people, which is why Trex warns against mixing its Select line with thicker lines. If you must use two products, keep them in separate areas or on separate levels, and check every joint with a straightedge.