Deck board thickness
Board thickness is not a durability spec. It is a stiffness spec, and it only means anything in combination with joist spacing. A 1-inch board at 12 inches on centre is stiffer underfoot than a 2-inch board at 24.
The common options
| Board | Actual thickness | Typical max joist spacing | Notes |
|---|---|---|---|
| 1×4 / 1×6 (5/4 nominal is thicker) | ¾″ | 16″ o.c. | Cheapest, most likely to cup |
| 5/4×6 | 1″ | 16″ o.c. | The standard treated deck board |
| 2×4 / 2×6 | 1½″ | 24″ o.c. | Stiffer, heavier, more expensive |
| Composite, 1″ | 1″ | 12–16″ per manufacturer | Less stiff than wood at the same thickness |
| Composite, 1¼–1½″ | 1¼–1½″ | 16″ | The "premium" boards that allow 16″ |
| PVC, 1″ | 1″ | 12″ | Least stiff of the synthetics |
What thickness actually buys
- Stiffness between joists. Deflection falls with the cube of thickness, so going from 1″ to 1½″ nearly halves the sag for the same span.
- Fastener hold. A 2½″ screw in a 1″ board has 1½″ of thread in the joist; in a 1½″ board it has 1″. Thicker board means a shorter fastener in the frame.
- Cupping resistance. Thicker boards resist the moisture gradient that cups a thin board.
- Not lifespan. A 1″ treated board and a 2×6 treated board rot at the same rate if water reaches them the same way.
The 2×6 deck board trade-off
A 2×6 deck board at 24″ centres is a legitimate system and it is cheaper on the frame: fewer joists, fewer hangers, less labour. What it costs: the boards themselves are roughly 30–40% more per square foot than 5/4, they weigh twice as much, and a 2×6 laid flat cups badly if it is installed green.
The flat-grain problem is why 2×6 decking has a reputation for cupping. A flat-sawn 2×6 has the growth rings running roughly parallel to the wide face, so the top dries faster than the bottom and the board curls upward. Vertical-grain or rift-sawn stock resists it, but it is not what comes off the pile.
Composite thickness is a different question
Composite boards are not structural in the same way, so thickness relates to the manufacturer's span table rather than to a species group. A 1″ composite board at 12″ centres is stiffer than the same board at 16″; there is no way to compute it from first principles because the material is not homogeneous.
The practical rule: if the frame is at 16″ and you want composite, buy the thicker board the manufacturer lists for 16″, or add joists. Do not assume the 1″ board will be fine because it looks solid in the showroom.
Where thickness stops being the variable
- It does not choose a board for you. Cost, appearance and climate decide that as much as thickness.
- It does not give span ratings for specific composite products — those are manufacturer tables.
- It does not cover board width, which affects appearance and the count, not the stiffness in the same way.
- It does not account for diagonal installation, which reduces the allowable spacing for every material.
Questions people actually ask
Is 5/4 decking thick enough for 24 inch joists?
No. 5/4 is a 1-inch board and is normally limited to 16 inches on centre. At 24 inches you need 2×6 decking or a composite rated for that spacing.
What is thicker, 5/4 or 2x6?
2×6 is thicker: 1½ inches against 1 inch. The 5/4 name refers to quarters of an inch of rough thickness, which surfaces to a full inch.
Can I use 1 inch boards at 12 inch spacing?
Yes, and the deck will feel stiffer than a 2×6 at 24 inches. Closer joists cost more in framing and less in decking.
Why do 2x6 deck boards cup?
Flat-sawn 2×6 boards have growth rings roughly parallel to the wide face, so the top surface dries faster than the bottom and the board curls upward. Vertical-grain stock resists it, but most retail 2×6 is flat-sawn.