Deck joist spacing for timber and composite boards

Deck joist spacing is the distance between the centres of the joists that carry the boards. Get it wrong and the deck feels springy, boards cup or sag between supports, and hidden fasteners lose their grip. The right figure comes from the board, not from the frame, and every board maker publishes a maximum span. This guide gives the typical ranges, the special cases and how spacing feeds into the number of pedestals or ground screws.

Typical deck joist spacing by board type

Board	Typical joist centres	What drives it
Softwood (pine, spruce, larch), 21–28 mm	about 40–50 cm	Thickness; 40 cm is the safe default for thinner boards
Hardwood (e.g. tropical species), 19–25 mm	about 40–50 cm	Dense and stiff, but thin profiles are common
Thermowood, 26–32 mm	about 30–40 cm	Heat treatment makes the timber more brittle, so many suppliers specify closer centres
Composite (WPC), solid or hollow	about 30–40 cm	Lower stiffness, creep under heat; hollow profiles are usually the strictest
Any board on stairs or in heavy-use areas	closer than on the main deck	Concentrated loads, edges and impact

These are typical ranges seen in board installation instructions, not Fastshifter specifications. The maximum span printed by the board maker is the binding figure; if it says 35 cm, 35 cm it is. Thickness and width matter too: the same species at 21 mm and 28 mm can have very different spans.

Why the board decides the spacing

Boards act as small beams between joists. Too long a span and three things happen:

• Deflection. The deck feels bouncy underfoot and boards may stay bent after loading, especially composite in summer heat.
• Cupping and cracking. A board that flexes every time someone walks on it works its fixings loose and opens end checks.
• Fastener failure. Clips and hidden screws are tested at the maker’s spacing; wider spans put more load on each fixing.

Closer joists cost a little more timber or aluminium, but they rarely cause problems. Wider joists save material once and cost repairs later.

Diagonal decking

When boards run at 45° to the joists, each board crosses a longer distance between supports: about 1.4 times the joist spacing. To keep the same real span, reduce the joist centres to roughly 70% of the straight-laid figure, for example from 40 cm to about 28–30 cm. Some board makers publish a separate diagonal value; use it if they do. Herringbone and chevron patterns also need extra joists or noggins under every board end.

Stairs, edges and board ends

Some parts of a deck need closer support than the field:

• Stair treads. Many board makers ask for tighter stringer or support spacing on stairs than on the main deck. Treads take heel impact on the nosing, so check the stair-specific value.
• Board end joints. Two board ends should never share a single narrow joist. Use a double joist or a wider joist so each end has its own bearing and fixing.
• Picture frames and borders. Edge boards running the other way need their own supports, usually extra joists or blocking along the perimeter.
• Heavy loads. Hot tubs, large planters and outdoor kitchens need a separate design for the joists and supports underneath.

Protect the top of every joist with joist tape, and keep board gaps and ventilation in mind: closer joists do not help if water sits on them.

How joist spacing drives the number of supports

The joist spacing sets how many joists you need; the joist profile and its span table set how often each joist must be supported. Multiply the two and you get the support count.

Example (assumptions, adjust to your project): a 5 m × 4 m deck on a slab, joists running 4 m, single joist layout, each joist supported at five points as its supplier’s span table requires.

• Joists at 40 cm: 5000 ÷ 400 = 12.5, so 13 spaces and 14 joists. 14 × 5 = 70 supports, about 3.5 per m².
• Joists at 50 cm (if the board allows it): 10 spaces and 11 joists. 11 × 5 = 55 supports, about 2.75 per m².

So a composite board that needs 33 cm centres instead of 40 cm adds several rows of supports on a single layout. A double layout breaks that link: the pedestals or ground screws carry bearers, the joists cross on top at board spacing, and the support count is driven by the bearer span instead. The full per-m² rates for both layouts are in how many pedestals per m².

On slabs and roofs, FT pedestals are rated up to 800 kg each, so the limiting factor is almost always the joist span, not the pedestal. On soil, ground screws give a fixed point under each bearer or joist line without digging.

Planning checklist

• Board maker’s maximum span found for the exact board, thickness and laying direction
• Reduced spacing for diagonal or patterned laying
• Stair and heavy-use areas checked against stricter values
• Double joists or wider joists at every board end joint
• Joist profile checked against its own span table for support spacing
• Support count calculated from joists × supports per joist, plus spares

For the bigger picture, read why the substructure matters more than the boards. Specifying a complete system for your customers? Request samples or apply to become a distributor.