Decking joist spacing and the subframe explained

In shortMost composite decking is laid on joists at 300 to 400 mm centres, closing up to 300 mm or less for diagonal boards and for hollow boards; 400 mm is the common figure for softwood. The exact spacing comes from the board maker's installation guide, not from habit. The frame itself is usually UC4-treated softwood joists, typically 47 x 100 mm or 47 x 150 mm, on posts or pads, with aluminium an option where rot or height matters.

The boards are what you see. The subframe is what you are paying for. A deck that bounces, sags between joists or rots at the posts within ten years nearly always failed underneath, and the usual causes are joists set too far apart for the board and timber treated for the wrong use.

This note covers the figures a homeowner needs to read a quote: joist centres, timber size and grade, treatment, and when a metal frame earns its extra cost.

Joist spacing by board type

Joist spacing is measured centre to centre. The figure depends on the board, because a hollow composite board, a solid capped board and a softwood board each span a different distance safely.

Two rules sit behind the table. First, the spacing in the maker’s guide is a condition of the warranty, so a frame built at 450 mm centres for a board that wants 300 mm has no cover if it sags. Second, heavy loads, a hot tub for instance, need closer centres and sometimes a different frame; tell the installer what the deck will carry.

Boards also need room to move. A composite board grows and shrinks lengthways with temperature, far more than timber does, so the maker’s guide sets side gaps (typically 3 to 6 mm, fixed by the clip), end gaps that depend on the temperature on the day, and roughly 10 to 20 mm of clearance at walls and posts. Those gaps differ by brand.

What size timber for a decking frame

The standard frame is pressure-treated softwood, strength graded C16 or C24, in joist sections of typically 47 x 100 mm or 47 x 150 mm. The deeper section spans further between bearers or posts. Span tables for each size are in the Timber Decking and Cladding Association’s Builder’s Guide, the reference most UK installers work from.

Posts are set in concrete or on pads, and a low deck over firm ground can sit on pads or adjustable pedestals with no posts at all. The ground is cleared, covered with a weed membrane and topped with gravel so soil never touches timber. Air should move beneath the deck, with about 150 mm of clearance under the joists and an open or ventilated perimeter, and the surface should sit about 150 mm below the damp-proof course.

Treatment class: the detail that decides lifespan

Treated timber is sold to use classes defined in BS EN 335. Use Class 3 covers timber outdoors and off the ground, which suits the boards and the handrail. Use Class 4 covers timber in contact with the ground or kept permanently wet, which is where joists, bearers and posts live. The TDCA and the treatment industry say a deck frame should be specified UC4, and that UC3 joists are a common cause of early failure.

So ask the installer, in writing, for UC4-treated joists and posts; much of the treated timber on a merchant’s rack is UC3. And insist that every cut end, notch and drilled hole is brushed with end-grain preservative, because that is where rot starts.

Aluminium and steel subframes

An aluminium subframe does not rot, does not move with moisture and can be very shallow, with joists from around 20 to 50 mm deep. That makes it the usual choice where a deck must sit low over a patio or a flat roof, where it goes on pedestals, and where a non-combustible frame is wanted under a balcony or terrace. Millboard, Composite Prime, Cladco and others sell aluminium systems for their boards. Steel is used for long spans and commercial work.

The cost is the drawback. Expect an aluminium frame to add something like £20 to £45 per m2; that is a rough figure of our own rather than a published one. For a garden deck on good ground, UC4 timber done properly is still the sensible choice.

Hidden clips or screws

Composite boards are fixed either with hidden clips, which sit in grooves in the board edges, screw to the joist and set the gap, or by face-screwing square-edged boards with colour-matched or stainless screws. Clips give a clean surface; screws are cheaper and stronger at board ends. Hollow boards need end caps or a fascia. Stainless fixings, A2 or A4 near the coast, are the rule for hardwood and any screw that shows.

Reading the frame in a quote

A proper quote names the joist size, grade and treatment class, the joist centres, the post or pad fixing method, the membrane and gravel, and the fixing type. If it says only “treated timber frame”, ask. The guide to choosing a decking installer lists the rest of what a quote should itemise, and the decking cost guide shows how far the subframe moves the price. If your garden falls away, the frame is where the money goes; see decking on a slope.

When you are ready for quotes, the town pages list installers whose websites we check: all towns.