What is different about fitting composite decking?

The four rules composite changes — joist centres, expansion gaps, hidden clips and no finishing — and the mistakes carried over from timber.

Difficulty: intermediate
Time: 1 hr
Read: 5 min
Safety: caution

Short answer

Follow the board manufacturer's span table, not the softwood default: many composites need 300mm centres or closer, and less for diagonals or stairs. Leave the specified end gap for thermal expansion, which is larger than a timber gap and varies with the temperature on the day you fit. Use the maker's clips, keep the frame ventilated, and do not oil, stain or seal it.

Composite is not timber with better weathering. It is stiffer in some ways and much less stiff in others, it moves with temperature rather than with moisture, and its fixings are part of a system rather than a choice. Fitting it like softwood produces the two failures composite decks are known for: sagging between joists and boards butting hard against each other in July.

Safety

Cutting composite boards produces a fine dust of wood flour and plastic that you should not breathe — wear a mask and cut outdoors. Composite boards get very hot in direct sun, hot enough to be uncomfortable underfoot on dark colours, which is worth knowing before choosing a colour for a south-facing deck.

Step by step

Read the manufacturer's installation sheet before designing the frame.This is not a formality. Joist centres, end gaps, side gaps, fixing type, minimum overhang and stair requirements are all product-specific, and warranties are void if they are not followed.
Close the joist centres.Where softwood decking takes 400mm centres, many composite boards specify 300mm, and 250mm or less for diagonal laying and for stair treads. Hollow-core boards are generally less tolerant than solid ones. Design the frame from the board's table.
Understand that composite moves with heat, not with damp.A timber board swells when wet. A composite board expands when hot, along its length. That is why the end gap is bigger, and why it depends on the temperature at the time of fitting — a board fitted on a cold morning needs more end gap than one fitted in the afternoon sun.
Leave the specified end gaps at every butt joint and every abutment.Against a wall, a post, a fascia and at every board end. The figure is on the installation sheet, often expressed per metre of board length and per degree. Do not eyeball it.
Acclimatise the boards on site.Lay them out flat, supported, in the position they will be fitted, for a day or more. It settles them to the ambient temperature and lets you sort the colour variation before it is permanent.
Use the system's hidden clips.Most composite is grooved for a clip that sets the side gap automatically and allows the board to move along its length. Clips are not interchangeable between brands. Face-screwing is allowed by some makers with pre-drilling and an oversized clearance hole, and prohibited by others.
Support the ends properly.Board ends need a joist under them within the specified distance, and many systems require a double joist at butt joints so each board has its own clip. Cantilevered ends are usually restricted to a short overhang.
Ventilate the frame at least as well as for timber.Composite does not rot, but the timber frame under it does, and a composite surface is less permeable than a timber one. At least 50mm of clear air below the joists and an open or vented perimeter.
Fit the fascia and trims last, with their own expansion allowance.Fascia boards expand too, and a fascia screwed tight at both ends will bow in summer. Use slotted holes or the maker's fascia clips.
Do not finish it.No oil, no stain, no sealer. Composite is finished as supplied, coatings do not adhere reliably, and applying one usually voids the warranty and looks worse than the weathering it was meant to prevent.

Tips

Mix boards from several packs as you lay. Colour varies between production batches and laying pack by pack produces visible bands.
Composite is heavier than softwood. Allow for it in the frame design and in how you handle it on site.
Dark composite in full sun gets genuinely hot underfoot. If the deck is south-facing and children use it barefoot, choose a lighter colour.
Keep the offcuts. Replacing a damaged board years later with a current batch will not colour-match.
Composite needs a fall like any deck. It does not absorb water, so standing water sits on the surface rather than soaking away.

Common mistakes

Using the softwood joist spacing — Most composite boards are less stiff between supports than 32mm softwood. At 400mm centres they visibly deflect, and on hollow-core boards they can crack.
Butting boards tight to a wall or a post — The board expands along its length in the sun and has nowhere to go. It bows upward, and the clips or fixings tear out.
Sealing or oiling it — The surface is designed not to absorb. A coating sits on top, wears unevenly, goes patchy within a season and in most cases voids the warranty.
Mixing brands of board and clip — Groove profiles differ between manufacturers. A clip that nearly fits gives an inconsistent gap and inadequate hold.

If it doesn't work

Boards have buckled or lifted in hot weather

Cause: Insufficient end gap, or boards pinned at both ends by face screws or a tight abutment. — Fix: Cut the end gaps back to the specified dimension. If face fixings are restraining the board, replace them with the system's clips or with oversized clearance holes.

The deck sags visibly between joists

Cause: Joist centres set to the softwood default rather than the board's table. — Fix: Add intermediate joists. There is no other fix — the board is doing what it was always going to do at that span.

Scratches and scuffs show badly

Cause: Uncapped composite scratches and the exposed core shows a different colour. — Fix: Some uncapped boards can be gently brushed along the grain to blend a scratch; capped boards cannot be repaired and the board is replaced. Choose capped boards if the deck will take furniture and pets.

White or grey mould-like patches

Cause: Composite is a wood-plastic mix and the wood flour at the surface can support surface mould in shade, particularly in the first year. — Fix: Clean with the manufacturer's recommended cleaner. Do not pressure wash close in — it damages the surface and voids most warranties.

Questions people ask

Can I lay composite on my existing timber deck frame?

Only if the joist centres meet the composite's span table and the frame is sound. In practice most existing frames at 400mm centres need intermediate joists adding, which is a straightforward job while the boards are off.

Does composite go slippery?

Less than timber, because it does not raise a grain and grows less algae, but it is not slip-proof — a shaded composite deck still gets a surface film in winter. Grooved and textured surfaces help; steps still benefit from anti-slip inserts.

How long does it last?

Warranties of twenty to twenty-five years are common on capped boards, covering structural integrity and, on better products, fade and stain. The frame underneath will usually be the limiting factor, which is a good argument for spending on the frame timber and the ventilation.