What is composite decking? A plain answer for Australian homeowners

Composite decking is a board made by blending ground-up wood with plastic and forming the hot mix into a decking profile. The wood gives it a timber-like look, and the plastic keeps most of the water out. The boards cost more than timber, and in return one board's product page promises no oiling, painting or sealing, and resistance to rot and termites¹. Most of what you'll read about composite comes back to it being half plastic.

What is composite decking made of?

One board sold through the big hardware chain is close to half and half, at 45 per cent reclaimed timber, 48 per cent recycled polyethylene and 7 per cent additives¹. Another Australian maker says a metre of its standard board holds about 37 recycled milk bottles and around two kilograms of pine dust, a by-product of timber milling². A big American brand puts its recycled and reclaimed content at 95 per cent, using recycled plastic film and wood left over from industries such as flooring and cabinet making⁴. Some composites use PVC as the plastic instead. A board with no wood in it at all is PVC decking, which is a different product (see PVC decking).

The additives get less attention than the recycled content. One Australian maker puts rice husks and hardwood flour in its boards, and adds pigments, UV inhibitors and fungicides⁶. The same maker says boards differ with the quality and amount of what goes in, and with the skill of the factory. Pigments and UV inhibitors are there for colour and fading, and fungicides for mould, so I'd ask a seller about the additives as well as the recycled content. Boards with the same wood-to-plastic mix needn't fade or mould the same way.

Capped or uncapped

The first generation of composite was uncapped, one mix all the way through. Wood fibre sits at the surface. The board picks up dirt, stains and mould more easily, and it fades as it weathers.

A capped board has a similar core with a protective skin extruded onto it in the same pass. One maker says the skin is what keeps stains and mould out and holds the colour longer⁵. Some makers cap only the top face, or the top and bottom, while others cap all four sides with the grooves included. The skin also changes how you repair a board. One maker tells owners not to sand its capped boards at all, because sanding can do irreparable damage⁷. I'd treat any capped board as one you replace instead of sanding back, and I'd read capped vs uncapped composite decking before choosing between the two.

Hollow or solid

Boards come solid or with hollow chambers down their length. Hollow boards weigh less, and I'd hide their open ends with end caps or a fascia. One Australian maker says solid boards are preferred to hollow ones in Australian conditions, pointing to moisture absorption and strength⁶. Solid boards are heavy, and one standard board 5.4 m long weighs a little over 22 kg¹. The same maker's manual warns that its boards are heavier than traditional timber. My advice is to plan the delivery and the carry around that, and hollow vs solid composite decking goes through the trade-off.

What composite decking is not

It is not timber, even in a colour named after one. Nor is it PVC decking, which has no wood fibre at all, and what is PVC decking covers how that changes its weight, movement and heat. It also has to be fixed down to a frame, and one maker's manual rules out laying it directly on a solid surface such as concrete⁷.

How it behaves differently from timber

Movement, heat and stiffness are the differences I'd plan around.

It moves along its length, not across its width. Timber moves mostly with moisture, and one composite maker notes that timber boards often shrink a little in width once laid. Composite takes up very little water, and its movement comes with temperature, almost all of it along the board. One maker puts that growth at about 1.1 mm per metre between 15°C and a hot day⁵, which comes to several millimetres over a full-length board. Install guides deal with it through the end gap. One maker's manual has an end-gap table, asks for full-length boards on decks up to one board long and wants breaker boards beyond that in place of butt joins⁷. It also asks for a gap at walls and posts so the growing boards don't damage themselves or what's around them. One Whirlpool member with a four-year-old composite deck wrote in December 2025 that it "expands a bit" in summer heat, which they said was to be expected¹⁰.

It gets hotter than timber in the sun. One maker says its boards heat up a little faster than timber in direct sun, and that darker colours hold more heat, up to 20°C or more above the air temperature⁶. A Whirlpool member who chose one of the lighter colours wrote in December 2022 that the surface still reached 70°C on a 40°C day, and there's a pergola over that deck now¹¹. I'd put colour and shade ahead of brand here, and I'd read does composite decking get hot before picking a colour.

It needs closer joists than hardwood. Composite sags further over time under a load that stays put, and one maker's engineering note assumes the long-term deflection under a heavy point load can reach four times the short-term figure⁷. That maker asks for joists at 450 mm centres on a home deck and 400 mm under stairs and commercial decks⁷. Timber Queensland's advice for timber decks lets 19 mm hardwood decking span 500 mm and 25 mm boards span 650 mm⁸, so an old hardwood frame can be wider than composite allows. I'd measure the centres before reusing one, and composite decking joist spacing has the detail.

Fading, stains and mould

New boards change colour a little at first. One maker says most of the colour change on its uncapped board happens in the first couple of months³. One ModWood owner wrote on ProductReview in November 2023 that their jarrah-coloured deck "faded to a soft pink" within three months and was fully discoloured by six⁹. Another owner on the same listing wrote in April 2025 that black mould appeared after the heavy rain of 2022, took a week on hands and knees to clean off, and came back⁹. The maker's own FAQ says mould can grow on its boards in some conditions, and that tannin water marks show up early in a deck's life³.

Capped boards resist stains and mould better, but they still need washing. One capped-board maker says mould usually comes from constant moisture and shade, made worse by a dirty deck. The same maker says its board isn't self-cleaning, asks for a general clean at least once a year and rules out pressure washers⁷. I'd budget for a wash a year whichever board you buy.

What I'd check before you decide

Get a sample of the actual colour and leave it in the sun where the deck will go, then touch it at three in the afternoon. Look at the cut end too. A capped board shows a distinct skin, and an uncapped board is the same all the way through. Ask the seller for the install guide and read the joist spacing and the end-gap table before you price a frame, because the frame is the part I'd want right before anything else. If you're weighing composite against the timber you had in mind, why Australians are switching from timber to composite sets out both sides. The deck designer will then show what the boards and frame add up to for your deck.