Composite versus timber decking

The short version

Choose composite if you want the least upkeep, the deck is close to the beach or around a pool, or you would rather never oil a deck again. Choose hardwood if you want the warmth and variation of real timber, are happy to oil it, or are on a bushfire prone lot where a listed bushfire-resisting species solves a problem. On price, composite boards usually cost more up front and less over the life of the deck. The table at the end sets the four options side by side.

How the materials behave

Composite is wood fibre and plastic, usually with a hard outer cap. It does not rot, split or splinter, termites ignore it, and it does not need oiling. It expands along its length in heat, more than timber, so end gaps and joist centres follow the manufacturer’s rules. Uncapped boards lighten in their first months; capped boards hold colour. Dark boards get hot in full sun.

Merbau (kwila) is the common Perth decking hardwood: red-brown, dense, stable and durable above ground. It bleeds dark tannin in its first months, staining light paving and render until it washes out. It is listed in AS 3959 as a bushfire-resisting timber.

Spotted gum is an Australian hardwood with real variation from board to board, pale to mid brown with a wavy grain. It bleeds less than merbau, holds colour well, and is also on the AS 3959 bushfire-resisting list.

Jarrah is the West Australian hardwood: deep red when new, silvering to grey quickly without oil. Heavy, hard and durable. It is not on the AS 3959 bushfire-resisting timber list, which matters on a mapped lot.

Upkeep

A composite deck is washed. That is the list.

A hardwood deck is oiled after its first weeks of weather, then re-oiled when the colour goes flat and water stops beading. In full sun that is sooner; in shade, later. Left alone, all three timbers go silver-grey. That does not hurt the boards if the fixings are right, and some owners prefer it, but it is a decision, not an accident.

The coast

Salt air and sand are hard on everything, and the boards are the least of it. On a coastal lot the fixings, hangers and stirrups are stainless steel whichever board you pick, because galvanised hardware pits and streaks within a few seasons. Composite has the edge for the board itself: nothing to rot, nothing to oil. Hardwood performs well when it is oiled and fixed properly, and the failures you see on old coastal decks are almost always the fixings and the frame, not the boards.

Heat underfoot

Perth summers make this a real question. Dark composite in full sun is the hottest of the options, hotter than hardwood. Light composite is close to timber. Among the timbers, spotted gum’s lighter tone stays coolest and jarrah and merbau are similar. A north or west facing deck with no shade should steer you to a lighter colour, whichever material you choose.

Movement

Timber shrinks and swells with moisture across the width of the board, which is why decking is laid with a gap. Composite barely moves across its width but expands along its length in heat, which is why the ends are gapped and the joist centres are tighter. Both are handled by a contractor who has laid them before; both go wrong when they are not.

Fixings and the look of the surface

Hardwood is usually face-screwed with two stainless screws per joist, in straight lines that become part of the deck’s look. Composite is usually hidden-fixed with clips in grooved edges, so the surface is unbroken. Grooved hardwood can be hidden-fixed too. Face screwing is stronger and easier to repair; hidden fixing is cleaner.

Bushfire prone lots

If your lot is on the DFES Map of Bush Fire Prone Areas, the deck has to meet AS 3959 at the assessed BAL level. At the lower levels a listed bushfire-resisting timber, such as merbau or spotted gum, can be used where an unlisted timber cannot. Composite is neither in nor out by default: it depends on the specific product’s BAL test report. At BAL-40 and flame zone the options narrow to non-combustible materials. The BAL-rated decking page has more.

Around a pool

Composite is the usual choice around pools because it does not splinter and does not need oiling where feet are always wet, and textured pool-grade finishes grip well. Hardwood works if you will keep up the oiling. Either way the fixings are stainless and the deck is designed around the pool barrier rules.

Cost, without numbers

Composite boards and their clip systems usually cost more than hardwood boards and screws. Composite needs more joists. Hardwood needs oil, time and eventually some board replacement. Over a long ownership the two get closer than the first quote suggests. Our guide to what affects the cost of a deck covers the rest.

Decide by site and owner, not by fashion

A shaded, sheltered deck on an inland lot owned by someone who likes the ritual of oiling is a hardwood deck. A pool surround three streets from the beach on a rental is a composite deck. Most decks sit between those, and the honest answer is that either will serve if it is built well. The contractor will give you a view on your site when they see it.