Most of the decks we build aren't standalone jobs. They come off the back of a home extension, a new living area or a reworked kitchen that now has bifolds where a window used to be. The client usually asks the merbau-or-composite question at the first meeting, and the honest answer is that it's the last thing we'll lock in. On an extension deck the board sits on top of a stack of decisions about the structure, the wall it meets and the floor level it has to hit, and each of those goes differently depending on what's going on top.
This is the builder's version of the comparison. If you want the material-by-material rundown of colour, feel and price, plenty of decking suppliers have that. What follows is what changes on the job.
The deck is the last thing decided, not the first
An extension deck has three things fixed before any board is chosen: the finished floor level of the new room, the wall or beam the deck hangs off, and where the ground is. The deck has to land within a step of the new floor, drain away from the house, and keep air moving underneath. Once those are set, the board choice is mostly about how the frame under it gets built and how the deck is finished at handover.
Getting that order wrong is how people end up with a deck a full step below the new sliding door, or a board that was chosen for its colour and then had its joists spaced for a timber that behaves differently.
Where the deck meets the house
On a standalone deck the connection to the house is a ledger bolted to whatever wall is there. On an extension we're building the wall at the same time, which is the one chance to do it properly. The ledger, its flashing and the cavity behind it are detailed into the new frame rather than cut into a finished wall, and the step-down from the new floor to the deck is set to suit the door we're fitting.
Timber and composite differ here in one practical way. A merbau board is usually run right up to the wall with a small gap for movement, and its first-year tannin bleed will mark a fresh rendered or painted wall if the flashing lets water sit against it. Composite doesn't bleed, but it expands and contracts more along its length with heat, so the gap at the wall and at every board end is set by the manufacturer's spec, not by eye. Either way, that junction is the part of the deck we spend the most time on, because it's the part that leaks if it's wrong.
The frame is sized for the board
What's underneath is the same family of structure whichever board goes on: posts on footings sized for your soil, bearers, joists, and stainless or coastal-grade fixings if you're anywhere near the water, which around the Illawarra is nearly everyone. What changes is the joist spacing and the fixing method.
- Merbau spans further between joists than most composites do, and it's screwed straight through the face with stainless screws or fixed with hidden clips.
- Composite generally wants joists closer together, because the board is less stiff, and most ranges use a proprietary hidden clip system that dictates the gap between boards.
If a client is undecided, we frame at the tighter composite spacing. It costs a little more in joists and it means a merbau deck goes on a stiffer frame than it needs, which is no bad thing. Framing at timber spacing and then switching to composite is the mistake, because the boards will bounce and every low spot in the frame shows through them. There's more on how we build the structure on our decking page.
Approvals when the deck comes with the extension
A deck built on its own can sometimes go through as exempt development if it stays low, small and clear of the boundaries. A deck built as part of an extension is assessed as part of the extension. It goes on the same plans and through the same approval as the new rooms, whether that's a Complying Development Certificate or a DA through council. That's simpler for you, because there's one approval rather than two, but it means the deck's height, area and setbacks count toward the whole job. Our guide to deck, pergola and carport approval in NSW covers what the standalone rules look at, and we sort which path applies before anything is drawn.
Merbau or composite decking for an extension?
For a deck coming off a new extension we lean to composite on coastal blocks, pool surrounds and anywhere the owners have told us they won't keep up the oiling, and to merbau where the owners want real timber and are happy to look after it. The build cost difference sits in the boards themselves, and on an extension deck it's a small share of the whole job, so it's rarely the thing that should decide it.
What should decide it is the next ten years. A merbau deck off a new living area needs a clean and a re-oil regularly to stay the colour it was at handover, and if it's not done the new extension looks finished and the deck looks tired inside two summers. A composite deck holds its look with a hose and a broom, but it runs warmer underfoot in full sun, so on a north or west-facing extension with no roof over the deck the board colour is a real decision. We cover the upkeep side in how often to oil a deck.
How does jarrah compare with merbau?
Jarrah comes up when someone wants a darker, redder deck than merbau gives and is prepared to pay for a Western Australian hardwood that's been carted across the country. From a building point of view the two go on the same frame, take the same fixings and want the same oiling. Jarrah holds its colour a little better between coats and bleeds far less tannin, which matters on an extension where the deck sits against fresh render or above new pale pavers. If that runoff is going to land on a surface you've just paid for, jarrah is the argument for the extra cost. If the deck sits over lawn or old concrete, merbau does the same job for less.
When the deck gets built in the job
The deck goes in late, after the extension is closed in and the external wall is finished, so the trades aren't walking new boards to bits and the wall behind the ledger is complete before it's covered. On a merbau deck the last thing we do is the first oil, so the timber starts protected rather than waiting on you to buy a tin. On a composite deck we leave the boards clean and hand over the manufacturer's care sheet, because the warranty on most ranges depends on it.
If you're planning an extension with a deck off the new rooms, the deck is part of the design conversation from the first site visit, not an add-on at the end. Call us on 0434 037 851 or go through the contact page and we'll look at the levels, the block and the aspect together.
Want a hand with this at your place? Get a free quote or call 0434 037 851.