Two offcuts ride around in the back of my ute and never get used for anything. One's a bit of 90x45 treated pine, green as a pool table. The other's the same size in blackbutt, and it weighs about twice as much.
I pull them out when someone asks why one quote is cheaper than the other. You hand a person both bits of timber and they work it out themselves in about two seconds. The pine's light, soft enough to dent with a thumbnail, and costs a fraction. The blackbutt feels like a piece of steel. Neither of them is the right answer on its own. What decides it is where the timber's going and what's going to be attacking it once it's there.
What the green stuff actually is
Treated pine is radiata pine that's had preservative forced into it under pressure. That's it. It's a soft, fast-grown, plantation timber that on its own would be gone in a couple of wet Illawarra winters, and the treatment is what buys it a life.
The number after the H is the bit worth knowing, because it tells you what conditions the timber was treated for. H2 is inside work only, protected from weather, treated for termites. H3 is outside above ground, so decking, pergola rafters, fascia. H4 is in-ground, which means posts going into the dirt. H5 is heavier again for things like retaining and permanent wet.
People get burnt on this constantly. A post that should be H4 gets ordered as H3 because it was two dollars cheaper a metre, and four years later it snaps off at ground level in a southerly. It doesn't rot in the middle, either. It goes right at the soil line where the moisture and the air meet, and from above ground the post looks perfect until you push it.
The other one nobody tells you: cutting a treated length exposes untreated heartwood at the cut. Every cut end going into the ground or sitting where water pools wants a coat of preservative before it's fixed. That's about thirty seconds of work per cut and it's the difference between fifteen years and six.
Where hardwood earns its money
Australian hardwoods, so blackbutt, spotted gum, ironbark, merbau if we're counting imports, are dense enough to resist rot and termites without anybody treating them. They're stiffer over a span, they hold a screw better, and they look like timber rather than like a green plank you're going to paint.
They also cost roughly two to three times the pine for the same section, they're heavy to work with, and they'll blunt a blade quickly. On a hot day, cutting spotted gum is genuinely unpleasant.
So I use hardwood where the timber is doing something pine can't. Bearers and joists on a deck that's a fair way off the ground. Anything exposed where the look matters, so exposed rafters on a pergola, posts you can see from the kitchen. Structural work near the water at Austinmer or Thirroul, where salt air chews through everything faster than it should. And any deck board that isn't going to be painted, because a pine board with a coat of oil never looks the way people picture it looking.
I've written about the board choice on its own in the decking timber guide, and there's a longer comparison against composite in the merbau versus composite piece if that's the decision you're actually making.
The one people get wrong on wall frames
Almost every new house frame in the country is treated pine now, and that's fine. Mill-graded pine like MGP10 or MGP12 is engineered to a known strength, it comes dead straight, and the span tables are written for it. Hardwood framing in a modern house would be an expensive way to make a builder's life harder. If you want the plain-English version of how lightweight framing works and why, the government's Your Home guide on lightweight framing covers it properly.
Where it goes sideways is the older places out here. A 1950s weatherboard at Corrimal or Balgownie is framed in hardwood, and the studs are usually seasoned to concrete over seventy years. You go to add a stud or open a wall up and your pine plate is soft against timber that's rock hard. Fixings behave differently. Hardwood needs pre-drilling or you'll split it, and a nail gun that sails through pine will just bounce.
That's not a reason to match old hardwood with new hardwood, mind. Mixing is fine and I do it constantly. It's a reason to have someone doing the work who knows the difference is coming, which is most of what the wall frames and structural side of the job is.
Termites, since it's the Illawarra
Treated pine is treated against termites. Untreated pine is termite food and shouldn't be anywhere near the ground on a house here. Hardwood species vary a lot, and the heartwood of something like ironbark is genuinely resistant, but the sapwood on the outside of the same board isn't. Termites will happily take the soft outer layer and leave the hard core.
So neither timber is a termite-proof answer by itself. What actually keeps them out is the boring stuff, which is keeping soil and mulch below the weep holes, not stacking firewood against the house, fixing the leaking tap that's keeping one corner of the yard damp, and getting the place inspected. Half the termite jobs I've been called to look at started with a garden bed built up against a wall.
What it costs you in practice
Ballpark, and timber pricing moves around more than most trades' materials do.
For a deck substructure, treated pine bearers and joists on a standard job run somewhere around $60 to $110 a square metre in materials. The same frame in hardwood is more like $130 to $220. On a 30 square metre deck that's a couple of grand difference before anyone picks up a tool.
Posts are where I'd spend regardless. The cost gap between an H4 pine post and a hardwood one on a normal job is small money, and a failed post means the whole thing comes apart to fix one component.
Deck boards is where the real money sits, because it's the biggest area and the most visible. That's covered in the deck cost breakdown with actual figures.
A fair compromise, and the one I build most often: treated pine frame, H4 pine posts on stirrups so the timber never touches soil at all, hardwood boards on top. You're paying for the part you can see and the part that's carrying the load, and not paying twice for timber buried under the deck.
Before you order anything
Check the end of each length for the treatment brand. Every treated piece is marked with its hazard level, and if you can't find a mark, treat the timber as untreated. Match the level to where the piece is going, order preservative for the cut ends at the same time, and keep the H4 gear separate on site so nobody grabs a post off the wrong stack.
Anything over $5,000 all up needs a licensed builder and a written contract under the NSW building and trade licensing rules, and most structural jobs clear that easily. Ask what's being specified before you sign, not after the timber lands.
Framing, posts or a deck substructure to sort out anywhere from Helensburgh to Nowra? Ring Dave on 0414 007 351 or send through what you're building and we'll spec the timber properly before a cent gets spent on it.
Want a hand with this at your place? Get a free quote or call 0414 007 351.
