How to pick the right timber, and what the H ratings mean

The difference between a job that lasts and a job you go back to

You are partway into a deck, the boards looked fine off the stack, and now they are cupping and splitting and drinking paint. Same supplier, same steps as always. What has usually happened is not that the timber was bad, it is that it was the wrong timber for where it ended up, and that is a much easier problem to avoid than to fix.

Timber is not one product. It varies by species, by treatment and by how much moisture is still in it, and our conditions here are hard on it. Dry wind, a sudden soaking, then weeks of sun.

1. Start with what the timber is doing

Before you get anywhere near species, work out the job.

• Structural. Frames, studs, joists. This is about strength and stress rating, the F grades, and about the timber staying straight under load.
• Exposed. Decks, fascias, posts. Has to look right and stay looking right, so you want it stable, straight, with few knots, and treated for outside.
• Non structural. Boxing in, shelving, trim. Light duty, and the place you can be least fussy.

The best timber is not the dearest one. It is the one rated for where it is going.

2. Softwood and hardwood, which is not about how hard it is

The names mislead people. Balsa is a hardwood.

• Softwoods like pine grow fast, cost less, cut easily and are the standard for framing. Outside, they need treating.
• Hardwoods like merbau and spotted gum are denser and last longer, so they suit exposed finish work. Allow more time for cutting and fixing, and predrill.

What actually decides it is whether the timber is treated for the exposure and seasoned for the use.

3. The H ratings, which are the bit worth memorising

If it is going outside, treatment is not optional. The H rating tells you the hazard level the timber is treated for.

• H3. Outside, above ground, protected from decay and termites. Decking, pergolas, fascia.
• H4. In ground contact. Posts, sleepers, retaining walls.
• H5. In ground contact in wet conditions, or in fresh water. Footings and permanently damp situations.

Going down a rating to save money on a post in the ground is the single most expensive saving in building. Ask what the rating is rather than judging by the colour, because the tint is not a reliable guide.

4. Moisture decides how much it moves

Green or unseasoned timber is fresh cut and still drying. It is cheaper and it will shrink and move after you fix it, so it belongs in framing where that movement is allowed for.

Seasoned timber has been dried and stabilised. It costs more and it stays where you put it, which is what you want anywhere the movement would show.

Either way, let it sit on site for a couple of days before you fix it so it can come to the moisture level of where it is actually going to live. That is 48 hours doing nothing, and it saves the callback on a deck that opens up gaps in its first summer.

5. Where it came from matters

Timber out of one mill does not always behave like timber out of another, even at the same grade. Worth asking whether it is kiln dried and where it was treated.

And local conditions are their own thing. Spotted gum is a beautiful board and it will split on you here if it is not predrilled and sealed properly, which is not a fault in the timber.

Common picks and what they suit

• Treated radiata pine. Cheap, easy to work, the standard for framing and battens, and fine for decking at the right H rating.
• Merbau. Dense, good colour, stable once seasoned. Decking, screens and stairs. It bleeds tannin, so seal it before it meets anything pale.
• Spotted gum. Tough and good looking, heavy to handle, and unforgiving if you do not predrill.
• Vic ash and Tas oak. Indoor work. Architraves, furniture, bench tops.
• Red gum. Heavy duty, good for rustic posts and beams, and hard work to cut.

Timber does not fail, the wrong choice does

Put it where it is rated for, let it acclimatise, fix it with the right fastener and seal what needs sealing, and it will outlast the person who built it. If you are not sure which rating a job needs, come in and ask us before you buy rather than after.

Shop related products

• Macsim 10-10 x 65mm Treated Pine Screws, C3. C3 coating, which is what treated pine needs or the fixing corrodes before the timber does.
• Macsim 8-10 x 42mm Treated Pine Screws, C3. The general purpose size for decking and battens.
• Diggers Timber Cleaner 1L. For getting a weathered deck back before you oil it.
• Diggers Raw Linseed Oil 1L. The traditional finish for exposed timber, and it is cheap.

Did you know we cut timber to length on the saw bench in store while you wait, and we carry framing, decking, sleepers, cement and sheeting that are not all listed online. Give us a ring on (08) 8536 4333 with your sizes and we will tell you what is in the yard.

What is online sits in building materials and fasteners and fixings.

Getting it to site

Long timber was never going in the car. Local delivery runs Tuesday and Thursday, $5 around Strathalbyn and $10 for the whole regional run, on goods bought from us. Trade accounts with monthly billing and job site delivery are on the trade page.

More from the shed

• How to prep surfaces like a pro without hours of sanding or scraping
• How to storm proof your shed without expensive repairs or complicated tools

Cheers,
Candeece

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