Treated pine versus hardwood: the honest cost of a deck

The cheapest sentence in outdoor construction is we’ll just do it in hardwood. The second cheapest is pine will be fine. Both sentences have wrecked good backyards. Treated pine and native hardwood are not two finishes of the same product. They are different materials with different stiffness, different ways of taking a fastener, different relationships to water, and different kinds of honesty about money.

This is a cost essay that tries to stay on the tools. Treatment levels first, because the stamp on the end of a pine post is doing more work than the stain on a hardwood sample. Then lifespan, look, stiffness and fasteners. Then the cases where pine is the right answer, and the cases where it is a false economy dressed up as pragmatism.

Treatment is not a species. H3 pine is still pine. It will not become spotted gum because the ticket said “premium decking”. What the treatment buys is resistance to decay and insects in a defined hazard. Everything you love or hate about the board — the grain, the give underfoot, the way it takes oil — is still the tree.

H3, H4 and H5, without the folklore

Australian treated pine is discussed in hazard classes. The ones that matter on a deck are H3, H4 and H5.

H3 is above-ground, exposed to the weather, a decay and insect hazard but not in soil. This is the stamp you want on pine decking boards, on pine joists that stay clear of the ground, on pine handrail components that never become a post in dirt. It is the workhorse rating of the suburban deck.

H4 is the in-ground and fresh-water rating for moderate to high decay hazard. This is the stamp you want on pine posts that enter the soil, on pine sleepers that behave like retaining, on any pine that will spend its life damp against earth. A surprising number of failed decks are H3 posts in holes that were “only a bit of dirt”.

H5 is the severe in-ground or fresh-water rating — the one you reach for when the post is in a wetter, nastier situation, or when the specification and the site both say the ordinary in-ground rating is not enough. It is not a magic upgrade for decking boards. Putting H5 on a wearing surface you could have built in H3 is spending copper and chromium on the wrong problem.

There are lower ratings for interior and protected timber. They do not belong on an open deck. There is also a habit of calling every green-tinged board “treated pine” as if the phrase were a specification. It is not. Read the stamp. If there is no stamp, you are guessing, and guessing is how a bearer and joist frame becomes a replacement job.

The National Construction Code will not save you from a stamp you ignored. AS 1684 will not invent a treatment that was never there. The standards assume the material is what the drawings said.

Lifespan is a system, not a species war

Ask a salesperson how long pine lasts and you will get a number that sounds like a warranty. Ask how long hardwood lasts and you will get a number that sounds like a river red gum. Both numbers are marketing until you talk about water, sun and termites.

A pine decking board is a wearing surface with a treatment envelope. Cut the end and you open untreated heart. Trench a groove for a clip and you cut through the zone that was easiest to treat. Leave the board in a puddle against a sliding-door track and the treatment is fighting a pond. In a dry temperate suburb, with airflow under the deck and an annual maintenance habit, H3 pine decking can look respectable for a long time. In a wet tropical set-down with leaf mulch in every gap, it can be tired, fibrous and holding mould before the furniture has been replaced.

Hardwood, chosen from the durable Australian species, does not rely on a factory envelope in the same way. The durability is in the wood. That is the genuine advantage. It is not an infinite advantage. Hardwood still cups, checks and supports decay where water sits. A merbau board with no gap and a blocked soffit is not morally superior to pine. It is just slower to admit the detailing was wrong.

The frame often decides the real lifespan. I have pulled up fifteen-year-old hardwood boards to find pine joists that were H3, wet, undersized and finished. I have also pulled up seven-year-old pine boards to find a hardwood frame that would have taken another generation of wearing surface. The honest cost of a deck includes the thing you cannot see from the lawn.

The look, which is not trivial

Pine looks like pine. Even when it is stained the colour of merbau, the grain is open, the knots announce themselves, and the board weathers into a softer, more fibrous grey. Some houses want that. A pale coastal cottage, a kids’ platform, a garden deck that is allowed to be a garden deck — pine can be handsome if you stop asking it to impersonate a eucalypt.

Hardwood looks like timber in the way Australians mean when they say “proper”. The density reads. The arris stays sharper. The silvering, if you let it happen, has a tighter grain. This is why designer decks default to spotted gum even when the structure underneath is pine. The wearing surface is what the body meets.

Stain on pine is not a crime. Film finishes on pine in full sun are a maintenance contract. If the client wants a dark, even, furniture colour on pine and also wants to forget the deck for five years, the conversation should move to a different material, not a darker stain.

Stiffness, bounce and the feel of a cheap span

At the same cross-section, pine is less stiff than the dense hardwoods. People describe this as bounce, spring, or “it feels a bit hollow”. Sometimes the species is the cause. More often the span is the cause and the species is the accomplice. A pine joist at the generous end of a span table, carrying a party and a spa step, will announce itself. A hardwood joist at the same span may stay quieter. A pine joist at a tighter spacing may be quieter still.

This is where false economy hides. Saving money on pine, then running joists at the maximum typical spacing because “the table allows it”, produces a deck that feels cheap even when the treatment is correct. The cost you should have spent on timber went into a feeling you cannot oil away. Deepen the joist, close the spacing, or shorten the span. The species can stay pine.

Hardwood boards on a lively pine frame do not fix the bounce. They add mass. Sometimes that helps. Sometimes it just makes a floppy frame carry more weight. Design the structure; do not buy density as a superstition.

Fasteners and the chemistry of treatment

Treated pine and metal have a relationship. Copper-based treatments are not kind to ordinary steel. Use fasteners and connectors rated for the treatment and the exposure. Stainless in coastal air. Connector hardware that the manufacturer will stand behind in treated timber, not a galvanised offcut from a fencing job. The cheapest screw in a treated joist is a rust bloom waiting for a wet January.

Hardwood wants pre-drilling more often than pride allows. Pine is more forgiving of a coarse thread driven fast, which is why weekend jobs look like pine even when the boards are not. The forgiveness has a cost: overdriven screws that crush the face, and clips that loosen as the softer face weathers.

If you mix the two — pine frame, hardwood boards — specify the fastener for the worst case, not the average. A stainless screw through hardwood into treated pine is a common, sensible assembly. A coated fastener chosen for pine and then asked to hold ironbark at the coast is a maintenance visit.

When pine is the right answer

Pine is the right answer when the deck is a structure first and a jewellery surface second. Raised platforms over a slope, big spans that would make solid hardwood framing a structural and financial event, posts that need a treatment rating you can actually buy in the section you need — this is pine’s natural country. It is also the right answer when the wearing surface will be covered, partly covered, or treated as a utility platform: the side path, the bin run, the lower landing that exists to get you to the lawn.

Pine is the right answer for many frames under hardwood or composite boards. There is no aesthetic purity prize for an all-hardwood substructure the client never sees, if the money would have been better spent on stainless connectors, a proper footing, or a thicker wearing board. A designer deck can be a hybrid. The original Queensland brief — a long-lasting outdoor room — was never a species oath.

Pine is the right answer when the owners are honest about maintenance and climate. A small, shaded, well-ventilated pine deck, oiled or stained on a calendar, in a temperate suburb, is a reasonable asset. Shame is a bad specifier.

When pine is a false economy

Pine is a false economy when it is asked to be a designer wearing surface in a climate that eats faces. Full western sun, no shade, no will to recoat, a client who pointed at a spotted-gum photograph — that job will look tired while the invoice is still in the drawer. The “saving” will be spent on a restoration or a replacement of the boards, while the frame, if it was any good, sits there watching.

Pine is a false economy when treatment is used as a substitute for detailing. H4 posts in waterlogged holes with no gravel, no dry zone, no termite inspection face — the stamp was not the problem. Pine is a false economy when the quote won by stretching spans, shrinking joists, and calling the bounce “normal for timber”.

It is also a false economy when a hardwood surface is specified in a thin, wide board on a pine frame that was never resized, using fasteners that were never upgraded, in a coastal postcode. That is not a hybrid. That is two materials failing in their own languages at the same time.

The honest way to price the choice

Price three assemblies, not two species.

• Pine frame, pine boards. Lowest first cost. Highest dependence on treatment, coating and the owner’s calendar. Fine when the architecture can accept pine.
• Pine frame, hardwood boards. The common designer hybrid. First cost sits in the middle. Lifespan of the surface rises if the species is durable and the gaps are real. Frame must be sized for the mass.
• Hardwood frame, hardwood boards. Highest first cost, highest stiffness for a given section, least reliance on a treatment envelope. Still requires stainless, ventilation and a termite plan. Not a substitute for those things.

Then add the money that does not appear on the timber invoice: footings, connectors, labour to pre-drill, the extra day to get the fall right, the first coat of oil, the scaffolding if you are up a storey. A cheap pine deck with expensive access is not cheap. An expensive hardwood deck that needed no restoration for fifteen years can be the frugal object.

If you are choosing with a pencil on the bonnet of a ute, write the climate next to the price. Coastal. Inland radiant. Wet tropics. Leafy and damp. Those four words change the winner more often than the mill’s discount.

Pine is not an apology. Hardwood is not a virtue. The honest cost of a deck is the cost of the system that will still be there when the furniture is on its second cover.