Steel vs timber deck frame for composite decking

A composite board is a long-life product sitting on whatever you put under it, and the frame decides the real life of the deck. For a deck with air underneath, away from the coast and outside a bushfire zone, a treated pine frame does the job. Steel asks more of the person building it, but it does not rot and termites cannot eat it. Its makers also sell it as straight and true to size¹. The question is not which material is better. It is whether your site is one where timber would be the first thing to fail.

What timber to use for a decking frame, and where it struggles

Deck framing in pine is sold by hazard class. Timber Queensland's data sheet on decks asks for H3 on bearers and joists more than 150 mm above the ground and H5 on posts that go into the ground⁷. H3 is meant for timber that gets wet and is then allowed to dry. That last part is the catch. A low deck over damp ground, or joist ends against a wall with poor flashing, can stay wet for long periods. Jack Norton of the Timber Preservers Association of Australia calls that the condition "where H3 is not H3"¹⁰. WoodSolutions says framing with less than 400 mm of clearance should step up to H4 or a Class 1 hardwood⁸.

Fixings matter as much as the timber. The same data sheet wants bolts and screws in hot-dip galvanised or stainless steel rather than zinc-plated, and it recommends standard galvanised connectors such as joist hangers only under a roof⁷. Where treated pine stays damp for long periods, it asks for stainless or epoxy-coated connectors instead of galvanised ones⁷. On an older deck, I'd check the hangers as closely as the timber.

So timber is fine when it can dry. Give it height, give it airflow, keep the soil away from it and seal every cut, and a pine frame will carry a composite deck for a long time. Put it close to the ground or against a wet wall and you are betting the life of the boards on the weakest part of the job.

Where steel makes sense

Steel does not care whether it dries. One maker's galvanised beam is roll-formed from two C-sections of Z450-coated coil and sold as termite proof². The same maker's deck page lists "no termite issues" and says the steel will not warp or rot¹. It also calls the steel "non-combustible" and pitches it at bushfire-prone areas¹. Its kits arrive pre-cut, with the brackets, the fixings and a set of construction drawings.

Where I'd choose steel over pine:

• Low decks. A frame close to damp ground needs H4 at least, and I'd rather have steel there than lean on the treatment.
• Termite country. H3 pine is treated against termites, but the preservative barely reaches the heartwood that a saw cut exposes. The deck data sheet also wants termite protection that can be inspected⁷, and a steel frame takes the timber out of that question.
• Bushfire zones. WoodSolutions' guide to bushfire-prone areas says an open deck at BAL-40 or in the flame zone needs its supports, framing and decking in bushfire-resisting timber or "suitable alternatives to timber"⁹. Treated pine is not on its list of bushfire-resisting timbers. Whether a frame and board are acceptable on your block is the certifier's call.
• When the frame should outlast the boards. Composite boards carry long warranties, and replacing a pine frame partway through means lifting every board to get at it.

Steel owners on Whirlpool made the case for it in a 2014 thread. One described a whole deck framed in galvanised steel sections and said he built it that way because it "doesn't rot, split or warp"¹¹. It had been up for more than seven years. The other had a galvanised steel frame that was eight years old and said that with steel "you do not get movement or flex" when walking on it¹¹. Straightness is the other advantage. Composite boards follow the joists, and steel joists arrive straight and stay that way.

What steel asks of the builder

Steel changes the job in a few ways worth knowing before the frame arrives. A pre-cut kit saves most of the cutting. Any cut you do make on site leaves an edge without the factory coating, so ask the supplier how they want it treated.

Fixing the boards is different too. ModWood's sheet on steel frames asks for self-drilling screws⁴, and its general fixing sheet does not recommend nail guns on steel joists³. The steel sheet warns that steel substructures move a good deal with heat and with stresses built into the steel. It asks for staggered joins and a pre-drilled hole in the board slightly larger than the screw thread, and says a concealed clip system will greatly assist in "de-stressing" the deck⁴. The owner with the eight-year-old frame put it plainly: it is "harder to drill the screws into the steel" than into timber joists, although once in they don't pop out or lift¹¹.

Clip systems differ by joist type. One board maker's clip guide says steel joists must be pre-drilled for the clip screws, with the drill size set by the thickness of the steel⁵. Check the clip against the steel section before you order either, and the clips guide covers which clips a maker allows on which frame.

The mistake I'd guard against first is putting aluminium and galvanised steel together. Where the two touch and stay wet, one of them corrodes faster. A Whirlpool member said in August 2018 that he had seen hot-dip galvanised brackets screwed to an aluminium gangway at a jetty "disintegrate within 12 months"¹³. He called that close to the worst case, and said a galvanised frame fixed to concrete 12 km from the sea "isn't a real drama"¹³. Another member in the same thread suggested a strip of bitumen flashing between galvanised steel and concrete.

Salt is the other limit. Timber Queensland's data sheet asks for higher levels of protection on connectors close to the coast or a swimming pool⁷. A member asking in February 2015 about a deck around a salt-water pool on a BAL-29 block, with heavy termite activity, was told that "steel around a salt pool has a short life" and that any steel there should be open to inspection¹². He ended up leaning towards a hardwood frame on steel stirrups. Steel is not automatically the right answer for a coastal pool deck. Compare it with aluminium or a hardwood frame on that site, and read the pool guide for the frame questions salt raises.

Where timber is still the sensible choice

For a deck at a comfortable height, with air underneath, on a block without heavy termite pressure and outside a bushfire zone, pine on galvanised stirrups is simple to build. It can also be adapted on the day when the deck turns out to need a step or a notch around a down pipe. A steel kit is cut to size from a drawing, which suits a site that matches the drawing and makes changes harder on one that doesn't.

Timber also forgives a change of mind. A sister joist, an extra bearer under a spa, a bay closed up to take a picture frame: all easy in pine with a saw and a box of batten screws. On a kit made to a drawing, I'd expect each change to mean new parts from the supplier.

A Whirlpool member asked in October 2014 what everyone was using to frame their composite decks, because "if you go treated pine, you'll have to pull it all up" after some years¹⁴. He was half right. Pine built badly fails early. Pine kept high and dry with the right fixings, the way the timber industry describes, lasts much longer. The site decides, more than the material does.

Joist centres are the same either way

Whatever you frame in, composite needs closer joists than hardwood decking. Timber Queensland's data sheet lets 19 mm hardwood boards span 500 mm⁷, while one composite maker sets 450 mm for its standard boards³ and another sets a 400 mm limit for timber and metal frames alike⁶. One steel kit maker drills the bracket holes in its bearers at nominated 450 mm or 600 mm joist centres¹. Tell the supplier which boards you are laying before the frame is drawn. A 600 mm layout is wider than both composite limits above, and a 450 mm one is still too wide for the board capped at 400 mm⁶. More on that in the joist spacing guide.

For timber frames, one board maker suggests its self-adhesive tape over the joists to prevent moisture damage⁶. WoodSolutions recommends flashing strips or damp-proof membranes over exposed beams and joists⁸. The tape protects the joist tops, where every fixing goes in.

Before you order

Walk the site with a tape and a spade. Measure the height from the soil to the underside of where the joists will sit, and compare it with the 400 mm line WoodSolutions draws for close-to-ground framing⁸ and with the clearance in the board maker's guide. Dig a small hole where a post would go and see whether the ground is damp. Check with the council whether the block is in a bushfire-prone area, and ask a pest inspector about termite pressure on your street.

If the frame will be low and damp, in termite country or on a BAL-40 or flame-zone block, I'd price a steel kit with the screws, the clip system for metal and whatever the supplier recommends for cut edges. If it will be high and dry, price H3 pine on stirrups with hot-dip galvanised fixings and joist tape. Either way, build it at the centres the boards need. Decide whether you want H3 or H4 pine in the parts that stay damp, and compare aluminium where height is tight. The bushfire guide covers what the certifier will ask about the frame. The steel framing page goes into the systems in more detail, and the deck designer will draw a concept plan for either frame type that you can take to a builder.