Aluminium is the frame I'd pick when the deck has nowhere to put a post. A rooftop with a membrane you can't drill is one example, and a balcony slab with a door sill a hand's width above it is another. A pine frame is too tall and too thirsty for either job. One Australian system is a kit of aluminium joists and adjustable pedestals, with clips that snap onto the pedestal heads and one screw per bracket1. Its makers sell it as rot proof and lightweight. Against that, expect to pay more than for pine, and to use clips made for metal. It must also be kept away from galvanised steel in the wet.
What an aluminium deck frame is
It is a composite decking subframe made from aluminium joists, sometimes with bearers in the same profile, carried on adjustable pedestals that set the height. In one Australian system the joists snap onto the pedestal heads. On pedestals the frame can sit on the surface below without being fixed into it. The product literature pitches that system at a waterproof membrane "that cannot be penetrated with fixings", as well as concrete patios and balconies2.
The system comes in three joist sizes, with spans quoted between 600 mm and 1800 mm depending on how the frame is laid out1. Its makers say it suits an ultra-low deck where the total height of boards and frame has to stay under 100 mm1. With bearers and taller pedestals, it reaches a finished deck height of a little over half a metre1. Stainless brackets and joiners tie the pieces together2. The published rating for its lowest joist and pedestal set is a safe working load of 2 kN per square metre spread evenly, or a 1.8 kN point load, when supported at 450 mm centres2.
Where aluminium makes sense
Low decks and doorsteps
Height is the usual reason. The maker of the system above says its lowest combination sits much lower than a stump, bearer and joist frame in pine or hardwood can. The pedestals adjust, so they can take up the fall in a slab while water still drains underneath. Pine that low, with little air around it, struggles to dry. In March 2023 a Whirlpool member planning low H3 pine joists over a pool slab was told the plan had "almost zero airflow" and was pointed to the designed low-deck systems "that use aluminium as the sub base" instead10.
Rooftops and balconies
On a rooftop or a balcony I'd talk to the body corporate or an engineer before anything goes up, about the added weight and about the waterproofing. A pedestal frame doesn't need fixings through the membrane. The same literature asks for a solid, stable, well-drained surface under the pedestals2.
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"6. One steel frame maker sells its kits as non-combustible for these sites7. Whether an aluminium frame is accepted on your block is a question for the certifier. So is the board. It needs a test report for the level on your certificate, because a frame that can't burn under a board that fails the test doesn't help you.
Curves and odd shapes
Some systems are sold on design freedom. One board maker's joist, which combines aluminium with recycled plastic, is pitched at "complex designs such as curves, cantilevers, and floating decks"3.
Where it does not
A plain backyard deck a metre off the lawn, outside a bushfire zone, is not an aluminium job in my book. The system above tops out at a finished height of about half a metre, and above that its maker recommends stumps instead of pedestals1. Aluminium is strongest where height is low, weight matters or the surface can't take a fixing, and on a site without those problems you'd be paying for an answer to a question you didn't ask.
It is also not a frame to improvise. A Whirlpool member in July 2019 had a stash of aluminium box section and wanted to make joists from it, screwing the boards up from underneath to hide the fixings9. The replies told him the screw head has to clamp the board from above and that he should use the systems that already exist. Leftover extrusion is not a deck frame. The published load rating above is for the maker's own joists and pedestals together.
Clips, screws and the metal you must not mix
Composite boards fix to an aluminium frame with clips or screws made for metal. One board maker's install guide lists a different screw for metal joists than for timber5. The joist centres are the board maker's, whatever the frame. One maker sets a 400 mm limit for timber, steel and aluminium frames alike4. Order the clips for the frame you are building, not the frame the shop assumes. The clips guide covers which clips a maker allows on which frame.
Then there is corrosion. Aluminium and galvanised steel in contact, with water present, can corrode fast. 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"8. He called that close to the worst case. Another member in the same thread warned that aluminium and zinc in contact "could be detrimental", particularly where moisture is involved8. The system above uses stainless brackets and joiners. Don't add a galvanised joist hanger or a gal batten screw to an aluminium frame because it was in the ute. Use the maker's stainless hardware, and if the frame has to meet a steel post or a galvanised ledger, put an isolating washer or a strip of plastic flashing between them.
Leave the end gaps the board maker's sheet lists at butt joins and against walls, whatever the frame is made of. The expansion gap guide explains why the end gap matters most.
Cost and what you get for it
Expect to pay more for an aluminium frame than for pine. One board maker is plain about the drawback of its aluminium-and-plastic joist3. It lists the higher upfront outlay as the con. In a 2014 Whirlpool thread, the owner of a galvanised steel deck said he had looked at aluminium and found it "Too expensive"12. Another member asked in the same thread whether aluminium could be used, because it is "a lot easier to drill than steel"12. On a different thread that year, a member weighing up a low composite deck was told to "think about aluminium" by someone who thought it looked like good value but imagined the makers "would charge a fortune for clips and screws"11.
The cost per square metre calculator works out the boards for your state, not the frame. Get the frame priced as one kit, from the pedestals to the clips, because the small parts are a big share of the bill.
Before you order
Measure the height you have from the slab or membrane to the finished level you want. Do it at the highest and lowest points of the surface. If it's under about 100 mm with the boards included, the ultra-low systems are built for that. Otherwise take off the board thickness and see what is left for the frame. On a roof, or anywhere the surface can't be drilled, I'd get an aluminium system priced as a complete kit: pedestals, joists, bearers if needed, brackets and the metal-fix clips for your board.
If it's a backyard deck on posts, read steel vs timber first. I'd keep aluminium for the hard sites. Either way the centres are the board maker's, not the frame maker's, and the joist spacing guide explains why. For a slab at the back door, the concrete guide compares pedestals with sleepers, and the bushfire guide covers what a certifier asks about the frame. The aluminium framing page goes into the systems in more detail.