My answer is usually no, not as it stands. An old frame can sometimes be made ready without much work. I'd still look underneath before planning to pull the old boards off and clip new ones on. The frame was built for timber boards, possibly at wider joist centres than a composite board allows. It has also had years to rot and move. Composite boards are less stiff than hardwood, and the clips hold them down tight to the joists, faults and all.
Can you put composite decking over a wood frame?
Yes, if the frame meets the board maker's rules, and those rules are tighter than the ones a timber deck was built to. One maker's installation page gives 450 mm joist centres for houses and 400 mm for stairs, landings and commercial decks1. Another sets a 400 mm limit for timber and metal frames alike4. Timber Queensland's deck data sheet lets 19 mm hardwood decking span 500 mm between joists and 25 mm boards 650 mm5. A frame built for hardwood boards can be perfectly sound and still too wide for composite. One maker also wants every piece of board to sit across at least two spans, which means three joists under it, offcuts included2.
The warranty depends on those figures. In May 2021 a member of the hardware chain's Workshop forum asked whether she could replace the timber boards on her deck with composite. The forum's resident DIY expert replied that she could, "as long as the framing is in good condition". In June a representative of the board maker added that the warranty needs joists no more than 450 mm apart, centre to centre, and a minimum clearance under the deck8. That is the whole question in two sentences: is the frame sound, and is it spaced for composite?
Why an old timber frame needs checking
The centres
Composite boards are wood fibre bound in plastic, and they keep deflecting under a load that stays put. One maker's engineering notes assume the long-term sag under a heavy point load can reach four times the short-term figure, and say heavy loads are best placed over joists rather than mid-span2. An Ekodeck owner on Whirlpool wrote in July 2019 that the boards "will sag (HDPE)", and that they wouldn't recommend them for a first or second floor deck unless the joists ran closer together14.
Measure the centres on the old frame before you do anything else: centre of one joist to centre of the next, not the gap between them. If any bay is wider than the figure in the guide for the boards you want, the frame needs more joists as well as new boards.
The treatment
Treated pine framing above the ground is sold as H3, which is meant for timber that gets wet and is allowed to dry after each wetting. Timber Queensland's deck data sheet puts H3 on framing more than 150 mm above the ground and asks for H5 on posts in the ground5. I'd check what your deck was actually built from, because an old frame may match neither rule.
Jack Norton, national secretary of the Timber Preservers Association of Australia, wrote in January 2026 that timber which stays damp, even out of the ground, is the condition "where H3 is not H3"7. His examples include a badly flashed ledger and a deck close to the ground with poor airflow.
The tops and ends of the joists are where I'd look first. A Victorian owner reviewing NewTechWood on ProductReview, in a review the site showed as two years old in October 2026, wrote that the joists under his old timber deck "were starting to rot at the top from screw holes" after fourteen years12. That and the yearly oiling were why he and his wife changed to composite. WoodSolutions asks for every cut end and drill hole in treated timber to be re-coated on site6, and on an old frame I'd assume the screw holes never were. Clip composite boards onto joists like that and you are putting a board with a long warranty on timber that has already started to go.
The level
Timber frames move as they age. Timber Queensland's data sheet notes that unprotected timber in the weather can distort and split. Footings can settle too. Hardwood boards bridge some of that because they are stiffer. Composite boards follow the joists, so I'd want the joist tops in one plane before any go down: a low joist shows as a dip in low sun and a high one as a hump. One maker's warranty excludes "movement, distortion, collapse or settling" of the ground or the structure the boards sit on2.
What to check before you decide
Pull a few boards off at the worst-looking corner and at the house. Then go through this list with a tape, a string line and a screwdriver.
- Joist centres, every bay, against the maker's figure.
- Joist width where boards meet end to end. One maker asks for 45 mm joists at butt joins, and a doubled joist or a laminated block where screws near the board ends need room3. Another says two board ends should not share one 45 mm joist2.
- Rot. Push a screwdriver into joist tops at old screw holes, into joist ends at the ledger and the fascia, and into post bases. Soft timber means that piece goes.
- Post bases. Posts should preferably sit on galvanised stirrups with a 75 mm gap above the concrete as a termite inspection zone, and any post set in the ground should be H5 on a bed of drainage gravel5.
- Bearers. Sight along each one. A bearer that has bowed will never give you a flat deck.
- Ledger fixings. The data sheet asks for 12 mm hot-dip galvanised bolts or coach screws into the house framing at a maximum spacing of 600 mm5. Check the flashing over the ledger too, because a badly flashed ledger is one of the failure points the preservation writer names.
- Air underneath. One maker asks for at least 40 mm of clearance where water drains away and 90 mm where it can't1. On an old low deck I'd check that soil or a garden bed hasn't built up against the frame.
If the frame passes on treatment, rot and level but fails on centres, you are in luck. That is the cheap fix.
Adding joists to a sound frame
Where the frame is sound but the centres are wide, add joists rather than replace the old ones. On a 600 mm frame, a new joist in the middle of every bay brings the centres to 300 mm, inside the 450 mm and 400 mm figures above1. The new joists sit on the bearers or in hangers, the way the data sheet describes for the old ones, and every cut end gets resealed. In a December 2021 thread on the same forum, the resident DIY expert said that laminating a second joist to an old one or adding one in between gives a similar gain in strength, but the boards are better supported by the joist in between because they don't span as far11.
A sister joist, bolted alongside, is how I'd deal with a joist that has gone soft at one end but is sound in the middle. It should bear on the bearer at both ends, so the old joist is no longer carrying the load. Be honest about how much of the frame needs this. If half the joists need a sister, you are building a new frame around an old one and paying twice.
One Workshop forum member took this route in July 2026 with a 26-year-old merbau deck of 82 m² that "was becoming unsafe"10. The owners inspected the joists first. Across the whole area they then replaced some and bolted new timber alongside others, and they taped the tops before laying the composite boards.
The new timber will be straighter than the old, so run a string line across the tops and plane or pack until every joist sits in the same plane. Fixing the centres and forgetting the level is the mistake I'd guard against here, because the boards will show it for as long as they're down.
When to replace the frame
I'd replace it when the posts are in the ground and soft, when the bearers have sagged, when more than a few joists are rotten at the ends, or when the whole thing sits so close to the soil that it will never dry. Two ModWood owners on ProductReview, in reviews the site showed as three years old in October 2026, wrote that they bought the boards to replace a timber deck that had rotted13. One said the old deck "still rotted within 10 years" despite constant upkeep13. I'd want to know what happened to the frame underneath, and the reviews don't say.
Bounce is the other warning sign. A member of the same forum built a new low deck in December 2023 with the spans pushed as far as he could, and felt a spring in it barefoot that he couldn't feel in boots9. His son jumped on it while he watched from underneath, and he could see the joists flexing mid-span. He fixed it with a laminated bearer under the longest joists on four new posts, and the spring went. That was a brand-new timber deck its builder believed was well inside the code. An old frame with tired joists at wide centres can do the same under composite, and you would feel it every time you walked to the barbecue.
If you are rebuilding, start with how to build a deck frame for composite decking and the joist spacing guide. Settle the steel or timber question before you order anything. If you stay with pine, the choice of H3 or H4 matters for the parts that stay damp. A deck that sits in the dirt has its own problems, covered in the ground-level deck guide.
What I'd do
Strip the boards off one corner and the edge nearest the house, and spend an hour under the deck with a screwdriver and a tape before you choose a board colour. If the centres are the only problem, add a joist in every bay and level the tops with a string line. If posts or bearers are soft, or the deck is sitting in the dirt, I'd build a new frame at the centres in the maker's guide. Don't burn the old frame: treated timber offcuts go through approved council disposal, never the fire5. The frame decides how long the deck really lasts, which the lifespan and warranty guide goes into. The deck designer will draw a concept plan at the right centres for the boards you pick, which you can take to a builder or certifier for the real thing.