The subframe is the part of a deck you never see. It's also the part responsible for most deck failures. Substructure runs $75–95 per square metre in Perth builds — a $20/m² swing that looks small in a quote and makes an enormous difference to the deck's structural life.
This guide is part of the Endure Perth deck cost series. It goes deeper on substructure specifically — what the spec means, why it varies between builders, and what an H4 frame actually costs.
The H3 vs H4 gap
Most Perth builders use H3 treated pine in their subframes. H3 is rated for above-ground use, protected from direct weather. H4 is rated for in-ground use and ground-contact environments — a meaningfully higher treatment standard.
A deck framed in H4 bearers costs $10–15/m² more than the same frame in H3. That gap disappears in the quote when a builder doesn't specify treatment level. It shows up 8–10 years later when the H3 frame starts to rot and the H4 frame doesn't.
What substructure actually includes
A complete subframe includes: bearers, joists, joist tape (applied to the top face of every joist to shed water), galvanised stirrups, concrete stumps, and footing depth to specification. Footing depth is site-specific — softer soils and sloping blocks need more concrete per pier.
Our subframe spec runs about $85–95/m²:
- H4 bearers (ground-contact treatment rating)
- Joist tape on every joist top face
- More concrete per footing than minimum spec
- Reduced spans versus standard
A builder working to minimum spec runs $70–80/m².
When aluminium subframes make sense
Aluminium subframes are corrosion-proof, which makes them the right choice for coastal properties (beachside suburbs, canal blocks), shaded understorey environments, and decks over water or in high-moisture areas. They cost $120–160/m², which is the full subframe rate — not an add-on to timber. The premium is real. So is the longevity difference in the right environment.