Deck subframe

A deck subframe is the structural frame of bearers and joists, anchored to footings or the building, that carries a deck’s boards and the foot-traffic (live) load. It sits beneath the visible decking timber and does all the structural work.

The subframe is a layered load path: the decking boards span across the joists, the joists span between bearers, the bearers sit on posts or footings (or are bolted to the building with a ledger), and the whole thing transfers the load to the ground. Member sizes and spacings come from AS 1684 (or an engineer) and depend on the span, the load, and whether the deck is low or elevated. Joist spacing in particular is set by the decking board, the thinner or softer the board, the closer the joists.

Durability is the other half of the job. A subframe is exposed to weather and ground moisture, so it is built from durable material, hardwood or H3-treated softwood (H4 or better in ground contact), with the right fixings: hot-dip galvanised or stainless brackets, bolts, and post anchors. Posts should sit on stirrups above the footing, not in the ground, and the framing should be detailed to drain and dry.

For a builder the practical points are to size bearers, joists, and spacings to AS 1684 (or the engineer) for the actual span and height, to set the joist spacing to suit the decking board, to use the correct durability class and corrosion-rated fixings (a deck subframe that rots or whose brackets rust is a collapse risk, decks failing under load is a known serious-injury cause), and to anchor a ledger to the building properly and flashed so it neither pulls off nor rots the wall behind it.

Also known as: Deck frame, deck substructure.

Category: Decks / Structure.

Related

• Residential decks
• Decking timbers

See also

• Face-fixing

References

• AS 1684 Residential timber-framed construction, Standards Australia (verified 2026-06-01)

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Last updated: 2026-06-01. Verified: 2026-06-01. Quarterly review for currency.