Decking Joist Spans
Maximum allowable spacing between deck joists, derived from the decking-board thickness, board span tables, and the decking material's structural properties.
The allowable centre-to-centre spacing of joists supporting decking boards. Spans depend on the board's thickness, species or composite type, and the load case (residential, balcony, or commercial). Common figures for 19 mm hardwood decking are 450 mm centres; 22 mm hardwood and 19 mm cypress typically allow 500 mm; thicker 32 mm hardwood may span up to 650 mm. Composite and modified-wood deckings have manufacturer-specific span tables that must be followed for warranty. AS 1684 provides joist spans for the joist members themselves under residential loads. For buyers, the wrong joist spacing is a common defect: too wide and the boards bounce or sag; too tight and material is wasted. Always cross-check the manufacturer's installation instructions and the engineer's design before setting out joists.
Values and references on this page are indicative and drawn from common industry sources. Always verify the current AS/NZS standard, manufacturer data sheet or NIAA policy publication before relying on figures for design, specification or compliance work.
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