Timber floor frame on bearers
The floor frame carries the building and sets its geometry, so it is worth more care than anything above it. Two bearers run across the slabs, the joist frame sits on the bearers, and the deck goes on last. Because the bearers run the short way and the joists the long way, the load path from the deck down to the ground is short and obvious.
Squaring the frame is done by diagonal, not by eye. With the perimeter joists nailed together, the two corner-to-corner measurements are compared and one corner is pushed until they match. Only then is the frame fixed off and the intermediate joists dropped in. If the frame is out of square here, every wall panel, every door and every sheet of cladding above it will be fighting that error.
Intermediate joists at regular centres keep the deck from flexing underfoot and give the sheet edges something to land on. Centres are chosen so that the deck sheets break on a joist rather than in mid-air, which usually means setting out from one end and accepting a narrower bay at the far end rather than spreading the error across the whole floor.
Bearers are the sacrificial part of the assembly and should be the most heavily treated timber in the building. They sit on the slabs, not in the gravel, and a strip of damp proof membrane between slab and bearer stops moisture wicking up. Leaving a clear air gap under the joists, rather than filling it, is what lets the frame dry out after wet weather instead of holding it.
1800 mm bearer centres
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