Pergola plans: the structural review process

Every pergola brief we take on gets a four-point structural review before we put a single line on the quote. The point isn't to slow things down. The point is that the cost of catching a problem at this stage is a fifteen-minute conversation; the cost of catching it at install is rebuilding.

Here are the four.

First, wind loading. Sydney's wind region is A2 for most of the metro area, A4 if you're near the coast. The wind load on a 4m × 5m pergola roof can be surprising once you actually calculate it. We work out the worst-case uplift first because if the connections can't hold the roof down in a winter southerly, nothing else matters.

Second, footings. Standard practice is 600mm × 300mm concrete piers, but standard practice is for standard soil. We get a soil read from the site visit (sometimes a hand auger if it's ambiguous) and adjust. Sandy soils need deeper. Reactive clays need wider with a polythene membrane to break the bond. We'd rather over-spec by 100mm than under-spec by 50.

Third, beam sizing and deflection. The span tables in AS 1684 will tell you what'll hold the load. What they won't tell you is how much it'll move under that load. A beam that satisfies the strength check but deflects 15mm at midspan reads as "sagging" in the finished work even though it's structurally fine. We size for L/300 deflection, not just L/200.

Fourth, connections. Post-to-beam, beam-to-rafter, rafter-to-batten. Each junction is its own little structural problem. For the visible junctions we use concealed steel where the loads need it and visible bolts (M12, hot-dip galvanised) where the structure is honest about itself. The connection design dictates the joinery, not the other way around.

The output of all four is a one-page sketch on graph paper that goes with the quote. It's not engineering certification (that's a separate document if the job needs DA approval), but it means by the time we're talking dollars, we've already had the argument with ourselves about whether the design works.