Rooftop solar and plant, and the structural check before it goes on

Rooftop solar and plant, and the structural check before it goes on

A solar array on a warehouse roof adds a modest amount of dead load. Most people stop the assessment there, decide the roof can obviously carry it, and install.

The load that governs is not the weight. It is wind acting upward on the panels, and on a large industrial roof that is a substantial force applied through a small number of fixings.

What actually has to be checked

Uplift on the array itself. Panels sitting proud of a roof surface generate suction in wind, and the worst of it happens at the edges and corners of the roof rather than in the middle. AS/NZS 1170.2 sets those pressures, and they are significantly higher in the edge zones than the average across the roof.

The fixing into the purlin or batten. Pull-out capacity of a screw into a thin steel purlin is a real limit, and it is usually the weakest link in the chain.

The purlin itself. A purlin designed for sheeting and its own wind load is now carrying point loads from the array frame, applied between its supports rather than uniformly.

The rafter, portal frame or truss below. Adding dead load to a truss designed with little spare capacity can matter, particularly on older sheds.

The connection back to the ground. On a building where the whole roof structure is already near its uplift capacity, additional suction area has to be traced all the way to the footings and the hold down bolts.

Existing buildings are where the risk sits

New buildings are designed knowing what is going on the roof. Existing ones were not.

An older industrial roof in Sydney may have purlins at a spacing chosen for the original sheeting and nothing else, corroded fixings, or a structure that has already been modified once for a mezzanine or an extraction unit. None of that is visible from a quote.

The assessment starts with finding the original documentation where it exists, then verifying it on site, because what was drawn and what was built diverge more often than not.

Mechanical plant is a different problem again

Air conditioning plant, extraction units and cooling towers add concentrated weight, and they add vibration.

Concentrated weight has to land on structure rather than on sheeting, which usually means a plant deck or a steel frame spanning between primary members. Vibration has to be isolated, or it travels into the structure and into the tenancy below as noise.

Access and maintenance load matters too. A plant deck is a working platform, and it is designed for people and equipment on it, not just for the unit sitting still.

Who asks for the certificate

Certifiers, for anything that needs approval. Insurers, increasingly, on commercial buildings. Network operators for larger connections. Landlords, when a tenant wants to install on a leased roof and the lease requires structural sign off.

The certificate has to say what was assessed, against which standards, and what conditions apply. A generic letter does not survive scrutiny when something comes loose.

The sensible sequence

Have the roof assessed before the array is sized, not after the panels are on site.

The assessment often comes back with a workable answer that costs nothing extra, a change to fixing type, a reduction in panel count in the corner zones, or additional purlin support in a defined area. Those are cheap when they are known in advance and expensive as a variation.

We handle this under structural engineering for commercial and industrial buildings across Sydney, and it usually takes less time than the approval process that follows it.