Structural engineers need to advise on solar installations

Redland on-top solar PV panels

I was interested to read Peter Caplehorn’s article about the installation of PV panels (25 May, page 52). As he says, such installations need to be reviewed by a structural engineer.

I was recently asked to check an installation in which the frames for the PV panels were to be fixed by means of 10mm diameter coach bolts through the roof finishes into rafters below. Eurocode 5 requires a minimum edge distance to screws of four or five screw diameters, which means that the maximum diameter of a screw, fixed into a typical 50mm wide rafter, is 5mm. For thinner timbers in trussed rafters the screws should be correspondingly smaller.

Given the distance between the frame of the PV unit and the face of the timber such screws are liable to be subject to local bending. If holes are pre-drilled through slate or tiles it is very difficult, if not impossible to guarantee that the hole will hit the rafter centrally. The use of such fixings is therefore likely to damage the structure.

It is essential that building control officers should require the installers to obtain advice from a structural engineer on both the overall load capacity of the structure and the adequacy of the fixings.

Henry Dalton, chartered structural engineer, Tottenham & Bennett