Free guide to retrofitting solar panels

Property owners across England and Wales are increasingly investing in photovoltaic and solar thermal collector panels to lower their energy costs. These panels usually come in a rigid, lightweight aluminium shell that can be screwed or clamped securely to a roof.

But once the individual solar cells are included, the panels can weigh up to 50kg depending on their size and manufacturer. Adding the panels to an existing roof needs careful consideration and workmanship as some roof structures may not be able to accommodate the additional dead weight.

The ability of an existing roof to carry the additional load or weight of the panels will need to be checked by an expert and proven - and some strengthening work may be needed before you’re allowed to install the panels.

Key points to consider when retrofitting solar panels

• Expect an increase in dead load of around 15%. Assess all roof structures for their strength and robustness to ensure they can accommodate additional dead load.
• If installing a single row of solar thermal or PV panels on a modern trussed rafter roof of up to 9m span, you don’t require a structural investigation. However, any loadings beyond this scale require a survey by a structural engineer.
• Get a structural engineer to investigate any proposed loading increase in both historic and more recent cut roofs as these are likely to be over-spanned if panels are installed.
• Wind effects on the overall roof structure will not be changed significantly, but adequate fixing of panels to rafters is necessary to prevent panel uplift.
• Connection of clamp brackets to the rafters is critical; fixing to tiling battens is not a suitably robust solution. On trussed rafter roofs, individual truss fixings may need strengthening in exposed locations.
• If you’re a competent person scheme member, you may self-certify all aspects of installation.
• You may be required to submit a Building Regulation application for certain structural aspects of solar installations. Speak to your local authority building control surveyor for guidance.