FAQs – Structural Calculations for Solar Panels

DIY solar panels are available, but you still need to be sure your roof can take the additional weight plus a full wind load and snow – it is best to consult a structural engineer to know the capacity of your roof. Installation can be complicated so if you are not competent with electrics then it is best to have a professional install your solar panels.

The only way to determine how much extra weight you can add to your roof is to have a survey completed by a structural engineer. If you have as-built drawings for your property these will help but calculations will need to be made to ensure your roof can not only take the extra weight of the panels and fittings but also a full wind load with heavy snowfall.

The roof is the best place to put solar panels, as opposed to surface or pole mounted panels, as they receive the most uninterrupted sunlight throughout the day. Solar panels on your roof are also out of the way and will not take up any space in your garden or on your property. If your roof doesn’t face south, then there are some alternative solutions;  like adding more or larger panels on the west side. Consult a solar panel provider to find out more.

You will need listed building consent to add solar panels to a listed building. You will need to contact your local planning authority who will help you. Historic England takes into consideration ‘minimum intervention and reversibility’  – so in order to obtain approval the solar panels must not damage the appearance of the building, cause major structural changes and once removed look like they were never there. Some historical buildings may not have roof materials suitable for solar panel installation like thatch, slate, or lead sheet.

The Government’s planning portal says that solar panels and equipment on residential buildings may be ‘permitted development’ with no need to apply for planning permission provided the following criteria is met:

• Equipment on a building should be sited, so far as is practicable, to minimise the effect on the external appearance of the building and the amenity of the area.
• When no longer needed equipment should be removed as soon as reasonably practicable.
• Panels should not be installed above the highest part of the roof (excluding the chimney) and should project no more than 200mm from the roof slope or wall surface.
• The panels must not be installed on a building that is within the grounds of a listed building or on a site designated as a scheduled monument.
• If your property is in a conservation area, or in a World Heritage Site, panels must not be fitted to a wall which fronts a highway.

The number of solar panels you can have on your roof will be determined by the additional load your roof can take and the size and weight of the panels you want to install. A structural engineer is needed to complete these calculations so you can then work out the number you can put on your roof.

If you know the number, weight, position, and array of the solar panels you want to have on your home a structural engineer will be able to calculate if your existing roof has the load capacity.

There are typically two ways of fitting solar panels to roofs, depending on the angle of the roof. If it is a flat roof then a ballast system will be needed, where the panels are on frames that can be elevated, and the frames are held securely in place with ballast. On a pitched roof, a direct fixed system can be used. Panels are positively fixed into the underlying roof structure. Structural calculations will be needed for both types of fittings to ensure the roof can support the additional loads.

The only way to determine if your roof is strong enough for solar panels is to have a structural engineer calculate the loads your roof can take. This will determine the number of solar panels you can have, or what you need to do to strengthen your roof to improve its capacity.