Solar panel weight and roof loading on farm buildings

Before anyone asks how much a farm roof can earn, the roof has to be asked how much it can carry. A modern module is not heavy on its own, but an array is a large permanent load laid across purlins that may be forty years old, and on many farm sheds nothing in the Building Regulations will make anyone check. This guide gives the module weights from manufacturer datasheets, the rule of thumb that building control uses, the farm-building exemption that removes that safety net, and the questions to put to an installer. It sits alongside the main guide to solar panels for agricultural buildings.

How much does a solar panel weigh?

Two current datasheets show where utility-scale and commercial-roof modules sit today:

Module (datasheet)	Size	Weight	Area	Module mass per m2
Trina Solar Vertex S+ TSM-NEG9R.28, 425 to 450 W (datasheet version TSM_EN_2023_D)	1762 x 1134 x 30 mm	21.0 kg	1.998 m2	10.5 kg
LONGi Hi-MO 6 Explorer LR5-72HTH, 565 to 585 W	2278 x 1134 x 30 mm	27.2 kg	2.583 m2	10.5 kg

The last column is arithmetic on the datasheet figures (weight divided by length times width), not a figure the manufacturers publish. The useful point is that a larger module is heavier but not much heavier per square metre. As an illustration only, 100 kWp built from 450 W modules is 223 modules (100.4 kWp): about 4.7 tonnes of glass, cells and frames spread over roughly 446 m2 of roof. To turn a roof area into a system size, the farm solar calculator gives the starting point.

What the module figure leaves out

The datasheet weight is the module alone. An array also carries rails, brackets, clamps and cable, and on a flat roof possibly ballast. Designing Buildings, which summarises the standards for rooftop PV, notes that ballasted flat-roof systems can add substantial permanent dead load. The mass of the racking should come from the racking manufacturer’s datasheet for the exact layout, and the sum goes into the engineer’s calculation, not a quote.

The load that matters is also not only dead load. The same summary lists the Eurocode parts engineers work from: BS EN 1991-1-1 for densities and imposed loads, BS EN 1991-1-3 for snow, and BS EN 1991-1-4 for wind, plus BRE Digest 489 on wind loads on roof-mounted PV. A panel is a surface that catches wind and holds snow, so the question to the engineer is the total, not the weight on the scales.

The 15% rule in Approved Document A

For England, Approved Document A (Structure) deals with changes to roof loading in its section on re-covering of roofs. Paragraph 4.3 says that where work significantly changes the applied loading, the roof structure and its supporting structure should be checked so the building is not less compliant than the original. Paragraph 4.4 defines a significant change as loading increased by more than 15%. Paragraph 4.5 says that if the check shows the structure cannot sustain the added load, for example overstressed members or unacceptable deflection, strengthening or replacement of members is classified as a material alteration. Paragraph 4.6 adds that increased stress in a member does not by itself mean non-compliance, provided an adequate factor of safety is maintained.

That is guidance written for re-covering, and building control bodies and PV guidance commonly borrow the 15% figure as the benchmark for adding panels. The document does not say what the 15% is measured against; the engineer sets the baseline from how the roof was designed. Designing Buildings adds a caution worth repeating: an assessment remains appropriate even where the increase is under 15%, because the threshold marks where a requirement applies, not where the risk starts. Scotland and Northern Ireland have their own provisions.

Many farm buildings are exempt, and the check does not go away

Schedule 2 of the Building Regulations 2010 (England and Wales) lists exempt buildings. Class 3 covers agricultural buildings, including buildings used principally for keeping animals, provided that:

• no part of the building is used as a dwelling;
• no point of it is less than one and a half times its height from any point of a building containing sleeping accommodation; and
• it has a fire exit no more than 30 metres from any point inside.

The exemption does not apply where the principal purpose is retailing, packing or exhibiting. A typical stock shed, grain store or machinery shed can fall inside Class 3. If it does, no building control body is looking at the roof loading, and the 15% test in Approved Document A is not what stops a bad installation. The responsibility sits with the owner who orders the work and the installer who fixes to it.

NFRC’s guidance sheet HSGS44, written for cement-sheet roofs, is blunt about who should do the check: engage a qualified structural engineer to assess the structure’s capability to support additional dead and live loads, and this applies whether the project strips the sheets or over-clads them. It names solar panels among the loads the original structure may not have been designed for, and notes that if the existing purlins, trusses or supports are degraded or overstressed, removal of the roof may be the only viable option.

Where the load actually goes on a farm roof

• Cement and fibre-cement sheets. HSE’s asbestos task sheet states that asbestos cement roofs are always fragile and cannot bear weight. The sheet is not the structure: brackets must pass loads to the purlins, and NFRC adds that on concrete purlins nobody should drill to fit a secondary member, because that can damage the reinforcement. The wider roof decision is on solar panels on asbestos cement farm roofs, and the choice between laying a new skin over the old and stripping is in overclad or strip an asbestos cement farm roof.
• Steel portal frames. Designing Buildings says purlins, cladding rails and fixings often govern whether an installation can proceed without strengthening. Corroded or overstressed members are where an old shed fails the check.
• Roofs being replaced anyway. When a roof is near the end of its life, the loading check can be done once for the new roof and the array together; see combined re-roof plus solar and the notes on farm roof types.

Questions to put to the installer

1. Who did the structural assessment, and where is it? A named engineer and a written assessment of the existing frame, purlins and fixings, covering dead, snow and wind loads.
2. What mass per square metre did they use? Both the module figure from the datasheet and the racking figure from the racking datasheet, for this layout.
3. Is the building exempt? If it is within Schedule 2 Class 3, say so in the paperwork, and say who is accountable for the structural check instead.
4. Which members carry the fixings? Purlins, rails or rafters, not sheeting. See the fixing notes in solar racking systems for farm roofs.
5. What was found on the roof surface? An asbestos survey before any fixing, and the fragile-roof precautions for the people doing the work.

For how the same buildings are treated for planning, see planning permission for solar on farm buildings; for the full picture of solar on sheds, stores and workshops, start at solar panels for farm buildings.