How much do solar panels weigh?

A typical 400W solar panel weighs approximately 18-22 kg. With mounting hardware (rails, clamps, and roof hooks), the total additional weight is around 23-25 kg per panel. For a standard 10-panel system, the total additional weight on the roof is approximately 230-250 kg spread across 17-18 m².

Can all roofs support solar panels?

Most UK roofs built to modern building regulations can support solar panels. The additional load of around 12-15 kg/m² is well within typical rafter capacity. However, older roofs, roofs with undersized rafters, roofs with existing damage, or flat roofs may require a structural assessment before installation.

What is the difference between on-roof and in-roof systems?

On-roof (rail-mounted) systems sit above the existing roof covering on aluminium rails fixed to the rafters via roof hooks. In-roof (integrated) systems replace the existing roof tiles in the panel area and sit flush with the roof surface. In-roof systems are lighter but require more waterproofing detail.

Do I need a structural engineer for solar panel installation?

For standard domestic pitched roofs with regular-sized rafters (at least 100mm deep at 400-600mm centres), a structural engineer assessment is usually not required. However, if your rafters are undersized, widely spaced, the roof is old, or you plan a large array on a flat roof, a structural check is recommended.

What rafter size is typical in UK houses?

Most UK houses built since the 1960s use 47x100mm or 47x125mm C16 timber rafters at 400mm centres. Older properties may have irregular sizing. Loft conversions and trussed roofs have different structural considerations and should always be checked by a professional.

Solar Roof Loading Calculator — Structural Assessment

Check whether your roof structure can support the additional weight of solar panels, mounting rails, and hardware.

Roof Type

Pitched or flat roof

Roof Covering

Existing roof covering material

Rafter Width

Narrow dimension of rafter

Rafter Depth

Deep dimension of rafter

Rafter Spacing

Centre to centre distance

Number of Panels

Total panels to install

Panel Weight

Weight of each panel

Roof Span

Rafter span (eaves to ridge)

How We Calculate This

This calculator estimates the structural impact of adding solar panels to your roof. It considers the panel weight, mounting system weight, existing roof covering weight, and imposed loads (snow, wind, maintenance).

Load assessment

Dead load: Permanent weight including roof covering, panels, and mounting hardware
Imposed load: Variable loads from snow (BS EN 1991-1-3), wind (BS EN 1991-1-4), and maintenance access
Rafter capacity: Bending strength calculated using C16 timber properties and rafter dimensions
Utilisation: Applied moment as a percentage of rafter bending capacity

Important notes

This is an indicative assessment only. A full structural calculation should consider rafter condition, bearing points, purlin support, and any existing defects. For flat roofs with ballasted systems, the additional weight is significantly higher and always requires professional assessment.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.