Solar Roof Loading Calculator — Structural Assessment

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 the relevant imposed (variable) loads.

Load assessment

• Dead load: Permanent weight including roof covering, panels, and mounting hardware (converted from kg/m² to kN/m² using g = 9.81)
• Imposed load: The roof maintenance/repair load and the snow load are mutually exclusive variable actions. BS EN 1991-1-1 clause 3.3.2(1) states the roof imposed load is not combined with snow or wind, so the calculator takes the worse of snow or maintenance, never their sum. The UK National Annex (Table NA.7, category H, slope <30°) gives a roof imposed load of 0.6 kN/m²; SCI Advisory Desk note AD 541 reduces this to 0.4 kN/m² where a PV array is already present.
• Wind uplift: Assessed as a separate case (BS EN 1991-1-4): uplift acts upward and is partially offset by the dead load, so it is never added to the gravity case.
• Bending check: Applied mid-span moment (M = wL/8) compared with the rafter bending capacity. A C16 permissible bending stress of 5.3 N/mm² (legacy BS 5268-2, used as a conservative indicative value) is applied to the rafter section modulus Z = b·d²/6.
• Deflection check: Instantaneous deflection δ = 5wL⁴/(384·E·I) against a span/250 limit, using C16 E0,mean = 8000 N/mm² (BS EN 338:2016). For slender rafters deflection usually governs rather than bending.
• Utilisation: The overall Pass/Fail verdict is driven by whichever of the bending or deflection checks is worse.

Important notes

This is an indicative assessment only and uses a simplified single-span model. A full BS EN 1995-1-1 (Eurocode 5) calculation should apply the appropriate modification factors (kmod, kh, ksys), partial factors (γM) and load combinations, and 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.