Estimate snow and wind loads

Per EN 1991-1-3 the load is s = μ₁ · C_e · C_t · s_k. The crucial point: once modules sit on the roof, the shape coefficient stays at μ₁ = 0.8 regardless of roof pitch, because the panels stop the snow from sliding off. Generic snow-load calculators apply the pitch-dependent reduction and thereby understate the load by up to 20 % — in the reference case at 35 degrees, 0.733 instead of the correct 0.880 kN/m².

Wind uplift is w = q_p · (c_pe − c_pi) per EN 1991-1-4, and the force per module is F = |w| · A. The required ballast follows from the partial safety factors as m = (1.5 · F_uplift − 0.9 · m_module · g) / g. Peak velocity pressure and the pressure coefficients are inputs taken from the national annex — there is deliberately no postcode lookup, because that annex data is not digitised here.

No — it is an order-of-magnitude check, NOT a structural verification. Drift surcharges, asymmetric load cases, the exceptional load case of the North German lowlands, edge and corner zones with substantially higher pressure coefficients, and the capacity of the existing rafters are not modelled. The stability of the roof structure, fixings and substructure must be verified by a structural engineer.

The tilt-dependent drop of the shape coefficient μ₁ above 30° relies on snow being able to slide off a smooth roof. Solar modules, snow guards and parapets prevent exactly that — the standard then sets μ₁ = 0.8 regardless of tilt. In the reference case (s_k = 1.10 kN/m², 35° tilt) the generic path yields 0.733 kN/m²; correct is 0.880 kN/m²: 20% more load, 1,440 N per module, 360 N per fixing point. That difference is the gap between “holds” and “does not hold”.

The formula and the shape-coefficient curve are stated as calculation rules in the standard and are implemented here. The load-zone map with zone s_k values and altitude formulas, however, is content of the copyrighted national annex — it is deliberately neither digitised nor offered as a postcode lookup here. The value for your site comes from the official source or the building authority; typical values range from 0.65 kN/m² to well above 3 kN/m² in alpine regions.

Both come from the national annex of the wind-load standard and depend on wind zone, terrain category and building height. The prefill (650 N/m², c_pe = −1.4) is an assumption for inland terrain and the flat-roof interior zone — edge and corner zones reach markedly higher suction coefficients, partly beyond −2.5. That is why both values are marked as visible assumptions with ranges, and why this rough check does not replace the mounting manufacturer’s ballast planning.

Self-weight is rarely the problem: a mechanically fixed standard module weighs 25 kg over 2.0 m², about 12.5 kg/m² — 600 kg of extra roof load for 24 modules. Snow is what makes it critical: in the reference case (s_k = 1.10 kN/m², 35° roof pitch) another 0.880 kN/m² arrives, roughly 90 kg/m², seven times the weight of the array itself. What decides the question is therefore not the array but the spare load capacity of your building, which is stated in its structural documentation. This calculator gives the order of magnitude; the verification is a structural engineer's job.

The load path — and therefore the answer to the same wind uplift. In the reference case the wind pulls with 1,818 N per module. Mechanically fixed, that force runs through clamp, rail and roof hook into the rafter: 455 N per fixing point that the hook has to take. Ballasted, mass alone holds it: that would take 255 kg per module, over six tonnes of extra roof load for 24 modules instead of 600 kg. That is exactly why pitched roofs are screwed down and why ballast is regularly the most expensive single item on a flat-roof installation.

Usually yes — the module is rarely the weakest link. Manufacturers state a test load to IEC 61215; common classes are 2,400 Pa as the baseline and 5,400 Pa for snowy regions. The reference case here is 880 Pa, comfortably below. In practice the limit arrives earlier: at the pull-out capacity of the roof hook, at the rafter section, or at the spare capacity of the building. So compare the force per fixing point against the figure from your mounting system's manufacturer, not against the module test load.

The 2025 edition of the snow load standard introduces dedicated provisions for tilted PV rows on flat roofs for the first time: an unbalanced load arrangement with drift accumulation behind every row, because wind scours snow in front of the rows and piles it up behind them. The governing load can therefore be considerably higher than the uniformly distributed value this rough check computes. We deliberately do NOT reproduce the new arrangement — the secondary sources available to us contradict each other on the details, and for a safety-relevant calculation a named gap beats a guessed formula. For flat-roof arrays this is one more reason for verification by a structural engineer.

The bare module at 25 kg over 1.134 × 1.762 m gives 12.5 kg/m². With substructure, roof-parallel systems typically land at 15 to 25 kg/m². Tilted, ballasted flat-roof systems leave that range far behind: in this calculator's reference case another 255 kg of ballast per module arrives — over 140 kg/m² of ballast alone. That is why the pressure coefficient is the single most important input here: with aerodynamic deflectors (c_pe −0.7 instead of −1.4) the ballast halves to 117 kg per module.