Snow and wind loads on solar panels, from map to c/c spacing.

Every load calculation starts with the site’s characteristic ground snow load and reference wind speed, read from national maps. The rest of the chain converts them into a load on this particular roof: the roof pitch, the building height, the terrain around the building and where on the roof the panel sits.

Snow presses the panel onto the roof and pulls it down the roof slope. Wind sucks the panel away from the roof, most of all at the corners and along the edges.

The loads are combined at the ultimate limit state. In Sweden, BFS 2024:6 chapter 3, § 6, table 3:1 applies: unfavourable self-weight γd·1.2·Gk, the leading variable load γd·1.5·Qk and other variable loads γd·1.5·ψ0·Qk. Load combination 2 is γd·1.35·Gk without variable loads. The partial factor γd is 0.83, 0.91 or 1.00 for safety classes 1, 2 and 3 (chapter 3, § 2).

Uplift is treated as a check of static equilibrium (chapter 2, § 3 and chapter 3, § 9, table 3:2), where the panel’s self-weight is favourable: the wind suction γd·1.5·w minus 0.9 times the self-weight perpendicular to the roof. The load per metre of rail is the load per m² times the rail’s tributary width. With two rails per panel, the tributary width is half the panel dimension across the rails. The largest c/c spacing is the fixing’s capacity divided by the load per metre, in each direction separately: onto the roof, away from the roof and along the roof.

In step 5 you enter the ground snow load (kN/m²) and reference wind speed (m/s) from Boverket’s maps. The site’s coordinates are shown next to the fields, and the regulations open via a link. The terrain is chosen as 0–IV, with III as the default. Under More load options you set the safety class (default 2), snow guards on the roof and the fixing’s capacity from the manufacturer’s report.

Ritwerk takes the ridge height and the building’s dimensions from the 3D model, applies the edge zone e/10 along every edge of the roof face and marks each panel as a corner, edge or centre panel. Once the fixing’s capacity is known, from the manufacturer’s datasheet or entered by you, the largest c/c spacing per zone is calculated against pressure, uplift, force along the roof and the rail’s span table, and the smallest value governs. The result is summed up on one line with the inputs, the number of fixings per panel and the widest c/c used on any rail. The bill of materials gives the c/c per rail: for the house in the worked example, at most 1.22 m in the corner and edge zones and 1.7 m in the middle of the roof, where the rail’s span table limits it. The calculation also gets its own page in the report.

The calculation is preliminary and follows the Swedish rules: BFS 2024:6 with Eurocode 1 (SS-EN 1991-1-3 and SS-EN 1991-1-4). Other countries’ National Annexes are not applied. Ballast for racking on flat roofs is calculated in the manufacturer’s tool, for example NM Planner or SOLTOP’s Systemstatik.

Sweden. BFS 2024:6 has applied since 1 July 2025 and replaced EKS (BFS 2011:10). The older rules could still be used if the application for building permission or the notification was submitted before 1 July 2026 (BFS 2024:14, transitional provisions, point 3). The snow map, figure 4:2, has zones of 1.0–5.5 kN/m² and is based on annual maxima for 1961–2020. Boverket’s open Klimatlaster API gives values per coordinate (version v2 = BFS 2024:6) as guidance, and the printed regulations apply.

Germany. Snow to DIN EN 1991-1-3:2010-12 with A1:2015-12 and the National Annex NA:2019-04, wind to DIN EN 1991-1-4:2010-12 with NA:2010-12. They become binding through the MVV TB (edition 2025/1) and the building law of the federal states. Snow load zones 1, 1a, 2, 2a and 3 and wind zones 1–4 are taken per municipality from DIBt’s tables, and the ground snow load is calculated from the zone according to the National Annex. In municipalities of the North German Plain that are marked in the table, buildings in snow zones 1 and 2 must also be checked for exceptional snow as an accidental action.

Spain. CTE DB SE-AE (April 2009 version) applies to buildings that require a building permit. Wind: qe = qb·ce·cp with qb = 0.5·δ·vb², δ = 1.25 kg/m³. Zones A, B and C have vb 26, 27 and 29 m/s and qb 0.42, 0.45 and 0.52 kN/m², and 0.5 kN/m² may be used as a simplification. Flat roofs have zones F, G, H and I (table D.4), and the tables of EN 1991-1-4 may be used (D.3.6). Snow: qn = μ·sk with μ = 1 up to a 30° pitch – higher than the Eurocode’s 0.8. Madrid has sk 0.6 kN/m² (table 3.8).

Italy. NTC 2018 (DM 17 January 2018) and Circolare n. 7 of 21 January 2019. Snow: qs = qsk·μi·CE·Ct with zones I-Alpina, I-Mediterranea, II and III. Up to 200 m above sea level, qsk is 1.50, 1.50, 1.00 and 0.60 kN/m², and above 200 m the value increases with altitude. μ1 = 0.8 up to 30°. Wind: vb = vb,0·ca in nine zones and p = qr·ce·cp·cd. Rome is in snow zone III and wind zone 3 (Lazio), with vb,0 = 27 m/s.