Glossary · Snow and wind
Ballast
Dead weight (concrete blocks, gravel trays) that keeps a PV table from wind uplift and sliding without anchoring into the ground or roof.
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Definition
A ballasted system is chosen when the roof cannot be penetrated (a flat roof with a membrane) or when piles are not wanted on a plot. Then only weight works against the wind: the structure and panels plus concrete blocks under the supports. Sizing ballast means the two checks described under uplift and sliding plus one more condition: the roof must carry the extra weight.
What sets the mass
- the city’s W₀ and the height of the array above ground or roof;
- the terrain type;
- the tilt, which sets the uplift coefficient cf;
- the position on the roof: edges and corners carry more than the middle;
- the surface under the supports, which sets friction.
Ballast weight works for stability, so the calculation reduces it by 0.90 — the bracketed value in Table 5.1 of DBN V.1.2-2:2006 for overturning checks.
Example: Kyiv, 30° ground table
For a 2 m height and terrain type II the uplift pressure is 466 Pa, and each square metre of panels needs ≈ 34 kg of ballast on top of the modules’ own weight. For a table of 10 panels of 2.6 m² each, that is around 880 kg. Those kilograms must also be split unevenly: the rear support takes more. On roofs a second limit appears — the load capacity of the roof.
How Solispec applies it
In the ballast calculator and the Solispec constructor the engine computes the mass for uplift and sliding from Wm and the panel cf, with the 0.9 factor on weight, and checks the ballast under the most loaded support line (resultant d/4 from the windward edge per EN 1991-1-4:2005). On roofs it accounts for the zone (middle, edge, corner) and checks the roof capacity. The result is kilograms per truss and per support line.
Norms and clauses
- DBN V.1.2-2:2006, Table 5.1 (values in brackets), cl. 9.4, 9.8
- EN 1991-1-4:2005, 7.3(6), Figure 7.16, Table 7.6
Related terms
- Wind uplift and slidingThe two checks of a ballasted table: wind must neither lift the structure nor slide it across the ground or roof.
- Aerodynamic coefficient Caer (cf)The multiplier turning wind pressure into force on a shape; for panels Solispec takes c_f from Table 7.6 of EN 1991-1-4:2005, as cl. 9.8 of the DBN allows.
- Characteristic wind pressure W₀The mean wind pressure at 10 m height exceeded on average once in 50 years; Kyiv has 370 Pa under DBN V.1.2-2:2006.
- Terrain type I–IVThe wind classification of a structure’s surroundings under cl. 9.9 of DBN V.1.2-2:2006: from open water (I) to dense urban areas (IV).
- Panel tilt angleThe angle between the panel plane and the horizontal; it affects yield, snow (μ), wind (c_f) and row pitch. Kyiv’s optimum is about 36°.
- DBN V.1.2-2:2006 “Loads and actions”: what matters for a solar plantNorms explained
Solispec’s calculation is preliminary: for a design it has to be checked by an engineer. The values and clauses follow the current editions of the norms on the date of the page.