A solar system stays outdoors for more than 20 years. Over that time the mounting structure that holds the modules on a roof or on the ground has to survive storms, heavy snowfall and earthquakes. A module torn off by the wind can drag others at the end of the row with it and damage the roof or injure people nearby. This article covers the loads that act on a mounting structure, the factors that make them larger, the role of the standards, the differences between rooftop and ground-mount work, and a pre-project checklist.
Which loads matter
- Dead (permanent) load: The weight of modules, rails, hooks, clamps and fixings. On a roof this is added to what the existing structure already carries; in ballasted systems the ballast is part of it too.
- Wind load: Pressure and suction (uplift) acting on the module surface. In most installations the governing effect is the suction trying to pull the module away, and fixings and anchors are chosen accordingly.
- Snow load: The weight of snow on the modules and the structure. On tilted modules snow can collect along the lower edge, and drifts can form next to parapets, taller adjacent walls or changes in roof level.
- Seismic action: Particularly for rooftop systems, the effect of the added mass on the building and the behaviour of the connections during an earthquake should be assessed.
These loads are not checked one at a time but in the combinations defined by the standards. Which combination governs depends on the site and the structure.
Factors that increase the loads
| Factor | Why it matters |
|---|---|
| Location | The regional wind speed and snow load values are the starting data for the project. Coastal and high-altitude sites need particular care. |
| Terrain and surroundings | Open flat land, a seafront or a hilltop sees more wind than a densely built-up area. |
| Building height | Wind speed increases with height above ground, so roofs of tall buildings see larger effects. |
| Roof type | Tiled roofs, trapezoidal sheet, sandwich panels and flat concrete roofs differ in fixing method and in how load reaches the structure. |
| Module tilt and height | Modules raised above the roof or tilted catch more wind; tilted systems on flat roofs need particular attention. |
| Edge and corner zones | Wind suction near roof edges and corners is markedly higher than in the middle of the roof. Extra fixings or a larger setback may be needed there. |
The role of the standards
In Turkey, load calculations follow the TS EN series, the Turkish adoption of the European standards. Actions (loads) are covered by the TS EN 1991 (Eurocode 1) family, with separate parts dealing with permanent loads, snow and wind. Steel members are designed to TS EN 1993 (Eurocode 3) and aluminium members to TS EN 1999 (Eurocode 9). Seismic action is assessed under the national earthquake regulation for buildings currently in force.
These standards define methods that take account of regional base values, terrain category, building height and surface shape. The choice of coefficients and regional values must be made by an authorised engineer, using the current edition of the standard and its national annex. We deliberately give no coefficients or zone values here; a wrong number is more dangerous than no number.
Rooftop considerations
- Capacity of the existing roof: However strong the mounting system, the load ends up in the rafters, purlins or concrete slab. On older buildings and light steel roofs this check must not be skipped.
- Fixing point: On tiled roofs the hook must be fixed to a rafter or purlin; on trapezoidal sheet the fixing must follow the manufacturer's method. A fixing that only goes into the roof covering may not safely carry uplift.
- Ballasted systems: For non-penetrating ballasted systems on flat roofs, the weight needed against uplift and sliding and the effect of that weight on the slab have to be assessed together.
- Module manufacturer's instructions: The module datasheet gives test loads for the front and rear faces, and these depend on the clamping zones. If clamps are placed outside the zones shown in the installation manual, the module's load rating may no longer apply.
- Setbacks: The distance kept from edges and corners matters for both wind and maintenance safety.
Ground-mount considerations
- Foundation type: Driven piles, screw piles or concrete footings are chosen according to the ground. Rocky ground, fill and soft soil cannot all take the same solution.
- Ground data: On larger projects a geotechnical survey and on-site pull-out tests mean the embedment depth is set from data, not from guesswork.
- Terrain slope: On sloping land, leg heights change from row to row, and buckling and bending checks become more critical for long legs.
- Row spacing: The spacing chosen for shading should also be checked for maintenance vehicle access and snow accumulation. See our article on row spacing in ground-mounted plants.
- Corrosion: Galvanising thickness, contact between dissimilar metals and salty coastal air determine how well the structure lasts over the long term.
Foundations and anchoring
The weakest link in a mounting structure is often not the profile but the connections and the foundation. When wind suction pulls a module upwards, the force passes from the clamp to the rail, from the rail to the hook or leg, and from there into the roof or foundation. Every link in that chain must carry the same load. On ground-mount projects, foundation size should be checked not only for vertical load but also for uplift and overturning.
Pre-project checklist
- Have the site location, altitude and surroundings (open land, coast, built-up area) been recorded?
- Have building height, roof type and the condition of the roof structure been documented during the site survey?
- Have module tilt, height above the roof and edge distances been decided?
- Have the module manufacturer's clamping zones and test loads been checked?
- Has fixing density in edge and corner zones been assessed separately?
- Have locations where snow may drift (parapets, level changes) been marked?
- For ground-mount projects, are the ground data and foundation type clear?
- Are materials and coatings (galvanised steel, aluminium, stainless fixings) suitable for the environment?
- Has the load calculation and structural project been prepared and approved by an authorised engineer?
Mounting calculations with PVAGE
The PVAGE structural calculation module produces rail, hook, screw and clamp quantities based on roof type, and calculates leg and profile dimensions and foundation sizing for ground-mount projects. It provides a pre-suitability check for buckling, bending, deflection and shear, with results in a PDF. This check does not replace an approved structural project; it is meant to steer the design early and produce quantities quickly.
See the structural calculation page for details, and the mounting system design page for ground-mount layouts.