Solar structures

Step 1: pitched roof, flat roof or free-standing

These three families behave differently under load, cost differently to install and put different demands on the building. Decide the family first, because it determines every component after it.

• Pitched roof: the cheapest structure per kWp, because the roof already provides the tilt and the load path. Modules sit close to the covering, so rear ventilation is limited and cell temperatures run higher. Suitable only where the rafters and battens are sound.
• Flat roof: the structure creates the tilt, typically 10 to 15 degrees for east and west layouts or 15 to 20 degrees facing south. Uplift is the governing load and is normally resisted by a ballasted load system that leaves the waterproofing intact.
• Free-standing: carports and ground frames carry all loads into their own foundations, so the building is untouched. Highest material and civil cost, but full freedom on tilt, orientation, row spacing and rear clearance for bifacial modules.

Step 2: the load path, clamp to rail to fixing to structure

Check every link, in order, against the component below it. A structure fails at the interface far more often than in the middle of a profile.

• Clamp to module: the clamp must match the frame height, commonly 30, 35 or 40 mm, and must sit inside the clamping zone the module manufacturer marks in the installation manual. Clamping outside that zone voids the module warranty regardless of how sound the rest of the structure is.
• Clamp to rail: the clamp bolt engages the rail channel, and the tightening torque comes from the system manual. Undertightened clamps let modules creep in thermal cycling, overtightened clamps deform the frame and crack cells.
• Rail to fixing: hooks, bridges and seam clamps each have a rated capacity in tension and compression. Uplift loads the hook screws in withdrawal, which is usually the limiting value, not the downward snow load.
• Fixing to building: a roof hook screw needs sound timber of adequate depth, a self-drilling screw needs the sheet thickness the datasheet assumes, and a ballast tray needs a roof that can actually take the added mass.

Step 3: wind zone, snow zone and the static calculation

Loads are derived from EN 1991-1-4 for wind and EN 1991-1-3 for snow, applied through the relevant national annex. Inputs are the wind and snow zone, terrain category, building height, roof pitch and the position of each module on the roof. Output is a maximum fixing spacing, a required fixing count or a ballast mass per position.

• Zones inside the roof: corners and edges see far higher suction than the interior field, sometimes double. Arrays are held back a defined distance from every roof edge for that reason, and edge modules get extra fixings or extra ballast.
• Snow drives bending, wind drives withdrawal: snow load bends the rail between supports, wind uplift pulls the fixings out of the roof. A structure can be fine for one and marginal for the other, so both cases must be run.
• The calculation proves the system, not the building: a configurator confirms the mounting kit. On older roofs, commercial halls and any penetrating flat roof solution, a structural engineer still has to confirm the building can take the load.
• Keep the output: the configurator report, with zone, terrain category and building height stated, belongs in the handover file. It is the evidence that spacing and ballast were designed rather than estimated.

Step 4: rail spans, cantilever and profile height

Rail choice is a trade between profile cost and fixing count. A taller profile spans further with fewer supports, which is almost always cheaper on a difficult roof where each fixing means lifting a tile, drilling and sealing.

• Span: the distance between two supports, set by profile section modulus and the snow load. On residential pitched roofs it commonly lands between 1.2 and 1.7 m, but the system table is the authority, not the habit of the last job.
• Cantilever: the rail overhang beyond the last support. As a working rule it should stay near a third of the adopted span, and it must never be left long simply because the rafter fell short. A long cantilever plus edge-zone suction is the classic bent-rail failure.
• Splices: rail connectors extend a run but have their own span rules, and a splice should not sit directly over a support unless the manufacturer allows it. Sliding splices double as expansion joints.
• Orientation: portrait and landscape module layouts load the rails differently and change how many rails a row needs. Fix the layout before ordering profile lengths.

Step 5: aluminium, stainless steel and galvanic separation

Structural profiles are typically EN AW-6005T6 or 6063 aluminium with A2 or A4 stainless steel fasteners. Aluminium keeps the dead load low, stainless carries the clamping force. That pairing is standard and the galvanic effect between them is small in normal atmospheres. Trouble almost always comes from a third material.

• A2 or A4: A2 (1.4301) is fine inland. Choose A4 (1.4401 or 316) within a few kilometres of the coast and in industrial, agricultural or swimming pool atmospheres where chlorides and ammonia attack A2.
• Copper and lead: the potential difference against aluminium is large. Runoff from a copper flashing or a lead soaker onto an aluminium rail is enough to start pitting, so separate with EPDM or divert the water.
• Fresh concrete and mortar: alkaline attack corrodes bare aluminium, so ballast blocks and foundation faces need a separating layer or a coated contact face.
• Tooling: do not cut aluminium with discs or blades used on steel. Embedded ferrous particles rust and stain the profile. Deburr and protect every cut end.
• Bonding: rails and module frames are bonded into the equipotential system, and separation distances to any external lightning protection must be kept.

Step 6: the mounting survey, in order

A structure quoted from a photograph is a structure that will be re-quoted on site. Run the survey in a fixed sequence so nothing is assumed.

• Identify the covering and its condition: tile type, sheet profile and thickness, seam type or membrane material, plus any brittleness or existing repairs.
• Measure the substructure: rafter or purlin spacing and section, batten condition, boarding and insulation depth, from inside the roof space where possible.
• Record the geometry: pitch, azimuth, ridge and eaves heights, usable field after edge setbacks, and every obstacle such as vents, dormers, aerials and lightning conductors.
• Establish the load inputs: wind zone, snow zone, terrain category and building height, then run the configurator with those values.
• Plan the routes: DC cable path, roof penetration point, earthing route, and access for lifting rails and modules onto the roof.
• Photograph and document: the survey record is what the static calculation and the bill of materials are built from, and what protects you if the roof later proves different.

Quick comparison of structure families

Swipe the table to the left