Pitched roofs

Step 1: identify the covering and its fixing

Get this wrong and nothing downstream works. Look at the profile, the material and the way the covering is held down, not at what the customer calls it.

• Clay or concrete tile: interlocking pantile, plain tile or Roman profile on battens. Fixed with an adjustable roof hook that passes between two courses of tile and screws into the rafter. Height and depth adjustment lets the hook clear the tile without loading it.
• Asphalt shingle or bitumen board: a continuous boarded deck under a flexible covering. Fixed with a hanger bolt through the deck into the rafter, or with a flashed mounting plate slid under a course of shingles. See shingle roofs.
• Trapezoidal sheet: profiled steel or aluminium sheet on purlins. Fixed with a short rail, bridge or L-foot bolted through the crown of the profile, never the valley, using a self-drilling fastener with a bonded EPDM sealing washer.
• Standing seam and folded metal: zinc, copper, aluminium or coated steel with an upstand seam. Fixed with a non-penetrating seam clamp that grips the fold and is tightened to a set torque. No hole in the roof at all.

Step 2: adjustable roof hooks or hanger bolts

These are the two penetrating fixings for timber roofs and they are not interchangeable.

• Adjustable roof hook: a stainless steel hook with a base plate that sits on the rafter or on a batten reinforcement and an arm that rises over the tile. Adjustment in two or three axes absorbs uneven tile courses and roof deformation. This is the correct fixing for tile because the load bypasses the covering entirely.
• Hanger bolt: a double-threaded stud, wood thread one end and metric thread the other, driven into the rafter and sealed against the deck with an EPDM washer or a flashing. Fast and low profile, but it depends entirely on the seal and on the thread engagement in solid timber.
• Thread engagement: the wood thread needs full seating in the rafter. A hanger bolt that has only entered boarding has almost no pull-out resistance, and uplift on a photovoltaic array is a tension load, not a shear load.
• Corrosion pairing: A2 or A4 stainless fixings with aluminium rails. Avoid direct contact between aluminium and copper or bare steel, and use an isolating pad where an aluminium foot lands on a metal roof of a different alloy.

Step 3: rafter spacing, hook spacing and rail span

Hook spacing is not a habit, it is an output of the static calculation for the site's wind and snow zone, the roof height, the pitch and the module weight. In practice it is quantised by the rafters: you can only fix where timber is.

• Find the rafters first: probe from the loft, measure a known rafter and step along the roof, or lift a tile and look. Do not read spacing off an old drawing.
• Typical spacing in European timber roofs runs from about 0.6 to 1.0 m. Wider spacing means fewer hooks per row, which means a bigger rail span and a heavier rail profile, not a cheaper job.
• Edge and corner zones of the roof see much higher local uplift than the middle field. The calculation usually forces closer hook spacing in the first bay along the eaves, the verge and the ridge.
• Snow load pushes down the slope, so on steep roofs and in high snow zones the calculation may add a hook or a snow stop rather than a thicker rail.

Step 4: tile lifting, dressing and watertightness

Where a hook passes out of the roof, the tile above it has to accommodate the arm without being loaded by it. That is the point at which most tile roofs start to leak.

• Lift, do not lever: unhook the tile above the fixing point, work the hook in, then relay the tile. Cold clay and concrete tiles crack easily, so on frosty days warm working is not a luxury.
• Dress the tile, not the hook: if the tile rocks on the hook arm, grind a shallow relief into the underside of the tile so it sits flat again. Bending the hook to clear the tile removes the adjustment you paid for and puts a permanent bending moment into the fixing.
• Keep a clearance gap: the hook must not bear on the tile below it. Contact transfers array load into a single tile and breaks it, usually two winters later.
• Restore the water path: underlay and any sarking membrane that was cut must be resealed. On boarded roofs, every hanger bolt gets an EPDM washer or a flashing collar dressed into the covering.
• Metal roofs: on trapezoidal sheet, fasteners go through the crown with a bonded washer, torqued so the washer compresses without dishing. On standing seam, use a seam clamp and keep the roof unpierced.

Step 5: rail direction, clamping and thermal expansion

On tile roofs the hooks run up the slope with the rafters, so the rails normally run horizontally across them. On trapezoidal sheet the fixings follow the crowns, so short rails or bridges run across the profile and the modules can often be mounted with very few components.

• Rail direction: horizontal rails on hooks, cross rails on sheet crowns, and a second rail layer only where module orientation demands it. Every extra layer adds height, weight and cost.
• Clamping zone: module clamps must sit inside the zone the module manufacturer permits, given as a distance from the frame corner. Outside it the frame and the glass take bending they were not tested for and the module warranty is void.
• Mid and end clamps are matched to the module frame height. Mixing frame heights in one row means mixing clamp heights, and that is a common cause of loose modules.
• Thermal expansion: aluminium moves roughly 23 µm per metre per kelvin. A 10 m continuous rail run swinging 50 K between a winter night and a summer afternoon moves about 11 mm. Use the manufacturer's expansion splice at the stated interval instead of a rigid joint, or the rail will bow and drag on the hooks.
• Earthing and bonding: anodised rail is not a reliable conductor through its clamped joints. Where bonding is required, use the toothed washers or bonding jumpers the system supplies.

Step 6: lightning protection separation distance

If the building has an external lightning protection system, the array is a large conductive object sitting inside its protected volume. The default approach is to keep the whole mounting structure isolated, at a separation distance calculated to EN 62305-3 from every air termination and down conductor. The separation distance depends on the class of protection, the number of parallel down conductors and the length of the conductor from the point of separation, so it is a calculated figure and not a fixed number. If that distance cannot be achieved, the alternative is to bond the structure into the lightning protection system deliberately, which then changes the surge protection requirements on the DC side. Neither choice is optional and neither should be made on site.

Fixings by roof covering

Swipe the table to the left