Roof hook and rail mounting

How an above-roof PV mounting system transfers load into a pitched roof without damaging its covering or weather performance.

An above-roof solar array normally uses brackets or hooks fixed through the roof covering to the structure below. Rails connect those supports, and clamps hold the modules to the rails.

The tiles or slates are weather covering, not a convenient structural support. A sound mounting system carries wind and downward loads into rafters, purlins or another verified structural element while leaving the roof able to shed water.

Match the kit to the roof

MIS 3002 requires products used above or integrated into pitched roofs to be tested and certified to MCS 012. All mounting components must be approved to work together unless a manufacturer declares universal compatibility.

The certificate and installation manual define the permitted use. The design should match the kit to:

the tile, slate, sheet or standing-seam profile
roof pitch and build-up
rafter, purlin or deck construction
module dimensions, frame and clamping zones
rail span and support arrangement
site wind and snow loads
material and corrosion environment

A rail that physically accepts another maker’s clamp is not proof that the combined system has been tested or can carry the calculated load.

Survey before fixing

The surveyor needs to see enough of the roof to identify its construction and condition. Useful evidence includes loft photographs, rafter or truss spacing, member sizes, covering type, underlay condition, previous alterations and signs of movement, decay or leaks.

MIS 3002 requires a suitably competent person to check that a pitched roof can withstand the PV loads. If the structure is unusual or there is any doubt, a qualified structural engineer is consulted. The fixing layout must then follow both that assessment and the mounting-system calculation.

Hook spacing is not set by a universal “every rafter” rule. Wind zone, roof edges, module layout, rail orientation and structural capacity all affect the number and position of supports. Rails and clamps also have maximum spans and approved clamping zones.

Transfer loads without breaking the covering

Traditional roof hooks are fixed into a structural timber and pass between tiles or slates. The hook should not bear onto a brittle covering under design load unless it is a certified design specifically tested to distribute that load without damage.

MIS 3002 recognises an MCS 012-tested bracket that transfers an evenly distributed load to the roof covering without causing damage. That exception belongs to the particular certified product, not to improvised packing beneath a normal hook.

The fixing must have the specified embedment, edge distances and corrosion resistance. Battens should not be mistaken for structural rafters. Where a roof’s construction or access prevents the required fixing, the contractor should redesign the mounting rather than use a shorter screw or miss the structure.

Keep tiles and slates weather-tight

A hook needs clearance through the covering so wind or snow load does not press it into a tile. MIS 3002 says tiles or slates must be notched or flashed where needed to avoid displacing them and creating gaps greater than those already present.

Notching should be limited to the amount permitted by the covering and kit instructions. A cracked tile, a raised water path or an excessive cut is not repaired by covering it in sealant. MCS 012 does not permit site-applied silicone or mastic to be the sole weather seal for a fixing system.

Purpose-designed flashing is often used on slate, plain tile and other coverings where a hook alone cannot preserve the water path. Broken or unsuitable coverings should be replaced, not hidden beneath the array.

The finished work should preserve:

tile headlap, sidelap and interlock
the underlay and any ventilation route
gutters, valleys and drainage paths
access to chimneys, roof windows and outlets
clearance for cables and connectors away from sharp or wet surfaces

Roof edges and higher wind loads

Wind uplift is normally greater near eaves, ridges and verges. MIS 3002 says modules should not be within 400 mm of a domestic roof edge unless the design takes specific measures for increased uplift, ridge security, rainwater, snow and wind noise.

That 400 mm is not an absolute prohibition and it is not permission to ignore other constraints. An edge-zone layout needs the additional calculation, fixings and sometimes roof timbers that its design requires.

The installed array must match the calculation. Moving a hook to avoid a difficult tile, changing rail overhang or adding a last module can change structural spans and edge loads.

Metal and standing-seam roofs

Metal roofs use different load paths. A standing-seam clamp may grip a verified seam without penetrating the sheet. A bracket fixed through profiled cladding may rely on the sheet, purlin or both, depending on its tested design.

MIS 3002 requires the sheet thickness and its fixing to the main structure to be checked where brackets are screw-fixed to metal cladding. The design must show that uplift can pass through the bracket, sheet fasteners and supporting structure.

Clamp torque, permitted seam shapes, thermal movement and incompatible metals all need checking against the product instructions. “No penetration” does not mean “no structural calculation”.

Installation quality checks

Before modules conceal the work, the contractor should check and record:

fixing locations and engagement with the intended structure
screw type, length and installation condition
hook clearance and any permitted notching
flashings, replacement tiles and underlay repairs
rail joints, spans, overhangs and alignment
fastener and clamp torques
bonding or earthing arrangements from the electrical design
supported cable routes and matched connectors

The installer should not drill or cut a module frame unless the module and mounting manufacturers expressly allow it.

Evidence to expect

The handover pack should include the mounting certificate, approved component schedule, site wind and snow design, structural roof check, fixing layout and relevant photographs. It should also record any strengthening, roof repairs or deviations approved by the system designer.

A photo of the completed modules cannot prove what each hook is fixed to. Photographs before the panels go on provide the useful evidence.