In-roof solar mounting

An in-roof or roof-integrated solar system replaces part of the normal tile or slate covering. The modules sit within a tray, flashing or interlocking system that drains water back onto the surrounding roof.

This is different from an on-roof array, where hooks and rails hold modules above an otherwise complete roof covering. With an integrated system, the mounting kit forms part of the weather and fire-performing roof assembly. Product compatibility and workmanship therefore matter as much as the module itself.

Use a complete, certified kit

For MCS work on a pitched roof, MIS 3002 requires mounting products tested and certified to MCS 012. The components must be specifically approved to work together unless the manufacturer declares them universally compatible. If certification names particular PV modules, only those modules may be used.

The designer should check the exact certificate and installation manual, not simply look for an MCS logo. The approved combination can depend on:

• module make, family and dimensions
• roof pitch and covering
• portrait or landscape orientation
• flashing and tray components
• batten and support arrangement
• wind and snow design limits
• the fire classification and its field of application

Swapping to a similar-sized module or mixing flashing from another range can take the installed assembly outside the evidence that supports it.

MIS 3002 provides an evidence-based route where no suitable MCS 012 system exists, but it is not a casual exemption. The contractor must demonstrate structural safety, wind resistance, fire performance, weather-tightness, component compatibility and installation to the manufacturers’ instructions. The evidence must relate directly to the installed design.

Weather-tightness is a system function

The visible module face sheds most rain, while the kit controls water around and beneath the array. Its side channels, head and sill flashings, overlaps and drainage paths have to work with the existing tiles, slates, battens and underlay.

The survey should establish the roof build-up and condition before tiles are removed. During installation, the contractor should follow the specified pitch limits, lap dimensions, batten positions, support details and flashing sequence. Common shortcuts create predictable faults:

• cutting or omitting a tray or flashing to make the layout fit
• flattening a water channel with a cable or connector
• leaving the lower edge unable to discharge onto the roof covering
• relying on surface sealant instead of the designed laps
• covering torn underlay or decayed battens
• using incompatible metals or membranes

Photographs taken at each layer are valuable because the drainage path cannot be inspected once the modules are fitted.

An existing roof warranty should be checked with its provider. The finished handover should state who covers the integrated kit, the modules, the surrounding roof and the workmanship, rather than leaving a gap between separate warranties.

Structure, wind and snow

Removing roof covering changes the dead load, but it does not remove the need for a structural check. MIS 3002 requires the pitched-roof structure to be checked by a suitably competent person. A qualified structural engineer is required where the structure is unusual or there is doubt, including signs of distress, later alterations, complex hips or valleys, a particularly shallow pitch or a layout prone to snow build-up.

The mounting design must transfer wind uplift and downward loads into the roof structure. Support spacing and fixing locations come from the kit design and the site wind and snow calculation. They should not be improvised from the dimensions of the old tiles.

An integrated array also changes drainage and snow shedding. Valleys, gutters, roof windows and lower roofs need checking for concentrated water or sliding snow.

Fire performance

MIS 3002 requires the contractor to demonstrate that installing the modules has not adversely affected the roof’s fire performance. For in-roof PV, that means using a kit with the appropriate fire-performance rating for the proposed array location.

There is no one classification that can be stated correctly for every UK roof. The required performance depends on the applicable building regulations, the nation, building type, distance from a relevant boundary and the tested construction. A classification for one module and kit combination does not automatically transfer to another.

As at 22 July 2026, government research has identified ways in which both above-roof and integrated PV can change fire spread, and England has consulted on further guidance. The published research expressly says its findings are not government policy. The design must apply the building requirements currently in force and the product’s actual test evidence, rather than treating a research finding or draft rule as law.

Any fire barrier, compartment line or fire-stopping at the roof still has to perform as designed. An integrated array should not bridge or conceal a defect at one of those locations.

Ventilation and operating temperature

An on-roof array has open air behind it. An integrated array has a shallower rear space and may run warmer, which can reduce electrical efficiency in hot conditions. The practical response is to use the ventilation and batten arrangement in the tested kit, not to add arbitrary openings that interfere with weather or fire performance.

The roof’s original ventilation strategy must also remain effective. Eaves-to-ridge paths, ventilated voids and vapour-control layers should not be blocked or punctured without a compatible detail.

Maintenance and module replacement

Integrated PV gives a flatter appearance and can avoid buying tiles for the area covered by a new-build or reroofing project. It also makes later work more dependent on the continued availability of matching modules and flashing parts.

The design should allow a failed module to be reached and removed without dismantling an unreasonable area of roof. Cables and connectors need support clear of drainage paths, and any connector left under the array must remain matched, correctly assembled and protected from water.

The owner should know how to report a roof leak or electrical fault and which contractor should attend first. A roofer unfamiliar with the kit should not lift modules or disturb live DC connections.

Evidence to expect

The design and handover records contain:

• the exact MCS 012 certificate or alternative compliance evidence
• the approved module and kit combination
• layout, support and fixing calculations
• the structural roof assessment
• fire classification evidence for the installed build-up and location
• the kit installation manual and completed checks
• photographs of underlay, battens, trays, flashings and cable routes
• confirmation of roof and product warranty positions
• access, maintenance and module-replacement instructions

The useful question is not simply whether the panels are “in roof”. It is whether the complete installed roof assembly has evidence for structure, weather and fire.

Related entries

• Roof hook and rail mounting
• Roof loading survey
• MCS 012 solar mounting standard
• MIS 3002 solar PV standard
• Firefighter PV isolation