Roof loading survey

A roof loading survey checks whether the building can safely support a proposed solar array and mounting system. It is not simply a visual look at the tiles or a comparison with a typical panel weight.

The assessment identifies the structure, its condition and the loads already present, then checks the additional PV loads and how the mounting system transfers them. It should be completed early enough for the design to change if the roof is unsuitable.

Who carries out the check

MIS 3002 requires the structure of a pitched roof to be checked by a suitably competent person. For the typical construction types covered by its calculation methods, that may be a person with the training and competence to inspect the roof and apply those methods correctly.

A qualified structural engineer must be consulted where the roof is unusual or there is any doubt. MIS 3002 examples include:

• a construction not covered by its typical methods
• signs of structural distress
• removed, cut or altered timbers
• a later change to a heavier roof covering
• a particularly shallow pitched roof
• hips, valleys, dormers, parapets or other features that increase snow build-up

Some roof forms listed by MIS 3002, including particular cut roofs with hips or valleys and asymmetric roofs, go directly to an engineer.

For a ballasted flat-roof system, MIS 3002 expressly requires a qualified structural engineer to confirm that the roof can withstand the combined load of the PV and proposed ballast.

What the survey records

A useful survey provides evidence, not a tick beside “roof looks sound”. It records as far as applicable:

• roof type, pitch, spans and support layout
• rafter, truss, purlin, joist or deck sizes and spacing
• the existing covering and roof build-up
• accessible connections, bracing and restraint
• loft conversions, water tanks, removed members or other alterations
• deflection, cracking, corrosion, decay, insect attack or water damage
• rooflights, chimneys, valleys, parapets and other load or access constraints
• the proposed array and the intended fixing locations

Not every element can be seen without opening the building. Where a critical detail is concealed, the assessment should state the assumption, obtain records, expose the detail safely or escalate it. Silence is not evidence that a connection is adequate.

Roof condition and structural capacity are related but different questions. A roof can be strong enough yet need its covering replaced soon; it can also look neat while containing altered or undersized structure.

Loads that matter

The calculation considers more than module weight. It needs to account for:

• the modules, rails, brackets, cables and other permanent dead load
• ballast and its concentrated bearing loads on a flat roof
• downward snow load and possible drifting
• wind uplift, which is often greater near roof edges
• the existing covering and other permanent loads
• the way each load is distributed into the supporting structure
• relevant combinations of loads under the structural design rules

There is no reliable national rule that says every domestic roof can accept a stated kilograms per square metre of solar. Two arrays with the same total weight can affect a roof differently if one spreads the load across many verified supports and the other concentrates it on a few weak points.

The module’s own load rating does not prove the roof or fixing system is adequate. Module, clamp, rail, bracket, fixing and structure are separate links in the load path, and the design is limited by the weakest one.

Wind and roof-edge zones

Site location, altitude, building height, surroundings, roof shape and array position all influence wind action. A calculation copied from a neighbouring project is not automatically suitable.

MIS 3002 says modules should normally not be mounted within 400 mm of a domestic roof edge unless specific measures address increased uplift and its consequences. A closer layout can be designed, but it may require additional fixings or roof timbers and checks on the ridge, rainwater, snow and wind noise.

The fixing plan should state the edge distances, rail spans and support positions used in the calculation. Installers need a route for reporting a hidden rafter, failed fixing or unavoidable moved bracket so the designer can assess the change.

Building regulations and notifications

Adding PV should not compromise the building’s compliance with the applicable regulations. Structure is covered by the building rules of each UK nation, with Approved Document A providing guidance in England. Fire, weather resistance and electrical safety also remain relevant.

The route for building-control notification differs with nation, work and installer registration. An MCS certificate does not by itself replace every statutory approval. The project records identify responsibility for each notification and include the resulting evidence at handover.

If the roof does not pass

A failed or conditional assessment does not always rule out solar. Possible responses include:

• repairing decay, leaks or damaged covering first
• reinstating missing bracing or strengthening members to an engineered design
• changing array size, orientation or location
• increasing or relocating supports where the verified structure permits
• selecting a lighter or differently loaded mounting arrangement
• replacing a ballasted design with an engineered fixed or hybrid system
• postponing PV until a planned reroof is complete

Changes should be recalculated. Reducing a few panels does not automatically solve a local fixing or wind-uplift problem.

Evidence to expect

The project file should make the conclusion reproducible. It should include:

• survey photographs and measured structural details
• the assessor’s name, competence and scope
• design assumptions and any limitations on access
• dead, wind and snow load calculations as relevant
• the mounting and fixing layout
• the structural engineer’s report where required
• details of repairs or strengthening
• final photographs confirming that the installed supports match the design
• building-control or competent-person notification evidence where applicable

An unexplained statement that the roof is “suitable for solar” is not equivalent to a recorded structural check.

Safe inspection

The survey itself is roof work if someone can fall from an edge, opening or fragile surface. HSE says all roofs should be treated as fragile until a competent person confirms otherwise. Loft access, ladders, roof access and any opening-up therefore need their own safe method; a structural question does not justify an unsafe inspection.

Related entries

• Roof hook and rail mounting
• Flat roof ballast systems
• In-roof solar mounting
• Scaffold and roof access
• MIS 3002 solar PV standard