Structural appraisal for rooftop solar on an existing roof

Most rooftop solar on a commercial building needs no planning application, and almost all of it needs a rooftop solar structural appraisal before a single bracket is fixed. Those two facts get confused. Part 14 of Schedule 2 to the General Permitted Development Order 2015 permits solar equipment on buildings within stated limits, and nothing in that permission says anything about whether the roof can carry the array. The structural question is separate, it is governed by the Building Regulations and Eurocode design, and on an older industrial unit it is the question that decides the project.

Start with the roof, not the panel layout. An installer's yield study on a roof that turns out to be a fragile asbestos cement sheet is wasted work.

Where permission is and is not needed

Part 14 permits solar photovoltaic and solar thermal equipment on buildings subject to limits, the familiar ones being that the equipment must not protrude more than 0.2 metres beyond the plane of the wall or roof slope, must not stand more than 1 metre above the highest part of the roof excluding a chimney where it is on a flat roof, and must be set back at least 1 metre from the external edge of the roof.

Part 14 has been amended more than once, including changes affecting capacity limits and the prior approval requirements for larger installations on non-domestic buildings, so read the current consolidated text of the Order rather than a summary. Check also whether an Article 4 direction applies.

Permitted development does not help you in four situations:

• A listed building, where listed building consent is required for the works and the permitted development right does not apply.
• A scheduled monument.
• A conservation area or World Heritage Site, where installation on a wall or roof slope forming the principal elevation and fronting a highway is excluded.
• A site where a planning condition on an earlier permission has removed the right.

Two further consents catch people out. Near an aerodrome, a glint and glare assessment may be required and the safeguarding authority consulted. And a roof above a shopfront or in a sensitive setting may attract an advertisement or design objection that has nothing to do with Part 14.

What the array adds in load

The load depends almost entirely on the mounting method rather than on the panels.

A mechanically fixed array on a pitched roof adds a modest additional dead load spread across the purlins, and on a sound modern roof it is rarely the governing case. A ballasted system on a flat roof is different in kind, because the ballast that resists wind uplift is itself a permanent load, and it is concentrated at the edges and corners where the uplift is highest. That combination of higher load in exactly the zones where the structure is often weakest is the usual reason a scheme has to change.

Three load questions need answering:

1. The additional permanent load from frames, panels, cable trays, inverters and any ballast, with the distribution shown rather than averaged.
2. Whether the array changes the imposed load case, including snow. Panels on a flat roof create obstructions behind which snow drifts, so the imposed load can rise locally rather than fall.
3. Maintenance and access loading on the roof covering and on any walkway provided.

Averaging the array weight across the roof area is the most common error in an installer's calculation. The structure does not experience an average.

Appraising an existing roof structure

The appraisal has three parts: find out what is there, check what condition it is in, and check the capacity.

Start with records. The original design drawings and calculations, any as-built information, and records of previous alterations. On an industrial unit built in the 1970s or 1980s, expect to find nothing and plan for a survey.

Then survey. A Structural Survey by a chartered engineer should establish the frame type and spacing, purlin sizes and spans, deck or sheet type and fixings, the condition of connections, and any corrosion, decay or deflection. Councils normally expect a suitably qualified author for this kind of report and professional review is strongly advised.

Several roof types carry specific problems:

• Profiled metal deck with hidden corrosion at fixings and laps, particularly where insulation has trapped moisture.
• Asbestos cement sheet, which is fragile, must not be loaded or walked on, and brings duties under the Control of Asbestos Regulations 2012. Our note on asbestos and the pre-redevelopment audit covers the survey obligations.
• Timber roofs with historic notching or previous service penetrations.
• Concrete decks where the capacity is adequate but the waterproofing is at the end of its life, which makes the array an obstruction to a re-roof within a few years.

That last point deserves a decision rather than a discovery. If the covering has five years left and the array has a twenty-five year life, re-roof first.

Fixings, penetrations and the waterproofing warranty

Penetrating a single ply or felt membrane to bolt a frame down is technically straightforward and commercially awkward, because the membrane manufacturer's warranty usually requires their approved detail and often their approved installer.

So resolve the fixing method with the roofing manufacturer before the structural solution is fixed, not after. The options in rough order of preference on an existing flat roof are a non-penetrative ballasted system where the structure can take the load, a bonded or adhered mounting system accepted by the membrane manufacturer, and mechanical fixing through the membrane with proprietary upstands and flashings installed to their detail.

On a pitched sheet roof, fixings go into the structure through the sheet with sealed fasteners, and the condition of the existing sheet fixings is usually the limiting factor.

Fire also belongs in this conversation. Roof build-ups with combustible insulation, cable routes and inverter locations all affect the fire strategy, and on a building with a fire engineered strategy the array should be reviewed against it rather than added to it. The same principle applies to external wall build-ups, as set out in our note on combustible materials rules and planning elevations.

Wind uplift and the edge zones

Wind is the dominant action on a rooftop array, and the design has to follow Eurocode wind loading with the UK National Annex, taking account of site exposure, building height and terrain.

The important point for the layout is zoning. Uplift pressures at roof corners are far higher than in the central field, and at the edges higher than the field but lower than the corners. That is why ballast requirements increase towards the perimeter and why the sensible layout sets the array back from the edges by more than the 1 metre the permitted development limit requires.

Illustrative example: a single storey warehouse where the initial layout filled the roof to within 1 metre of the parapet. Ballast in the corner zones pushed the local load beyond the purlin capacity. Setting the array back 3 metres at the corners and 2 metres at the edges reduced the array by around 8 per cent of its area and removed the need to strengthen anything. The figures are illustrative and used to show the trade-off.

Parapets change the picture, generally reducing edge uplift, and a low parapet is not the same as a high one. Have it assessed rather than assumed.

Evidence to submit with the application

Where an application is needed, the pack is short:

• A roof plan showing the array, the set-back from each edge, and inverter and cable routes.
• An elevation or section showing the height above the roof and the protrusion beyond the roof plane.
• A structural appraisal letter or report from a chartered engineer confirming capacity, with any strengthening described.
• A statement of the fixing method and the roofing manufacturer's acceptance.
• Where relevant, a heritage statement, a glint and glare assessment, or an assessment of visibility from the street.

Where the works are permitted development, keep that pack anyway. A lawful development certificate is the only formal way to confirm that works are lawful, and the evidence that the limits were met is worth having on file if the question is asked later.

Conditions and maintenance access

If a permission is granted, expect conditions on the approved layout, on the array not exceeding the stated height, and sometimes on removal when no longer needed.

The practical issue at handover is access. Working on a roof to clean or replace panels is work at height, and the design has to provide for it: walkways that protect the covering, edge protection or an anchor system, and a safe route to the roof. Those arrangements belong in the design and in the health and safety file under the Construction (Design and Management) Regulations 2015, not in a note handed to the facilities manager at completion.

And record the array on the building's structural information, so the next engineer asked about a mezzanine or a new plant item knows what is already up there.

This is general guidance, not legal advice, and permitted development limits and structural requirements depend on the building and its location, so check the position with your local planning authority and a chartered structural engineer before relying on it. PlanScope produces AI-assisted drafts of the planning documents that support this kind of installation, flagging the structural and technical facts it needs from your engineer. Enter your postcode to see which reports your site needs.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.