Roof area arithmetic: photovoltaics against everything else

On a typical flat roof, the area actually available for photovoltaics is closer to half the gross roof area than to three quarters of it. Roof area competition for photovoltaics is the quiet reason energy statements over-promise: the array is sized from a plan area that has not yet lost its plant zone, its green roof, its access routes, its edge set-back and the row spacing between panel banks. The shortfall then appears at technical approval, or worse, at discharge of conditions, when the carbon commitment is already a condition and the roof has already been priced.

Do the arithmetic at the same time as the energy strategy rather than after it. The order matters, because plant sizing depends on the heating strategy and the heating strategy depends on how much generation the roof can carry.

What competes for a flat roof

Six demands land on the same surface, and on a residential block above six storeys most of them are non-negotiable.

• Mechanical plant: air source heat pumps or chillers, ventilation units, their acoustic enclosures and the clearances the manufacturer specifies for airflow.
• Vertical service terminations: lift overrun, stair pressurisation, dry riser, soil vent pipes and the smoke shaft head, each of which fixes its position on the roof from the core below.
• Green roof or biodiverse substrate, usually driven by an urban greening or biodiversity requirement rather than by choice.
• Maintenance access: a route from the roof hatch or stair to every item of plant, wide enough to carry a replacement unit.
• Fall protection: a guarded or restricted zone along the perimeter and around every opening, which cannot be built over.
• Photovoltaic array, including the row pitch between banks and the standing space to clean and replace panels.

Amenity terraces and water tanks add a seventh and eighth on some schemes. London Plan policy G5 sets an urban greening factor target score, and on a constrained site the roof is where the score has to be found, which puts green roof and photovoltaics in direct competition for the same square metres.

Calculating the usable area

Work from the structural roof plan, not the gross building footprint. Subtract in this order, because each deduction changes the next.

1. Plant footprints with manufacturer clearances, taken from the mechanical schedule rather than from an allowance.
2. Service terminations and the smoke shaft head, positioned from the core below.
3. The perimeter zone where fall protection applies, plus a zone around each roof opening.
4. Access routes linking the hatch to every item of plant, and to the array for cleaning.
5. Any area where the parapet or an adjacent taller element casts shade for a meaningful part of the day.

What remains is the area available to share between green roof and array. A south-facing array tilted at around 30 degrees occupies roughly two to three times the plan area of the panels once row pitch is set to avoid inter-row shading at the winter design sun angle. An east-west array at around 10 degrees packs far closer to the panel area, and generates a flatter daily profile that often suits a block with evening and morning demand. The lower tilt loses annual yield per panel and gains yield per square metre of roof.

Record the deductions in a table in the energy statement. A reviewer who can see how 1,400 m² of gross roof became 520 m² of array will accept the output figure. A reviewer given only the output figure will ask for the workings, and that is a month.

Biosolar arrangements that do both jobs

A biosolar roof puts panels on raised frames above a green roof substrate. It is not a compromise so much as a mutually useful arrangement: the substrate keeps the panel underside cooler in summer, which lifts electrical output, and the shade and microclimate under the panels increase the variety of the vegetation. Substrate depth under the array has to be specified with the frame, and the ballast calculation has to account for saturated substrate weight.

Two details decide whether it works. Panel height above the substrate must allow vegetation to grow without shading the cells, and access strips must be left for maintenance rather than assumed. Where the green roof is carrying an urban greening or biodiversity commitment, agree with the ecologist which parts of the biosolar area count.

Where the trade-off becomes a policy question

Once the roof is oversubscribed, the choice is a planning judgment rather than an engineering one. Generation reduces regulated operational carbon and helps meet the on-site reduction expected before any offset payment. Green roof serves greening, biodiversity and surface water attenuation objectives. Both are policy requirements, and the adopted plan rarely says which yields.

Three arguments carry weight in practice. Surface water attenuation can often be delivered below ground, so a green roof that exists only for attenuation is the easier concession. Urban greening can sometimes be found at podium or ground level, though that conflicts with play space and tree pits. Generation is harder to relocate on a constrained urban site, which usually pushes the array up the priority order for the upper roof and the green roof onto lower flat roofs and podium.

Whichever way it resolves, the reasoning belongs in the Energy Statement and should be repeated in the design and access narrative so the two documents agree. For smaller schemes the question is different: the constraint there is the permitted development limit rather than competition for area, and the flat roof photovoltaic permitted development limits set what can go on without an application at all.

The roof plan that settles the argument

Submit a dedicated roof plan, drawn at a scale where a 600 mm access strip is legible, with the zones hatched and their areas annotated. Add a short schedule alongside it: gross roof area, each deduction, the residual, the split between array and green roof, and the resulting installed capacity.

The figures above are illustrative and will not match any real building. What transfers is the shape of the table.

Fixing this before submission also fixes what the conditions can sensibly ask for. Conditions that require an array of a stated capacity, on a roof that cannot hold it, produce a section 73 application later. Getting the wording right at the outset is the point of securing an energy strategy through planning conditions rather than through an aspiration in the supporting text. Where a whole life carbon figure is also required, the Whole Life Carbon Assessment draws on the same roof build-up, so the two should be issued from one set of quantities.

A last practical note: Approved Document O may drive external shading on south and west elevations, and that shading interacts with the roof edge detail. Coordinate the two rather than resolving overheating after the roof is fixed.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice.