Flat Roof Solar Panels for Commercial Buildings

Can a commercial flat roof take solar?

Three things decide it: whether the structure can carry the panels (and any ballast), the condition and age of the waterproofing, and whether people can work on it safely. MCS requires the building to be assessed by a competent professional, and the installer must be able to show a structural assessment and a wind loading calculation for the specific mounting system (MIS 3002, 3.1.2 and 3.5).

• Structure. Concrete decks, steel frames and timber joists each carry load differently. Ballasted systems add weight across the whole roof.
• Waterproofing. A membrane near the end of its life is best replaced before panels go on, because lifting a system to re-roof later costs time and money.
• Access and fragile areas. Rooflights and fragile sheets are a fall risk. HSE says work on or near fragile surfaces needs careful planning (HSE), and MCS asks designers to consider walkways and permanent protection over fragile elements such as skylights (MIS 3002, 3.6.2).
• Shading and plant. Parapets, air handling units and lift rooms cast shade and need clear space around them for maintenance.

Ballasted or mechanically fixed: what the roof decides

All of this is from MIS 3002 section 3.9 (MCS). The practical upshot: a lightweight roof that can't take ballast usually points to a fixed system, and a roof whose membrane you don't want pierced usually points to ballast, if the structure allows. The survey is where that is settled.

Roof membranes and warranties

MCS requires the installer to be able to show how the roof's weather-tightness has been kept after the work, which may include photographs, method statements, or confirmation that existing warranties remain valid (MIS 3002, 3.5.5(d)). On a ballasted system, the slip or protection layer under the frames has to be compatible with your waterproofing.

Before you sign, find the roof's guarantee and ask its manufacturer or roofing contractor what they need to keep it valid. If the membrane has only a few years left, ask for a price to replace it first: panels typically stay put far longer than an ageing membrane.

East-west or south-facing layouts

On a flat roof the installer chooses the angle and direction. South-facing rows tilted up make more per panel but cast shadows backwards, so they need gaps between rows. East-west layouts place panels back to back at a low tilt, fit more on the same roof and spread generation across the morning and afternoon.

Which suits you depends on when you use power. A business busy from early morning to late afternoon may use more of an east-west system's output on site. We don't publish a rule of thumb: the installer models both for your roof and your half-hourly data. See how much solar your business needs.

Metal roofs: profiled sheet, standing seam and composite panels

Many industrial units and warehouses have low-pitch metal roofs rather than true flat roofs. MCS requires the installer to check that the roof sheet thickness suits the brackets and screws and can withstand the extra wind uplift, and that the sheets are fixed to the main structure well enough to carry the extra loads (MIS 3002, 3.9.8 and 3.9.9).

• Profiled sheet. Brackets fix through or onto the crowns of the profile.
• Standing seam. Clamps fix to the seams, so the sheets themselves are not pierced.
• Composite (insulated) panels. The outer skin, the core and the fixings all need checking.
• Asbestos cement sheets. A different job altogether: see solar on an asbestos roof.

Planning limits for flat roofs

In England, solar on a non-domestic building is permitted development under Class J if the highest part of the equipment is no more than 1 metre above the highest part of a flat roof and no part is within 1 metre of the roof's edge. Listed buildings, buildings in their curtilage and scheduled monuments are excluded. Over 50 kW, you must ask the council first whether it needs to approve the design or appearance, in particular glare (Class J).

Wales sets the same 1 metre height and 1 metre edge limits for non-domestic buildings (GOV.WALES). In Scotland, panels on non-domestic buildings must not protrude more than 1 metre from the building's surface (circular 2/2024). Full detail by nation: planning permission for commercial solar.

What the survey checks on a flat roof

• The structure. Deck type, spans and what the roof was designed to carry; whether a structural engineer is needed (always for ballast).
• The waterproofing. Type, age, condition and guarantee.
• Usable area. After the edge zone, plant, rooflights and maintenance routes.
• Access. How panels and people get up there safely, and whether edge protection is needed.
• The electrics. Where the cables run, the board, and the grid connection. See the site survey.