Solar Panels on Flat Roofs

A structural question before an electrical one

On a pitched roof the array follows the roof plane and the mounting fixes into the structure. On a flat roof neither is true: the frame creates its own angle, and in most cases it is held down by weight rather than by fixings. That makes flat roof solar a loading exercise first and a solar design second.

The three questions in order: what will the structure carry, what state is the waterproofing in, and how much wind will the array have to resist.

Ballasted versus mechanically fixed

Ballasted. Frames sit on the roof on protection mats and are held in place by concrete blocks or paving slabs. Nothing penetrates the membrane, which is the strongest argument for the approach — the waterproofing stays as the roofer left it. The cost is weight, and it is not trivial: a ballasted array distributes a meaningful additional dead load across the roof.

Mechanically fixed. Frames are bolted through the waterproofing into the deck or structure below. Used where the structure will not take ballast, where exposure is high, or where the roof build-up will not tolerate concentrated point loads. Every penetration then becomes a roofing detail — sleeved, flashed and sealed to the membrane manufacturer’s method — done by or with a roofer, with the covering warranty position understood first.

Hybrid arrangements, partly ballasted and partly fixed, are common on larger roofs where perimeter zones need more restraint than the field.

Wind loading governs

An array is light. A tilted panel in a gale is not. Uplift on a flat roof array routinely exceeds the weight of the panels several times over, and the calculation depends on site exposure, building height, parapet arrangement and where on the roof the array sits.

Roof edges and corners see substantially higher pressures than the middle, which is why arrays are set back from perimeters and why ballast or fixing density increases towards the edges. This is calculated for the specific building. It is not a matter of applying the same block count everywhere.

Tilt angle is the main lever. Shallow tilts generate far less uplift, need less ballast and allow tighter row spacing, at the cost of some peak output.

Row spacing and layout

South-facing rows on a flat roof shade each other. Spacing has to be set so the front row does not cast a shadow onto the row behind at low winter sun angles, and that spacing consumes roof area — often more than people expect.

East-west facing frames, with panels back to back in a shallow A-frame, avoid most of that. Rows pack closer, ballast is lower because the profile is more aerodynamic, and generation per square metre of roof is frequently higher than a south-facing layout even though each panel produces less.

Around all of that, the layout has to keep clear of outlets, upstands, rooflights, plant, and access routes to anything that needs servicing.

Membrane, structure and access

Membrane type sets the method: single ply, felt, asphalt and liquid systems each behave differently underfoot and under load, and each has its own repair detail. Remaining life is the deciding factor — an array on a covering with a few years left will have to come off again.

A structural loading assessment is normally required before ballast is specified, particularly on timber decks, older concrete and anything with a long span.

Access is usually the easy part: work is done from a level surface rather than a pitch, though edge protection is still needed and materials have to reach roof level.

Honest limitations

Ballast weight rules out some roofs entirely. Row spacing means a flat roof supports fewer panels than its area suggests. Debris and standing water collect around frames. And an array makes future roof maintenance more involved than it was before.