Flat Roof Solar Installation

A different set of problems

A pitched roof gives you the angle and the fixing points for free. A flat roof gives you neither, and adds a third problem: a large, light, tilted object on an exposed surface is exactly the shape wind likes to lift.

So flat roof design starts with two calculations — what the structure can carry, and what the wind will try to do — and works backwards to the array.

Ballast or mechanical fixing

Ballasted systems hold the array down with weight. Frames sit on the membrane on protection mats, and concrete blocks or paving slabs provide the resistance to uplift. Nothing penetrates the roof, which is the main attraction: no new holes in a waterproof surface, and no argument with a roofing warranty.

The cost is load. The ballast has to be sufficient for the worst wind case, and on an exposed or tall building that is a lot of concrete. Whether the structure can carry it is the deciding question.

Mechanically fixed systems bolt through to the deck or structure. Far less weight, but every fixing is a penetration through the waterproofing that has to be detailed and sealed properly. On some membranes this is routine work; on others it is a specialist job that should involve the membrane manufacturer.

Hybrid arrangements — mostly ballasted with a modest number of fixings at the corners and edges where uplift is worst — are common and often the sensible answer.

Wind loading, briefly

Uplift is not uniform across a roof. The corners see the highest pressures, the edges next, and the middle of the roof the least. A competent ballast layout reflects that: more weight at the perimeter, less in the field. A layout with the same ballast everywhere is either over-loaded in the middle or under-restrained at the edges.

Building height and exposure matter as much as the site. The same array on the same roof in a sheltered suburban street and on an exposed ridge are different calculations.

Tilt and row spacing

Panels on a flat roof need a frame to give them an angle. The temptation is to replicate a pitched roof angle, but that is rarely right.

• A steeper tilt produces more per panel in winter and more wind load all year.
• A steeper tilt casts a longer shadow, so rows must be spaced further apart to avoid shading each other in low winter sun.
• Wider spacing means fewer panels on the same roof.

For most UK flat roofs, a shallow tilt in the region of ten to fifteen degrees gives more total generation from a given area than a steep one, because you fit more panels. East-west facing arrays in an A-frame arrangement go further still, packing panels tightly with almost no inter-row shading and producing a flatter output curve across the day.

Protecting what is underneath

The membrane was there first and it has to outlast the array. That means protection mats under every foot, compatibility checked between the mat material and the membrane, and a layout that keeps outlets, upstands and existing details clear.

Drainage is part of the design. A ballast block sitting across the fall of a roof creates a dam, and standing water shortens the life of any covering. We set the array out so water still reaches the outlets.

We also leave you the ballast layout drawing. When someone comes to inspect or repair the roof in ten years, they need to know what can be moved and what cannot.