Solar panels on a flat roof: ballast, layout and yield

On a pitched roof, panels hook onto the roof itself. A flat roof works differently: the panels sit in plastic or aluminium frames held in place by ballast, usually concrete tiles in the frame tray. Nothing has to pierce the roofing membrane, which reduces the risk of leaks. Only on roofs too light for ballast do we choose mechanical anchoring, and only with a properly detailed connection into the roofing.

We usually set the frames at an angle of 10 to 15 degrees. That is flatter than the ideal pitch of a sloped roof, but it keeps wind load and the required ballast manageable and the rows compact. Under the frames we lay protective mats, so the ballast and frame do not damage the bitumen or EPDM layer.

On a flat roof you are free to choose the orientation, and that is the main design decision. A south-facing layout catches the most sun per panel, but the rows shade each other and need spacing: count on about 2.3 square metres of roof per square metre of panel, according to Milieu Centraal (Dutch). In an east-west layout the panels lean against each other like a small roof. You give up roughly 10 percent yield per panel, but you fit more of them and generation spreads better across morning and afternoon.

In practice we recommend east-west as soon as the roof is the limiting factor: if you want to use your roof surface to the full, more panels facing east-west often beat fewer panels facing south on total yield. How many panels you need is worked out in our guide how many solar panels do I need; what a panel yields per orientation is covered under solar panel yield.

Wind does not hit a flat roof evenly. Along the edges and especially on the corners the wind forces are highest; the middle of the roof gets off lightly. That is why we calculate ballast per position using the frame manufacturer's wind calculation: panels on corner and edge positions get more weight than panels in the middle field. Building height counts too, because a four-storey roof catches more wind than an extension.

For the same reason we keep distance from the roof edge. That avoids the heaviest wind zone and has a side effect many homeowners appreciate: a free strip along the edge makes the installation much less visible from the street. A panel sticking out over the edge or pushed right against it is asking for trouble in a storm.

Panels, frames and ballast together quickly add up to tens of kilos per panel. A concrete roof can almost always take that. With timber roofs, such as on many extensions, garages and sheds, we look more critically: what is the span, how old is the structure and is there already gravel or decking on it? When in doubt we request a structural check before placing anything. Better a calculation upfront than a sagging roof afterwards.

Also look at the age of your roofing. An installation sits there for 25 years; if your bitumen or EPDM is due for replacement within a few years, renovating the roof first and installing panels after is the logical order. Otherwise you pay for mounting work twice. These points belong in a good quote; what else to check is covered in buying solar panels: what to look for.

A 10 to 15 degree setup yields slightly less per panel than the ideal tilt, but the difference is smaller than many people think and the flatter angle keeps wind and ballast manageable. The Netherlands also generates a lot from diffuse light, where the angle matters less.

One honest note: at a low tilt, rain washes dirt away less effectively than on a steep tiled roof. So count on roughly a yearly cleaning check, especially along the bottom edge of the panels where algae can build up. How to do that safely is covered in cleaning solar panels.

During the site survey we measure the roof surface, check the roofing and load capacity, and look at the cable route to the fuse box. Then we draw a layout plan with the setup that fits your roof and consumption, including the ballast calculation per zone and the distance to the roof edge.

You get a no-obligation quote within 24 hours, no deposit required, and after approval we usually install within three weeks. Not sure whether your flat roof qualifies? Put your roof to us or start on our solar panels page.