Roof Types and Mounting: Flashing Decides Everything

Mounting is where a solar installation meets a building, and it is the part most likely to cause a problem years later. The array will still be there in two decades. So will the holes made to attach it.

Sequencing Comes First

Before any question of hardware, establish how much life the roof covering has left.

An array expected to operate for decades sitting on a covering with a few years remaining creates a predictable and expensive event: the array has to be removed, the roof replaced, and the array reinstalled.

Where the covering is near the end of its life, replacing it first is almost always cheaper than the removal and reinstallation later. Where it is new, the question does not arise.

This is a conversation to have before design begins, not after a quotation has been prepared.

Attachment by Covering

The pattern across all of them is that the attachment has to become part of the covering’s own water-shedding arrangement rather than a hole with sealant over it.

Flashing Is the Whole Job

A sloped roof does not keep water out with sealant. It keeps water out with overlapping layers, each shedding onto the one below.

A correct attachment inserts a flashing plate into that overlap so that water running down the roof passes over the flashing and continues on its way. The fixing is then protected by the same geometry that protects everything else.

An incorrect attachment relies on a bead of sealant around a bolt. Sealant is a maintenance item with a finite life, exposed to ultraviolet light and thermal movement, and when it fails the leak is under an array and hard to trace.

The distinction is invisible once modules are fitted, which is why it is worth asking how attachments will be flashed before work starts, and worth looking at the attachments before the modules go on.

Finding the Structure

Fixings need to land in structural members rather than in sheathing. Locating rafters accurately matters, and a missed fixing is both a weak attachment and an unnecessary hole.

Structural capacity is the other half. The array adds weight and, more significantly, transfers wind loads into the structure at discrete points. On an older building, or where the array is large, that warrants a structural opinion rather than an assumption.

Tile Roofs

Tile deserves separate attention because the failure mode is breakage rather than leakage.

Tiles are brittle and walking on them cracks them, sometimes invisibly. An installer working on tile needs staging that distributes load and a stock of replacement tiles.

Attachment is usually by replacing selected tiles with purpose-made flashed mounts, or by hooks that pass under the course above and bear on the batten. Both preserve the water path when done properly.

Expect some tile replacement as a normal part of the work, and agree beforehand who supplies replacements, particularly for an older roof where matching tiles may be difficult to source.

Flat Roofs

Two approaches, and the choice is structural.

Penetrating mounts are flashed into the membrane and provide a positive attachment. Ballasted racking sits on the membrane and is held by weight, avoiding penetrations entirely but adding substantial load.

Ballast is attractive where penetrating a membrane would affect a warranty, and it is constrained by what the structure can carry and by wind exposure. Tall buildings and exposed sites need more ballast, which is exactly where structural capacity is most likely to be the limit.

Either way, membrane warranties frequently require work to be carried out by an approved contractor, which is worth confirming before anyone goes on the roof.

What People Get Wrong About This

The first mistake is installing over a covering near the end of its life. The removal and reinstallation cost arrives later and was avoidable.

The second is treating attachment as a bolt with sealant. The flashing is what keeps the roof dry.

A third is fastening a corrugated metal roof at the low point of the profile, where water runs, instead of at the high point.

A fourth is underestimating tile breakage and the difficulty of matching tiles on an older roof.

And the last is assuming a flat roof can take a ballasted array without a structural check. Ballast is load, and load is the constraint.

Questions Worth Asking

How will the attachments be flashed for this covering? Where will fixings land, and how will the structure be located? What does the roof covering warranty require? Is a structural opinion needed for the loads involved? And how much life does the covering have left?

The answers are specific to the building, and an installer who works on that covering type will have them ready. See residential solar for the wider process and ground mount systems for the alternative when the roof is the wrong place.