Which Roof Is Suitable for Solar Panels: Assessment Guide

When considering a solar array, the first calculation is usually how many modules will fit and how much they will generate. But there is a question that comes earlier: whether the roof is suitable at all, and how much service life it has left. The answer determines whether the array is an investment or a problem in ten years’ time. This article covers what to check before installation, how to fix an array without harming the covering, and where things usually go wrong.

The key question: how much roof is left

Start with the thing that governs everything else.

A solar array is long-lived equipment — it will work for decades. The roof covering has a service life of its own. And here lies a simple piece of arithmetic that is regularly skipped:

If the roof has ten years left and the array is expected to run for twenty-five, at some point the whole array must come off so the covering can be replaced.

Dismantling, storage, recovering and reassembly cost enough to swallow several years’ worth of saved electricity. And with a ballasted array, all that weight has to come off and go back too.

So the rule is simple: if the roof’s remaining life is shorter than the array’s, the roof is dealt with first. Not in a year, not “when it becomes necessary”, but before installation.

How to assess what is left is covered in our warranty and service life article.

Will the structure carry it

The second question, answered neither by the electrician nor the roofer but by a structural engineer.

The load has several components, and they are assessed together:

• Modules and frames. Modest in themselves.
• Ballast. On a ballasted system this is the largest part — concrete slabs or blocks on every foot.
• Snow load. And here is a nuance worth knowing separately.
• Wind load. Modules act like a wing — the wind does not only press on them, it lifts them.

On snow in more detail. On a bare flat roof snow generally distributes evenly. But rows of modules act as barriers: wind carries snow and it accumulates behind the rows, forming drifts. Instead of an even layer you get concentrated zones, which the original design may never have allowed for.

On older buildings, therefore, a structural assessment is not a formality but a necessity. Particularly where the deck is profiled steel or timber.

Three ways to fix an array

This choice determines how much risk the array adds to the roof itself.

Method	Advantage	Drawback
Ballasted	No penetrations	Adds substantial weight
Welded or torched-on feet	No penetrations, no added weight	Must match the covering type
Mechanically fixed	Most secure hold	Every point is a detail

Ballasted is the most common on flat roofs and logical: no penetrations means no new leak points. The limit is the structure’s capacity.

Welded feet are an elegant solution: a component is torched or hot-air welded to the covering and the frame attaches to it. The covering stays continuous and no weight is added. But the component has to belong to the same system as the covering.

Mechanical fixing is used where nothing else works — usually on light structures that cannot take ballast. Every point becomes a detail to be sealed and maintained thereafter.

Covering and compatibility

A technical detail that, if skipped, surfaces as a problem a few years later.

Ballasted system feet often have rubber pads. On bituminous covering that rarely raises questions. On PVC membrane it does.

The reason is the same one that prevents PVC being laid directly on bitumen: plasticiser migration. Where the materials touch they act on one another, the membrane changes locally, and the load is concentrated at exactly that spot.

The solution is simple but has to be planned:

• Protection mats under every foot — compatible with the covering type.
• A continuous protection layer under the whole array where there are many feet.
• A manufacturer-approved combination — most covering manufacturers publish what is compatible with what.

The second reason for mats is point loading. All the ballast weight concentrates under the foot, and the insulation below can settle. A mat spreads the load and protects the covering from abrasion as the frame moves with temperature.

Where you cannot build: edges and corners

An array is never laid out to the very edge of the roof, and that is not an aesthetic decision.

Wind uplift is considerably higher at the corners and along the edges than in the field. Modules placed at a corner see the greatest load — and would need the most ballast there, which the structure may not take.

So clear zones are left:

• Around the entire roof perimeter.
• Around parapets and rooftop structures.
• At every outlet, so it stays reachable.
• At the overflow.

These strips are not lost area but a maintenance and safety zone. An array built over an outlet will sooner or later end in standing water that nobody notices.

The same logic applies to the gaps within the array. If the spacing between rows is only what shading requires, nobody will get through there with equipment. In practice that means part of the roof goes unmaintained — not because nobody wants to, but because it cannot physically be reached.

So the layout is agreed not only with the electrical designer but with whoever will maintain the roof. A few modules fewer is cheaper than a roof half of which nobody sees for ten years.

Shading: usually from your own roof

On a pitched roof shade comes from trees and neighbouring buildings. On a flat roof it usually comes from whatever else stands on the same roof.

Sources of shade, in order:

1. Air handling units and shafts. The largest and closest.
2. Parapets. A tall parapet shades the whole first row morning and evening.
3. Chimneys and vents. Small, but numerous.
4. The module rows themselves. The front row shades the next if spacing is too tight.
5. Stair exits and lift shafts.

That last point about rows is a trade-off: the wider the spacing, the less shading but the fewer modules fit. Flat roofs therefore increasingly use east–west layouts at a shallower tilt — more modules fit, generation is flatter across the day, and inter-row shading practically disappears.

Access for maintenance

An array is installed once; the roof is maintained forever. The two have to be compatible.

• Outlets must stay reachable without dismantling modules.
• There must be a route between rows for walking and inspecting the covering.
• Walkways where the roof is crossed regularly — otherwise the covering wears more from maintenance than from weather.
• Cables must not lie directly on the covering. They abrade, and in winter they freeze into ice.

That last point looks trivial, but a cable resting on the covering and moving in the wind will scour the mineral surfacing within a few years.

How an array changes the covering itself

Rows of modules do not merely sit on a roof — they change the conditions beneath them. The effect cuts both ways.

The helpful side. Under the modules the covering is in shade. It sees no UV and far smaller temperature swings, and those are what age it most. In that zone the material stays in better condition than in the open.

The unhelpful side. A module is an inclined plane, and water leaves it not evenly but along one line — the lower edge. Instead of rain falling across the whole area you get a concentrated stream, always at the same place.

Within a few years that produces:

• Washed-off surfacing. On bituminous covering, pale stripes appear beneath every module row.
• Localised wear. Falling water carrying grit acts as gentle abrasion.
• Ice building up in winter along that same line.

The remedy is simple: a protection strip is laid beneath the lower edge of each row — an additional layer of covering or a purpose-made mat. It costs little, and it is only done at installation.

So when surveying a roof with an array, look not only at the feet but at the line beneath each row. That is where the years of service show.

When the covering needs replacing and the array is already there

A situation best avoided, but one that does occur — particularly where an array went onto a roof that had little life left.

The options, cheapest first:

• Phased replacement in sections. The array is taken down zone by zone: one part off, the covering relaid, the part returned, then on to the next. Slower, but there is no need to store the whole system and generation does not stop entirely.
• Lifting without dismantling. Some ballasted systems allow the frame to be raised and shifted with little disassembly. It depends on the system.
• Complete removal. Technically the simplest and practically the dearest — it needs storage space and time to rebuild.

In every case one more thing needs assessing: modules and mounts age over ten years too. Before returning an old frame onto new covering it is worth checking the fixings and cables, because the next chance to reach them will be a long way off.

Which brings us back to the first section: none of this expense would exist had the roof been dealt with before installation.

The warranty question

A situation that arises constantly and that nobody discusses in advance.

One contractor gave the roof warranty. A different one installs the array. If the installer pierces the covering or damages a detail, the roof warranty no longer applies at that point — and sometimes across the whole roof, depending on the conditions.

How to avoid that:

1. Inform the roofing contractor beforehand and obtain written approval of the approach.
2. Photograph the roof before the works — the whole area, not just the installation zone.
3. Give the covering work to a roofer, not the electricians: welded feet and details are roofing work.
4. Photograph afterwards and add it to the roof file.
5. Agree explicitly who is liable if a leak appears in the installation zone.

The fifth point matters most. Without it, when a leak starts the electrician points at the roofer, the roofer at the electrician, and the owner pays the bill.

When a roof is unsuitable

1. The covering is due for replacement. Sort the roof out first.
2. The structure cannot take the load. Neither ballast nor penetrations will help.
3. The roof is full of plant. When the free area is smaller than the shaded area.
4. Much of the day is in shade. A taller neighbouring building changes the whole calculation.
5. Ponding and an uncorrected fall. The array will hide the problem, not solve it.
6. No safe access. Maintenance becomes impossible.

What to do before installation

• A roof survey with a finding on condition and remaining service life.
• A structural assessment, where the system is ballasted or the building is old.
• Checking the fall and the outlets — if anything needs correcting, now is the time.
• Verifying compatibility between feet and covering type.
• Agreeing the layout with the roofer: edge zones, outlet access, walkways.
• Documentation — photographs and agreements before starting.

The most common mistakes

1. Installing on a roof with a few years left. The most expensive mistake of all.
2. Not allowing for snow drifting behind rows. The load is concentrated, not even.
3. Rubber pads directly on PVC. A compatibility problem that surfaces later.
4. An outlet built over. Water accumulates where nobody looks.
5. Modules right to the edge. Wind load is highest at the corners.
6. Electricians doing the covering work. A detail is roofing work.
7. No agreement on liability. When the roof leaks, the owner is left with it.

What drives the cost

• The fixing method — ballasted, welded or mechanical.
• Whether protection mats are needed and over what area.
• Whether the roof is put right first.
• Structural strengthening, where required.
• Walkways and access provision.
• Investigations — survey, structural assessment, shading analysis.

How preparation proceeds

Related reading

• Solar panel installation on a roof
• Roof warranty and service life
• Commercial solar on a flat roof
• Green roofs
• Roof renovation