Solar Plant Foundation on Flat Roof

A solar array on a flat roof begins not with the modules but with the roof beneath them. We do not mount modules — that is the electrical contractor’s job. We prepare the roof so the array can be mounted without damaging the waterproofing and so the roof beneath stays watertight. This article explains what that means as a service: which bases, what load and why the order matters.

The core idea is simple: an array is only as good as the roof it stands on. If it generates power but leaks beneath, you have a more expensive problem than before. So we treat the roof-side preparation as a standalone, serious service, not an afterthought to mounting modules. An investment in solar pays back over many years — and the roof beneath it has to last just as long, or saving on the roof eats up the array’s benefit.

Why the roof comes first

The most common mistake with flat-roof solar arises not from the modules but from the roof nobody looked at first. An array stands for 20 to 30 years, so the waterproofing beneath it has to last just as long.

Mount on an aged covering and an expensive problem arises: to renew the covering, the whole array has to be dismantled. Dismantling, recovering and reassembly often cost more than a timely renewal would have, and the array stands idle in the meantime. So we always begin with a survey and the question of the covering’s remaining service life, before any talk of bases. If the covering is near the end of its life, the right moment to renew it is before the array — not after.

Ballasted or mechanically fixed base

This is the fundamental decision, directly affecting the roof’s watertightness. There are two ways to hold a module frame on a flat roof.

• Ballasted bases. The frame is held by weight — concrete slabs or trays — without penetrating the covering. The great advantage: no holes in the waterproofing. The price: extra load on the structure, which has to be checked. For the roof’s watertightness this is almost always the gentler solution.
• Mechanically fixed bases. The frame is screwed into the structure. Lighter, but every fixing is a penetration of the covering that must be sealed properly. This applies where the structure cannot carry ballast weight or where wind load demands firm anchoring.

In practice we prefer ballasted solutions for watertightness, where the structure’s capacity allows — every penetration avoided is one potential leak point fewer. Where ballast is not possible, the penetrations are executed with the same care as any other roof detail.

Why penetrations are the main risk

The field of a flat roof is rarely the leak — leaks almost always start at the details and penetrations. A solar array can mean dozens of new penetrations if it is mechanically fixed.

Each is a place where water can get in if it is not done properly. So we treat every module fixing as a separate detail: with a collar, dressed neatly into the covering and checked, not covered with a layer of sealant. The difference shows up not on installation day but after a few winters. That is exactly why the number of penetrations is itself an argument: where ballast is possible, the roof is spared dozens of potential weak points at once.

Load: a question for the structural engineer

A solar array adds weight to the roof — considerably so with ballasted systems. Whether the structure carries it is not a matter of eye but of the structural engineer.

We say when an assessment is needed, and without one we do not propose a ballasted array on an unclear structure. It is not only the dead weight that has to be considered but the wind and snow loads: modules present an area to the wind, and higher forces act at the corners and edges than in the field. A base that holds in the middle can fail at the corner if the zones were not calculated. Snow behaves differently under and between modules than on a bare surface, so the load assumption must fit the specific roof and location.

What we prepare

Our service is the roof-side preparation — everything between the existing covering and the point where the electrical contractor sets the modules.

• Assessing and, if needed, renewing the covering before mounting, so it lasts as long as the array. This is the most important point and the only moment when it is cheap.
• Reinforced zones under the bases, where ballast or fixing applies pressure.
• Proper details at every penetration, if mechanically fixed — with a collar and a check, not sealant.
• Sealing the cable penetrations — often forgotten and a common leak point because they are small.
• Maintenance paths, so the roof can be walked and inspected without damaging the covering.

The goal is simple: after mounting, the roof should be as watertight as before — and just as accessible for maintenance. Everything we prepare on the roof side serves that one goal, and it is by that goal that our work can be judged.

Working with the electrical contractor

A good flat-roof array comes from the coordination of two trades. So the boundaries are clear:

• We assess and prepare the roof, build the details and penetrations, and ensure watertightness and access.
• The electrical contractor mounts the frames and modules, runs the cabling and the electrical connection.
• The structural engineer assesses the load where needed.

Both trades are best planned together, not one after the other. Setting modules on a finished roof without involving the roofer is the road to later leaks — and to the question of who is responsible. If a leak appears, the solar firm points to the roof and the roofer to the penetrations; if it was coordinated from the start, that question does not arise.

Remaining service life of the covering

This point decides more than any other detail. An array binds the waterproofing beneath it for decades.

If the covering is new or nearly new, mounting can proceed directly. If it is mid-life, an honest calculation is worth it: if it will need replacing in five years, it is cheaper to do it now than to dismantle and rebuild the array later. If it is at the end of its life, there is no way around a renewal — more on the installation page, and what governs service life in the warranty article.

Maintenance and access

Modules make roof access harder — and a flat roof needs access, above all to the outlets. This is often overlooked in planning and regretted later.

• Outlets must stay reachable. A module row directly over an outlet makes cleaning impossible without dismantling.
• Paths are needed between the rows so maintenance is possible without stepping on modules or fragile covering.
• Leaves and debris gather under modules, where they are harder to remove — regular checks become more important.
• An overflow stays essential. If a hidden outlet clogs, water must have a second way out before it builds up under the modules.

Solar and a green roof together

Increasingly, greenery and modules are wanted on the same roof. It is possible and even beneficial: a green surface cools the modules’ surroundings in summer, and cooler modules work more efficiently. What matters is that the bases do not penetrate the waterproofing — ballasted bases are usually used. Such combined build-ups are worth planning together, not adding modules onto an already-installed green roof, because reconciling two finished layers later is difficult and risky for the waterproofing.

Common mistakes we see when taking over sites

Almost every problem with flat-roof solar repeats the same mistakes — and none of them comes from the modules themselves. Worth knowing, even if another contractor does the work.

• Mounting on an aged covering. The most expensive mistake: the covering fails a few years on, and renewing it means dismantling the whole array.
• Penetrations “solved” with sealant. Instead of a proper detail, a layer of sealant; it lasts a season, then begins to drip.
• Wind zones not calculated. Uniform ballast across the whole area means the corners and edges are too weak where the forces are greatest.
• Outlets under module rows. Drainage becomes inaccessible, and a blocked outlet goes unnoticed until water stands.
• No roofer involved. When a leak appears, it is unclear who is responsible — the installer points to the roof, the roofer to the penetrations.

What they share is that they do not show on installation day but appear a season or two later, when a leak is hard to trace back to the mounting. That is why the roof side is worth entrusting to a roofer.

What we do and do not

It is only fair to state the boundaries at once.

We do: inspect and prepare the roof, renew the covering if needed, build the base zones, details and cable penetrations properly, and ensure access and drainage.

We do not: mount modules or frames — that is the electrical contractor’s job; run the electrical cabling; produce structural load calculations — that is the structural engineer’s field, and we say when it is needed. If the roof is not suited to an array, we say so at once, even if it means the roof comes first.

How the preparation runs

Want the roof side prepared by professionals? See our service Rooftop solar installation.

1. Survey and remaining life. Covering condition, details, drainage, the type of array planned.
2. Coordination with the installer and structural engineer. Who does what, what load, which module rows.
3. Renewing the covering, if needed, before mounting.
4. Preparing the base zones and details.
5. Handover to the installer for mounting.
6. Post-mounting check that all penetrations are watertight and the outlets accessible.

Season

• Roof work — in the usual bituminous-covering season, from late spring to autumn, when the base is dry.
• Survey and planning — year-round; it is practical to prepare the roof at the start of the season and schedule the mounting after.
• Best planned early, so roof preparation and module mounting do not delay each other and both trades arrive coordinated.

Whether a roof suits solar at all is covered in the guide on which roof suits solar panels. The full scope of services is on the services page, and how to get in touch on the contact page.