Flat roofs are a natural place for solar panels. They are out of sight, there is room for a framed array, and the panels can be tilted towards the sun. The installer will talk about orientation and output. A roofer asks something different: what is this roof, how old is it, and what will it be like to repair underneath the panels in ten years?
The answer matters because a flat roof is a waterproof covering on a deck, and everything that is added to it either rests on it or goes through it. Panels add weight. Their frames need either ballast or fixings. If the membrane under them wears out before the panels do, someone has to lift the array to renew it.
This post covers the roof side of the question: loading, the choice between ballast and fixings, the covering and its lifespan, falls and drainage, and the checks to make before anything is fitted. What you need first is an accurate view of how your own roof stands.
Our flat roofing page explains how we survey and repair flat roofs, and the findings from a survey are what an installer needs before design begins.
Loading: what the roof has to carry
A solar array is an extra load on a roof that was designed for its covering, the people who maintain it and the weather. Whether it can take the added weight is a structural question, and the answer has to come from the structure, not from the panels.
Why weight matters
Approved Document A treats a load rise of more than 15 per cent as a significant change to roof loading. The Planning Portal's re-roofing guidance sets a near-identical line, at 15 per cent or more extra weight, and recommends asking a structural engineer or surveyor for an opinion. An array with its frames, and with any ballast, is a load that has to be checked against the deck and the structure below.
Different loads, different checks
The load comes in several forms: the permanent weight of the panels and frames, the ballast if the array is held down by weight, and the force of wind trying to lift it. The deck and the walls beneath must resist all three.
Who checks
An installer should say how the load has been assessed, in writing. A verbal assurance is not enough when the deck is the only thing between the panels and the room below. For an older roof, or a lightweight deck, an engineer's calculation is the sensible way to show it. If a roof cannot carry the load, the choices are to strengthen it, to spread the load, or to leave the roof alone.
What a survey records
Our survey records the deck type, the covering, the condition of the upstands and the outlets, and photographs the roof and the structure where it can be seen from inside. That is the information a designer needs.
Ballast or fixings: two ways to hold it down
A flat-roof array has to stay put in wind. There are two broad ways to do that, and each has consequences for the roof.
Ballast
A ballasted system is held by weight, usually blocks or slabs sitting on the frames, so that nothing has to pass through the waterproof layer. The benefit is that the membrane is not pierced. The cost is the extra load on the roof, which has to be checked, and the work of moving the ballast when the roof needs attention.
Fixings through the covering
The alternative is to fix the frames to the deck or the structure. This keeps the load lower, but each fixing is a hole through the covering that has to be made watertight, usually with a purpose-made detail. Every penetration is a place where a leak could start if it is done badly.
Mixed systems
Some arrays use a combination, with fewer fixings and some ballast. The roof then has both issues, though in smaller amounts.
Which is better?
Neither is right for every roof, and the loads involved change with the building's height and position. The choice depends on the deck, the covering, the loads and the wind exposure, and the installer and roofer should agree it before work. Ask how each fixing will be sealed, who is responsible if it leaks, and what happens to the detail if the covering is renewed. A system that cannot be lifted without damage creates a problem later.
The covering underneath and how long it has left
The most important check is the condition of the covering that the panels will sit on. If it is near the end of its life, the panels will have to come off to replace it, which costs money and time.
Typical lifespans
Published guides give ranges for flat-roof coverings, and they do not always agree. HomeBuilding gives EPDM 25-30 years, where Less gives 30 to 50. For GRP, HomeBuilding gives 25-30 years and Less 25 to 40. For felt, HomeBuilding gives 10-15 years and Less 10 to 20. HomeBuilding gives lead on a flat roof 60-100 years. These are typical ranges, not promises.
What the numbers mean for solar
A covering with several years left is a good base, and one at the top of its range is a poor one. If the roof is old, renewing it before the array goes on is often the sensible order, because it avoids lifting a working system later. Our post on the longest-lasting flat roof material compares the options.
Different materials, different jobs
Lead sits outside BS 6229 and follows BS 6915, so a lead roof needs its own approach. Single-ply membranes such as EPDM and GRP have their own methods for the details around fixings.
Look at the condition, not just the age
A younger roof can be in poor condition, and an older one can be sound. The survey looks at blisters, splits, lifted seams, ponding marks and the state of the upstands, which is a better guide than the date alone.
Falls, ponding and drainage around the array
Water must be able to leave the roof. An array can change how it does.
The fall
BS 6229:2025 sets out the design of flat roofs with flexible waterproof coverings, for roofs with a pitch up to 10 degrees. Its minimum finished fall for general areas stays at 1:80, while IKO advises designing for 1:40. A roof laid to those falls drains well, and one that is flatter holds ponds.
Panels and ponds
Standing water under or beside an array is a problem. It sits on the membrane, adds weight and can reach details at the frame feet. Frames and ballast should be placed so that they do not block the path of water to an outlet, and the outlets should stay clear and reachable.
Access for clearing
Leaves and debris collect in outlets and gutters, and someone has to clear them at least once a year, more often near trees. If the array leaves no safe way to reach an outlet, or no room to work, the roof will drain badly. A walkway and a clear margin make maintenance possible.
Keep outlets reachable
Whatever the layout, leave outlets and gutters where a person can reach and clear them.
If standing water is already a problem, solve that first. Our guide to GRP fibreglass roofs and the pages on falls explain what a repair involves, and a survey shows whether the fall is adequate.
Checks before anything is fitted
A short sequence of checks avoids most of the problems that arise when solar goes onto a flat roof.
On the roof
- Survey the covering, upstands, outlets and deck, and record them with photographs.
- Decide whether the covering has enough life left, or should be renewed first.
- Check that the falls drain properly and that outlets are clear.
- Have the loading assessed for the deck and structure.
- Agree the fixing method, and the detail at every penetration.
On the paperwork
Ask the installer how the roof has been considered, and who is responsible if it leaks where the array is fixed. Ask what happens if the covering has to be repaired or renewed, and who lifts and refits the panels. Keep the roof survey with the installation records, because a future buyer or roofer will want to see it.
On the building
Check the position on consents. A flat has no permitted development rights, a listed building needs listed building consent for work that touches its character, and a conservation area or Article 4 direction may apply. For example, Basingstoke and Deane's directions for four town-centre conservation areas cover the installation of any solar panels, including to the roof. For other addresses, ask the council.
Fire and compatibility
Approval is also needed when a new covering would react in a markedly different way in a fire. Ask that the covering and the panel system be compatible.
Roof work later, with panels in place
A roof under an array still needs maintenance. Planning for it at the start saves cost later.
Inspection and repair
A roof with panels needs a survey from time to time, and a repair under or around the array can mean lifting part of it. Frames that are easy to unfasten, and cables that can be disconnected by the installer, make that simpler. Agree who carries it out and who pays.
Renewal
When the covering reaches the end of its life, the array has to come off. The cost of lifting and refitting should be allowed for in planning. A covering renewed before the array goes on is the best way to avoid this, and a covering chosen with a long life reduces it.
Who to call
The roofer and the installer are different trades. Roof repairs under panels should be agreed with both, so that the electrical side is dealt with by the person who fitted it, and the waterproofing by someone who can see the whole roof.
What we do
We survey the roof before an array goes on, and we record its condition in photographs and measurements. If a repair will do, we recommend a repair; where the covering needs renewing first, the survey gives the reasons. One written price follows.
If you are weighing up the roof against the panels, the sensible first step is the survey, and the flat roof building regulations guide explains the approvals that apply.
