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Waterproofing membrane being laid on a flat roof
Flat Roof Membrane Testing

Electronic Leak Detection London

The stain on the ceiling is rarely under the hole in the roof. Electronic membrane testing follows an electrical signal through the waterproofing to the breach itself, so the roofer repairs the right spot the first time instead of patching guesses.

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Quick Answer

Electronic leak detection finds holes in a flat roof membrane by passing a small electrical signal across it: the membrane insulates, the deck below conducts, and the current flows through any breach. Low voltage testing works on a wetted surface and through some ballast; high voltage testing sweeps a dry membrane with a charged brush. It locates the breach rather than the stain, and each one is marked for the roofer with a written report.

Why Flat Roof Leaks Are So Hard to Find

A flat roof is a membrane laid over insulation and a deck, finished with a fall so water drains to an outlet. When water gets through a pinhole, a split or a lap that has lifted, it does not drop straight through. It runs along the top of the deck, soaks sideways into insulation and follows the falls, timbers and joints until it finds somewhere to come out. By then it can be several metres from the hole.

That is why roofs get patched over and over. The roofer goes up above the stain, finds nothing obvious, adds a patch or a coat of sealant where the damage looks likeliest, and the ceiling is wet again after the next heavy rain. Our guide to a damp patch on the ceiling with no obvious leak covers why the stain misleads.

Electronic testing removes the guesswork. It reads the membrane itself, so the result is the breach, marked on the roof, whatever route the water took afterwards.

Low Voltage and High Voltage Testing

Low voltage testing (electric field vector mapping)

The roof surface is lightly wetted and a wire loop is laid around the test area. A low voltage applied between the wet surface and the deck sets up an electrical field that is drawn towards any breach, and the engineer reads its direction with a pair of probes, following it in until the arrows converge on the hole. It suits larger roofs, roofs with gravel ballast and some green roof and paving build-ups.

High voltage testing

The membrane must be clean and dry. A charged brush is swept across the surface, and where there is a hole, split or thin spot the charge jumps through to the deck and the detector sounds. It is fast and precise on exposed membranes, including detailing around outlets, upstands and penetrations where many leaks start.

Supporting methods

A moisture meter and thermal imaging show how far water has spread into the insulation and ceiling, which tells you whether insulation needs replacing as well as the membrane repairing. Where a leak turns out not to be the roof at all, such as a gutter, a parapet or a plumbing pipe in the ceiling void, the survey says so.

Roofs We Test

  • Flat roofs on houses, extensions, garages and dormers
  • Roof terraces and balconies over living space
  • Commercial flat roofs on offices, shops, schools and blocks
  • Single-ply PVC and TPO, bituminous felt, liquid-applied and hot-melt systems
  • Ballasted, paved and some green roofs, depending on the build-up
  • New roofs before handover, to prove the membrane before it is covered

Some roofs cannot be tested electronically: carbon-loaded black EPDM that conducts, and older build-ups with no conductive layer under the membrane. If yours is one of them, we will tell you before booking rather than after.

How a Flat Roof Leak Survey Works

1

Ask about the roof first

Before we book we ask for photographs, the rough size, the membrane type if known and where the water is showing inside. That tells us whether the roof can be tested electronically and which method suits it.

2

Inspect the roof and the ceiling below

Outlets, upstands, rooflights, flashings and penetrations are checked by eye, and the damp area inside is mapped with a moisture meter so we know the zone the water is reaching.

3

Test the membrane

For low voltage testing the surface is wetted, a perimeter wire is laid and the field is read with probes across the roof. For high voltage testing the dry membrane is swept with a charged brush. Either way, the reading leads to the breach.

4

Mark every breach

Each breach is confirmed, marked on the membrane and photographed. A roof with one leak often has more than one hole, so the whole test area is covered rather than stopping at the first find.

5

Report

You receive a roof plan with every breach located, photographs and the likely cause of each, ready for your roofer, managing agent or insurer.

What It Costs

The price depends on the roof area, the membrane and what sits on top of it: a small extension roof is a short visit, a large ballasted commercial roof is a day. Send photographs, the approximate size and, if you know it, the membrane type, and we will give you a fixed price for the survey before we book. It is agreed before we start and the written report is included.

For leaks inside the building rather than through the roof, our published prices cover every plumbing leak survey, and our damp and moisture surveys cover water that is not coming from a pipe at all.

Flat Roof Leak Detection Questions

What is electronic leak detection on a flat roof?
Electronic leak detection uses the fact that most roofing membranes do not conduct electricity while the deck beneath them usually does. A small electrical signal is applied across the membrane, and at any hole, split or failed seam the current finds a path through to the deck. Following that current leads the engineer to the breach itself, often a hole smaller than a pencil point, rather than to the stain on the ceiling below.
What is the difference between low voltage and high voltage testing?
Low voltage testing, often called electric field vector mapping, is done on a wetted roof surface: a wire loop is laid around the area, a low voltage is applied, and probes read the direction of the electrical field, which points towards any breach. High voltage testing is done on a clean, dry membrane: a charged brush is swept over the surface and an alarm sounds where the charge jumps through a hole to the deck below. Which one suits a roof depends on the membrane, the deck and what is on top of the waterproofing.
Why can't a roofer just find the leak by looking?
Water that gets through a flat roof membrane rarely drops straight down. It runs along the deck, soaks into insulation and follows timbers or falls, and appears inside wherever it finds a way out. The stain on the ceiling can be metres from the hole. Visual inspection finds obvious splits and failed upstands, but a pinhole or a lifted seam hidden under chippings can go unseen, which is how roofs get patched repeatedly without the leak stopping.
Which roofs can be tested electronically?
Most single-ply membranes such as PVC and TPO, bituminous felt systems, liquid-applied coatings and hot-melt systems can be tested, provided there is a conductive layer underneath, which a concrete deck normally provides. Timber and metal decks can be tested where a conductive layer was built in. Some black EPDM rubber contains carbon and conducts, and some older build-ups have no conductive layer at all, so we ask about the roof before booking and tell you if another method is the better choice.
Can you test a roof with ballast, paving or a green roof on it?
Often, yes. Low voltage testing can work through gravel ballast and some green roof and paving build-ups, because the test relies on water carrying the signal across the surface rather than direct contact with the membrane. Where the overburden is deep or includes an insulating layer above the waterproofing, some of it may need moving at the suspected area. We confirm this from photographs or drawings before the visit.
Is electronic testing better than a flood test?
A flood test shows whether a roof leaks by holding water on it for a period and watching the ceiling below. It does not show where the leak is, it loads the structure with water, and it can make an existing leak worse. Electronic testing locates the breach itself, needs far less water or none, and can be done on a finished roof in a few hours.
What do I get at the end of the survey?
Every breach found is marked on the roof so your roofer can repair it, and the written report includes a roof plan with the breach positions, photographs and notes on the likely cause such as mechanical damage, a failed lap or a detail at an outlet or upstand. The report suits a roofing contractor, a managing agent or an insurer.
How much does flat roof leak detection cost?
It depends mainly on the roof area, the membrane and what is on top of it. A small domestic roof or extension takes a fraction of the time of a large commercial roof with ballast. Send us photographs, the approximate size and the membrane type if you know it, and we will give you a price before we book. It is agreed before we start and does not change on the day.

Get a Roof Survey Price

Send photos of the roof and the ceiling below, and the rough size. We will tell you if it can be tested and what it costs.

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For Roofers and Managing Agents

Every breach is marked on the membrane and plotted on a roof plan, so a roofing contractor or managing agent can price and scope the repair from the report.

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