A leak detection system is a system that signals and locates a breach in a waterproofing layer without opening up the construction. On flat roofs this matters because the point where water appears indoors is rarely the point where the leak is: water travels along the vapour control layer or the deck before it drops through.
Two families are commonly grouped under the same name: permanently installed monitoring systems, and one-off measuring methods used to find a leak after the fact.
Permanent monitoring
In a permanent system, sensors or a measuring grid are built into the roof build-up, usually in or beneath the insulation layer. The system measures continuously or at intervals whether moisture is present anywhere in the build-up — for example by determining the electrical resistance or the potential difference between measuring points. With enough measuring points the system indicates not only that moisture is present but where.
The big advantage is time: moisture is reported before it becomes visible indoors, so the amount of wet insulation stays limited. The main limitation is that such a system essentially has to be included when the roof is built or refurbished — retrofitting means opening the roof up.
Electronic leak detection after the fact
For existing roofs there are measuring methods that need no built-in sensors:
- High-voltage spark testing. A conductive brush is passed over a dry membrane; at a pinhole a spark jumps to the conductive substrate below. Suited to unballasted, dry roofs.
- Low-voltage vector mapping. The surface is wetted and a voltage applied between a loop around the test area and the substrate. Where water passes through a hole, a current path forms that can be traced with a probe. This works under gravel, paving or a green roof.
- Vacuum testing. A clear bell with a vacuum placed over a soaped seam reveals leaking seams as bubbles — mainly used for checking welds.
- Smoke or tracer gas. Smoke or a tracer gas is introduced beneath the membrane and escapes at the leak.
- Thermography and moisture metering. Wet insulation cools differently from dry; a thermal camera can pick out saturated zones. This shows the wet area, not always the exact hole.
Requirements and practical points
- Electronic methods require an insulating membrane above a conductive substrate. On a concrete deck this works; where the membrane is separated from any conductive layer by dry, insulating materials, it does not work without a purpose-laid conductive layer.
- Match the method to the condition of the roof. A high-voltage test on a wet roof gives false readings; a low-voltage test on a bone-dry roof gives nothing at all.
- Detection does not replace inspection. A system reports moisture but says nothing about the remaining life of the membrane, blocked outlets or detached perimeter trims.
- Where it pays off. Above all on roofs where repair is expensive or disruptive: green roofs, ballasted or paved roofs, parking decks, roofs with solar panels, and roofs above spaces where water damage is costly (server rooms, archives, museums).
- Keep the survey report. A plan showing the located leak positions is valuable evidence in any warranty discussion with the roofer.
International guidance for electronic leak detection exists (from ASTM among others), setting out the methods and their preconditions; ask a contractor which method and which guidance they work to.
Related terms
- Flat roof
- EPDM
- Bitumen
- Green roof
- Vapour control layer
- Roof boarding
- Load-bearing insulation
Want to know more about building terms? Take a look at our knowledge base on fredsdiyplans.com.