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This Technical Note continues the series discussing non destructive tests for identifying leakage paths through roof membranes. Previous issues have covered infra red thermography and the electrical conduction technique whilst this Technical Note covers a third form of testing - the electrical capacitance technique, which has been in use for almost fifteen years.

Principle of Operation

Wet insulation has different electrical properties from that which is dry. This can be determined from above the roof and without puncturing the membrane by the use of an electronic capacitance meter. The base of the instrument is fitted with two rubber electrodes which transmit low frequency signals through the roof covering. When the signals come in contact with a conductive layer, in this case moisture, the circuit is completed and an audible and visual signal is transmitted. The area of a leak can be identified by simply moving the instrument across the roof surface following the strongest signal.


There are two types of instruments available. The hand held Leak Seeker is suitable for covering small areas and close up around details such as upstands. A telescopic handle helps to reduce backache when covering bigger areas!

For continuous operation, the larger Dec Scanner can be used.. This wheeled trolley which covers the full width of a standard roll of bituminous felt, weighs just under 10kgs and is pushed along the roof much in the same way as a carpet sweeper. However, it is more difficult to use on built up roofs covered with chippings.

Both pieces of equipment are battery operated and don't require mains power supplies or additional cabling.

Method of Operation

Before starting, the roof surfaces must be dry.
The instrument is switched on and the battery level checked. For the Dec Scanner there is a simple calibration procedure to be run through over an area of dry roofing. The depth of field can be varied on the Leak Seeker.

It is important to adopt a systematic method of work, moving the instrument across the roof to ensure that all of the relevant areas are tested. On large roofs it may be appropriate to look at a selective sample of the roof, say every fifth roll or every grid line, although this of course introduces the risk that not all areas of wet insulation will be identified.

The extent of wet areas can be marked directly onto the roof surface using a yellow wax crayon, or alternatively by marking onto a roof plan as shown in the illustration. This enables the roof inspector to concentrate his attention on selected 'wet areas' such as at the highest point of the slope or perhaps around penetrations and detail work.

If a more general pattern of wet insulation is found, then there could be an interstitial condensation problem instead of one of rainwater leakage.

For inspections where the extent of remedial works is being determined, it is recommended that sample cuts are made in a wet and in a dry area, to confirm readings and to agree a wet/dry threshold.


1. The equipment cannot be used on wet roof surfaces, or those which are electrically conductive.
2. The technique doesn't actually pinpoint a leak - it simply identifies the area for close examination.
3. It is important that the results are interpreted properly. In the wrong pair of hands, the instrument can be a dangerous tool when, for example, an over zealous inspector demands that large areas of roof with moist but not saturated insulation, are dug up and replaced.


Suitable Not Suitable

- Mineral surfaced bituminous felts

- pvc single ply

- Metal faced bituminous felts

- epdm single ply

- other electrically conductive surfaces

Suitable roof surface finishes for capacitance technique.


1. There is no cutting of the roof membrane or damage caused by sharp probes.
2. The technique clearly identifies areas of wet insulation, which otherwise would not be apparent beneath level roof surfaces.
3. The technique can be used on unheated buildings as a thermal gradient through the roof is not required for the equipment to work.
4. Large areas of roofs can be covered relatively quickly, enabling an overall picture of the roof condition to be built up. This can be particularly useful for identifying the extent of a roof in need of repair.


In conclusion, the electrical capacitance technique is a useful tool to assist in the tracing of leaks through membrane roofs and identifying wet insulation. The smaller hand held device is considered by some to be an essential part of a roof inspector's tool kit, whilst the larger and more expensive Dec Scanner can be hired in for covering larger areas of roofing.


1. RCI Technical Note No.12, May 1990 - Detecting Moisture in Roofs.
2. RCI Technical Note No. 34, November/December 1993 - Infra Red Thermography.
3. RCI Technical Note No.35, January/February 1994 - Flat Roof Leak Detection: Using Portable Electrical Conductance Technique.

© Keith Roberts, 1998

First published in Roofing Cladding Insulation, June 1994

RCI Technical Notes

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