FLAT ROOF LEAK
DETECTION: ELECTRICAL CAPACITANCE TECHNIQUE
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
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
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.
|The equipment cannot be used on wet
roof surfaces, or those which are electrically conductive.
|The technique doesn't actually pinpoint
a leak - it simply identifies the area for close examination.
|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.
- Mineral surfaced bituminous
- pvc single ply
|- Metal faced bituminous
- epdm single ply
- other electrically conductive
Suitable roof surface finishes
for capacitance technique.
|There is no cutting of the roof membrane
or damage caused by sharp probes.
|The technique clearly identifies
areas of wet insulation, which otherwise would not
be apparent beneath level roof surfaces.
|The technique can be used on unheated
buildings as a thermal gradient through the roof is
not required for the equipment to work.
|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
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.
|RCI Technical Note No.12, May 1990
- Detecting Moisture in Roofs.
|RCI Technical Note No. 34, November/December
1993 - Infra Red Thermography.
|RCI Technical Note No.35,
January/February 1994 - Flat Roof Leak Detection:
Using Portable Electrical Conductance Technique.