Dampness diagnosis
Building Surveyors, when carrying out inspections for
Building Societies or homeowners, will check for
dampness using an electrical conductance moisture meter
most commonly manufactured by "Protimeter".
These electrical moisture meters can only be used to
accurately assess the level of moisture in timber as
this is the primary task they were designed for. When
used in other materials it can only record the presence
of surface moisture and is unable to determine the true
moisture content existing below the surface, i.e. within
the wall. High readings ( often mis-diagnosed as
dampness) can also be obtained from plasters containing
black ask mortar and also foil-backed wallpaper so the
you should be wary of any surveyor or damp-proofing
company relying solely on the use of moisture meters.
The Surveyor is therefore only able to detect surface
moisture, and will often report: "there is evidence of
dampness and remedial damp-proofing work in this respect
is required". Alternatively: "you need to instruct a
damp-proofing and timber treatment contractor to carry
out a full investigation to investigate the full extent
of the problem and the necessary repairs required,
together with an estimate for any damp-proofing and
timber treatments."
The purchaser may then proceed to commission specialist
remedial treatment contractors offering a ‘free damp and
timber survey’’ found on Google or perhaps recommended
by the Estate Agent to inspect and provide a report and
estimate.
If you are a house-owner and invite a damp-proofing
company to inspect your home for dampness problems, the
surveyor will inevitably confirm that confirm that you
do have a damp problem which can be cured with a new
chemical damp-proof course.
These contractors, who normally offer a free survey,
also use electrical moisture meters and they are likely
to solemnly declare the presence of rising damp in a
report which usually states : "Random tests were carried
out with a moisture meter to both internal and external
wall at ground level. At the time of our inspection,
high levels of dampness were obtained in the following
areas... It is our opinion that these high levels of
dampness found at the base of the aforementioned walls
are attributed to rising dampness due to the absence of
an effective damp proof course’.
The funny thing is that when damp-proofing companies
undertake their free survey they will only use a
moisture meter but if there are any call-backs on their
20 year guarantee they will always take a plaster sample
from the wall to get the true level of dampness.
They would then recommend installation of an injected
replacement damp proof course and associated internal
re-plastering. This work would substantially disturb the
existing internal decorations.
The contractors quote for the work they propose might
well be in excess of £3,000. Nowadays with lenders
having very strict lending criteria there will probably
be a retention on the mortgage until the damp-proofing
and timber treatment works recommended by the Contractor
have been completed. This is fair enough if
damp-proofing and timber treatments are needed but in
most of the houses that we survey there is never any
need for chemical damp-proofing or timber preservation
works.
The contractor's opinion that rising damp exists has not
been substantiated by any in-depth testing as
recommended within Building Research Digest 245 "Rising
Damp in Walls - diagnosis and treatment".
Building Research Digest 245 recommends that samples of
brickwork are taken from within the wall and laboratory
analysis undertaken to determine the actual amount of
capillary moisture which is present. This test is
invasive as holes are drilled (10mm diameter) in walls
to obtain plaster and brick samples but it is a lot less
destructive than having plaster chopped off to a height
of approximately 1 metre all round the house in order to
install a chemical d.p.c. when the treatment is not
necessary. After collecting samples we can make good
walls and the cost of accurate diagnosis usually results
in avoiding unnecessary expenditure on disruptive, messy
work, which often results after incorrect diagnosis.
Moisture content of samples can
be determined by 2 methods.
1. Carbide or Speedy test
A measured sample of brick dust or plaster and a measure
of calcium carbide are placed in a special pressure
cylinder. Any dampness in the test sample reacts with
the calcium carbide to form acetylene gas. This gas
creates a pressure, which registers percentage moisture
content on an appropriately calibrated pressure gauge.
The carbide meter reading is not affected by salts and
moisture content readings from within the thickness of
the wall and can be obtained in approximately five
minutes. Actual moisture content is determined rather
than the Wood Moisture Equivalent (WME) readings
displayed on a moisture meter.
2. Gravimetric or oven-drying method
The Carbide test provides actual moisture content but
does not determine Hygroscopic Moisture Content (HMC)
and Capillary Moisture Content (HMC).
All building materials are hygroscopic and absorb a
certain amount of moisture depending on humidity levels
etc and no amount of ‘damp-proofing’ will remove this
and it is not doing any harm. The moisture that concerns
people is actually capillary moisture i.e. within the
capillaries and pores of the wall.
Hygroscopic Moisture Content is determined by allowing
the sample to come to its equilibrium weight under
controlled conditions and the sample is then oven dried
to determine the Capillary Moisture Content. If this
sample is taken from the base of a wall and has a
moisture content of less than 5% it is unlikely to be
affected by rising damp.
Other sources of damp ingress, such as leaking gutters
and downpipes, bridging of physical damp-proof course by
external renders and paving etc., should also be checked
and repaired before.
In over 95% of the surveys that we undertake no remedial
damp-proofing works are required and we can still
provide reports and guarantees that you may require for
your mortgage company.
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