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.
 
The cost of a timber survey can be found here and can be arranged by phoning 0800 028 1903 or click the enquiry button to the left of the screen.