These electrical moisture meters can only be used to accurately assess the level of moisture in timber. This was the task it was 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.

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: "that a specialist contractor on rising damp and timber defects be instructed to inspect and to carry out their recommendations in full."

The purchaser may then proceed to commission specialist remedial treatment contractors either selected from say, "Yellow Pages" 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’

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 £3000. In addition, the mortgage lender may hold back a retention until the work the Contractor recommends is completed!

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. The moisture 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 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 damp 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.