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.
 
A Chartered 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 damp 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 damp-proofing companies and surveyors, often members of the Property Care Association (PCA), 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 Property Care Association has published two leaflets with very similar titles. On is entitled ‘The use of moisture meters to establish the presence of rising damp’ and the other ‘Establishing the Presence of Rising Damp’ and both state that “moisture meters do not provide a percentage reading of the moisture content since readings may vary from material to material” and that “the electrical method does not give a direct reading of moisture content” but despite this guidance and proof that moisture meters are not calibrated for use on plaster most PCA members rely solely on the use of moisture meters to diagnose rising damp.
 
They then recommend the installation of an injected chemical damp proof course and associated internal re-plastering. This work causes a fair amount of mess internally as it involves chopping off plaster, removing skirting boards, re-plastering and also injecting a damp-proof course into the walls. After this the householder has to wait up to a year to allow the walls to dry before any redecorating can take place.
 
TBuilding 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.
 
A Chartered 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 damp 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 damp-proofing companies and surveyors, often members of the Property Care Association (PCA), 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 Property Care Association has published two leaflets with very similar titles. On is entitled ‘The use of moisture meters to establish the presence of rising damp’ and the other ‘Establishing the Presence of Rising Damp’ and both state that “moisture meters do not provide a percentage reading of the moisture content since readings may vary from material to material” and that “the electrical method does not give a direct reading of moisture content” but despite this guidance and proof that moisture meters are not calibrated for use on plaster most PCA members rely solely on the use of moisture meters to diagnose rising damp.
 
They then recommend the installation of an injected chemical damp proof course and associated internal re-plastering. This work causes a fair amount of mess internally as it involves chopping off plaster, removing skirting boards, re-plastering and also injecting a damp-proof course into the walls. After this the householder has to wait up to a year to allow the walls to dry before any redecorating can take place.
 
The contractors quote for the work they propose might well be in excess of £4,000. 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 chemical damp-proofing and timber treatments are needed but in most of the houses that we survey there is never any need to inject a chemical damp-proof course or chemical timber treatments.
 
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 using a Carbide ‘Speedy’ meter.
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 with the volume and pressure of gas being directly proportional to the amount of moisture in the sample. 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.
 
Another alternative is to put a plaster or brick sample in a petri dish and have it weighed then dried in a moisture balance ( such as Adams AMB50 ) which will give a true percentage reading of the actual moisture content.
 
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.
 
By following the detailed instructions in BRE Digest 245 the moisture content due to the presence of hygroscopic salts can be determined and if this is subtracted from the total moisture content the amount of capillary moisture can be calculated.
 
A sample from the wall is weighed to give us the wet weight (WWET) and then the sample is placed in a humidity chamber for 24 hours to allow it to come to its equilibrium weight under controlled conditions (RH 75%, temperature 20oC) and the sample is weighed again to give us the weight under equilibrium conditions (WRH75)
 
The sample is then oven dried to remove all the moisture and then weighed give us the dry weight (WDRY ).
 
From these weights we can then calculate the Total Moisture Content, Hygroscpoic Moisture Content and Capillary Moisture Content.
 
Total moisture content (TMC) is calculated as follows :
 
TMC = WWET- WDRY/ WWET X 100
 
and Hygroscopic moisture content (HMC) is calculated by:
 
HMC = WRH75 - WDRY/ WRH75 X 100.
 
The Capillary Moisture Content (CMC) ,the moisture that causes dampness is calculated by subtracting Hygroscopic moisture content(HMC) from Total moisture content (TMC),
 
So,
 
Capillary Moisture Content = Total moisture content - Hygroscopic moisture content
  
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 considering installing a chemical damp-proof course.
 
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.
PDF copies of the leaflets published by the Property Care Association- ‘The use of moisture meters to establish the presence of rising damp’ and ‘Establishing the Presence of Rising Damp’ are available on the request
  
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.