HOW EXTENSIVE IS RISING DAMP?
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It is a frequent worry for home owners and purchasers, but is “rising damp” really as common as we are led to believe?
“Householders and even some damp surveyors are too quick to assume that problems with dampness are caused by rising damp. In fact, true rising damp is not very common.
Because the remedies for the control of rising dampness are so expensive it is doubly important to ensure the diagnosis is correct before starting damp control work.” Building Research Establishment (BRE) Good Repair Guide 6 – Treating Rising Damp in Houses – January 1997
The BRE, which until recently was UK Government funded, have been highlighting that dampness was being misdiagnosed since the early 1980’s!
“Because of the high cost of remedial work, it is essential that the diagnosis is as positive as possible to distinguish between rising damp and other sources of damp.”
BRE Digest 245 – January 1981 “ How Extensive Rising Damp in Walls: Diagnosis and Treatment”
“Investigations have revealed many instances in which systems intended to combat rising damp have been installed in buildings where rising damp is not occurring.
A frequent reason for this has been a wrong interpretation of high readings obtained when using an electrical moisture meter. Another reason was the failure to recognize other causes of the damp conditions.” Building Research Advisory Service, Technical Information Leaflet TIL 47 August 1982
“The diagnosis of rising damp needs careful and systematic thought because it can easily be confused with penetrating dampness and condensation. The Building Research Establishment (BRE) have suggested that only 10% of the dampness problems it investigates are attributable to rising damp. Unfortunately, there are a number of damp- proofing companies specializing in d.p.c. replacement who obviously have a commercial interest in finding problems with rising damp. The diagnosis needs to be treated with caution.
Although there are several reputable companies working in this field, it may be wise to seek independent advice. Further “encouragement” to find problems of rising damp is provided by banks and building societies who often request that house purchasers obtain a damp & timber report which will include a price for damp proofing and timber treatment as a condition of a mortgage loan advance. Understanding Housing Defects (Estates Gazette) 1998
There are many damp-proofing contractors advertising specialist services to remedy dampness by installing damp-proof courses. Yet most apparent rising dampness cannot be attributed to the absence or failure of a damp- proof course.” The Remedial Treatment of Buildings by Barry Richardson 1995
THE NEED FOR ACCURATE DIAGNOSIS
“Dampness of one sort or another is the most common problem in housing. It results in visible wetting of walls, ceilings and floors, blistering paint, bulging plaster, mould on the surfaces and fabrics and sulphate attack on brickwork It can also lead to less obvious problems – thermal insulation is reduced in effectiveness or brickwork because metal components embedded in it have corroded. As with all remedial repair work, the first step to solving any damp related problem is to diagnose the cause correctly.” B.R.E. Good Repair Guide 5 – Diagnosing the Causes of Dampness, January 1997
“Before any measures are undertaken, the problem should be analysed in order to identify the cause properly. In the first instance professional advice should be obtained rather than that of a specialist damp-proofing contractor.” The Repair of Historic Buildings (English Heritage) by Christopher Brereton
Often specialist remedial treatment companies report “they have diagnosed rising damp” and specify remedial chemical damp-proofing treatments which is possibly inappropriate, to be carried out by themselves.
If a specialist damp contractor is to be used they should be a member of the Property Care Association who are more likely to provide an accurate damp assessment. However, an Independent damp and timber surveyor with specialist knowledge of Historic Buildings will look at the property holistically using his/her residential survey experience to provide the broader picture without the potential influence of profit on the opinion given.
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 damp 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 meter all round the house in order to install a chemical d.p.c. when the damp 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
A speedy carbide survey damp test in masonry 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 damp 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. The formula for determining the percentage of gravimetric moisture content is: ww – wd x 100% – ww – where ww is the wet weight and wd is the dry weight.
Hygroscopically bound moisture content is significant in most masonry and plaster due to water bound to salts. This is defined in BRE Digest 245 as the moisture bound to a material at equilibrium with air at relative humidity of 75% at 20°C.The formula for the percentage of hygroscopic moisture content is: W75 – Wd x 100% – W75 – where W75 is the weight after equilibrium at 75% RH at 20oC
PCA/Property Care Association-Code of Practice- The Installation of Remedial Damp-Proof Courses in Masonry Walls
A positive diagnosis of rising damp cannot be done if it is being obscured by other faults such as leaking gutters and down pipes, bridging of the physical damp-proof course by external renders and high ground levels etc . Checks of these defects should be carried out and remedial repairs undertaken.
A specialist damp surveyor should recommend that the client remedies these first and then allow a period of time for the areas of wall to dry out naturally before going to the expense of installing a chemical damp-proof course to treat rising damp and the cost of disruptive damp-proofing and re plastering to the internal walls of your house.
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