What is Damp Proofing?

What exactly is damp proofing and how is it carried out?

by David Morton, Senior Surveyor, CSSW, CSRT

Damp proofing in construction – new or existing buildings – is a type of moisture control applied to the walls, floors and roof structures to prevent moisture from passing into the interior spaces. Dampness problems are among the most frequent problems encountered in residences.  Essentially, everything surrounding the design of a structure is based upon preventing water ingress into the fabric of the building.

When considering rising damp problems, this can occur for a variety of reasons, including:

• Failure of an existing damp proof course (DPC)
• Bridging caused by raising of external ground (flower beds) or internal floors. This can also occur where internal suspended timber floors have been replaced with solid floors.
• Absence of a DPC, often in older buildings.
• The occurrence of dampness as a result of hygroscopic salt damage.
• Salt efflorescence on wall surfaces.

Permeable building materials such as brick, stone and mortar allow damp from the ground to rise by capillary action, carrying with it ground salts, which become deposited in the wall structure.  Due to their presence, when the humidity in the building is high, dampness may become more apparent.

Rising dampness within a wall, in its true form, rarely extends higher than around 400-500mm above the floor level.  However, in some situations, the signs of dampness can sometimes go significantly higher.  This is generally due to the presence of hygroscopic salt contamination of the plaster.  These are chlorides and nitrates, which are deposited in the wall substrate as a result of water rising from the ground.  The other type o f salts we have to consider are efflorescent salts, which are sulphates.

Hygroscopic (moisture absorbing) salts are very different to salt efflorescence.  Hygroscopic salts will remain in the wall substrate and migrate into the plaster.  They will continue to absorb relatively high amounts of atmospheric moisture, such as condensation, or residual moisture from the wall substrate, resulting in the appearance of visible damp patches.

Salt efflorescence can appear on the surface as fluffy white deposits, or white marks on the plaster, which indicate moisture has been present but has since evaporated and dried.  These salts will not continue to absorb moisture in the way that hygroscopic salts do but can appear on the surface of plaster during the drying process following remedial damp proofing works.  Usually, they can just be brushed off the surface.  Sometimes, it may be useful to use a salt neutraliser, such as a mixture of clean water and white vinegar (ratio 1:10) to clean off the salts and prevent them reoccurring.

It is important to mention that the amount of water that is absorbed by (hygroscopic) salt contamination will vary with the humidity of the atmosphere within the room/property.

Some of the questions we are asked include:

1. How much does damp proofing cost?
2. How long does damp proofing last?
3. How long does the work take?

How much does damp proofing cost?

This is always such a difficult question to answer, as “damp proofing” is dependant on so many things, which include:

• The type and structure of the walls, such as: whether the walls are solid brickwork, which is most properties built before the 1920’s, cavity construction (after the 1920’s – 1930’s), solid stone or rubble-filled stone and the overall thickness of the walls.
• The extent of damp proofing required within a property. We generally would treat the walls affected by rising damp with a new, injected damp proof course but would not necessarily treat walls which are not affected by damp.
• The extent of internal re-plastering which needs to be carried out to remove existing, salt contaminated plaster.
• Whether or not we use a salt retarding render product on the internal walls, or a lime-based renovating plaster.
• Then, of course, there are the associated costs, which could be applicable, such as; the cost of removing radiators, electrical fittings and other fixtures and fittings to facilitate the requires damp proofing works.

The most important thing to do is to arrange an established company with years of experience carrying out damp proofing work, who are members of the Property Care Association (PCA).  They will arrange for one of their CSRT (Certified Surveyor Remedial Treatment) Surveyors to carry out a detailed inspection of the specified area and provide a detailed report and quotation.  This report should list the type, age and construction of the property, together with their Observations both internally and externally, together with their Recommendations for works required i.e. installation of a Chemical Damp Proof Course, together with associated works, such as rendering / replastering using specialist products.

How long does damp proofing last?

The guarantees issued in respect of damp proofing works will be for a ten year period, which is the industry standard damp proofing guarantee period.  There was a period when guarantees issued were for 30 years, which then dropped to 20 years.  Approximately 7-8 years ago, the directive was that guarantees for such a long period of time were unreasonable and so were dropped to TEN YEARS.

In reality, damp proofing materials, when used correctly by experienced, registered installers, will last considerably longer than the guarantee period.  Aging tests carried out by the manufacturers of damp proofing products have proved this to be the case.

How long does damp proofing work take?

This is very dependent upon the extent of the damp proofing work.  Generally, the procedure, once we arrive at the property, would be; to remove the existing render and plaster from internal wall surfaces, clear up and bag up the resulting debris, inject the new chemical damp proof course, apply the salt retardant render coats, or lime-based renovating plaster and apply a skim / finish plaster.  Even for the smallest jobs, the work will be done over two days but typically, work is likely to take between 3 and 5 days.