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“Signs of dampness were detected, and it is advised to have the damp proof course checked by a specialist contractor.” Not the words you want to read on your home-buyer's survey when you have found your dream property, adding extra pound signs to those already accrued. But is it as bad as it looks? Mortgage surveyors tend to be very cautious, and the slightest blip on their ‘moisture meter’ makes them recommend calling in the specialists.

Most properties are built with protection to stop moisture from the ground from travelling up in to the walls above ground level. These damp proof courses include slate or flint layers, a molten bitumen coat, a sheet of a damp proof membrane, and courses of impervious bricks.

This old bitumen damp course has expanded over the years.

The problem is caused when, over the years, these originally incorporated systems become damaged through thermal or ground movement, or by the bonding materials breaking down through weathering and age.

During a general house-buyer's survey, ground floor walls are checked, most usually, with a two-pin type conductivity, or moisture, meter. A qualified remedial surveyor will have been trained to recognise the pattern of readings obtained with one of these meters, and be able to differentiate between rising damp, penetrating damp, and condensation.

Rising damp is moisture rising within the masonry, carrying salts which then cause the plaster to deteriorate, and wall paper or paint finishes to become stained. This is usually in a band at the lower part of the wall, of not really higher than one metre. If confirmed, a remedial dampcourse will need to be installed, along with replacement of plaster in the affected areas.

Rising damp salt band seen on external wall. (Photo courtesy The Wykamol Group.)

What’s The Treatment?

A damp-proofing contractor will install a dampcourse, usually through drilling a series of holes along the lower part of the wall, and injecting the fluid, cream or mortar that they have chosen as the best material for a particular masonry. Some contractors chose, on a cavity wall, to check that the cavity is clear and then treat only the internal leaf. On a solid wall, it is possible to inject from one side only, as long as the dampcourse material is injected in to the wall in sufficient quantity and to a sufficient depth to allow penetration along the whole wall thickness. This method can avoid unsightly drill holes on the outside of the house.

If a property has its dpc installed retrospectively, such as the chemical injection type, the plaster should also be replaced. If the wall has suffered from rising or penetrating dampness for any period of time, ground or masonry salts may have contaminated the plaster.

It is reasonable, though, for a contractor to recommend leaving the existing plaster on the wall after the treatment, and checking several months later to determine the extent of plaster replacement required. During the drying out, the salts will surface, and can be removed with the old plaster, and therefore not migrate in to the new plaster. The only problem with this is that home owners, understandably, want all of the work done at once so that the mess is out of the way!

Is It Really Rising Damp?

If you wall looks or feels damp, eliminate other causes first before either paying for treatment, or, if it has already a dpc installed, before complaining to the contractor!

A typical rising damp pattern has stained this wall paper. Check though, that it is not caused by the plaster's contact with a damp solid floor - there should be a gap behind the skirting board.

If your plaster is completely broken down, i.e. you can almost take it off the wall with a spoon, then it is unlikely to be to the recommended specification. Did you pay the damp-proofing contractor to also do the plastering? If yes, call him back!! If you suspect that the property has not had a remedial damp proof course, then it may be the original plaster.

Do you have patches of mould on the walls? Mould generally indicates condensation (see Condensation page). The salts associated with rising damp usually cause the wall surface in that area to be unattractive to mould growth.

Check outside. Is the render cracked? Do you have any leaking gutters or downpipes? Look out for lichen growing on the outside of walls. Are fixing brackets rusting? How sound is your pointing? Any of these can allow rainwater to penetrate from the outside. Keeping on top of general repair and maintenance can help you to avoid many costly problems, including wet and dry rot.

Dampness in the lower parts of the wall is sometimes attributable to the outside ground level being higher than the internal floor level. The property may be built in to a slope; sometimes over the years garden paths or flower beds increase in height, causing them to bridge the existing dampcourse, rendering it ineffective. If your external ground and internal floor are about the same height, it may be worth digging out a channel around the house, about 200mm wide and at least 300mm deep, and then filling with a pea gravel. This will act as a French drain (soakaway), and water laden soil, and any rainwater running down the outside of the property, will be taken away, thus the moisture is not held against the foot of the wall.

Plastering & Plasterboards

If 'plain' plasterboards are used on external damp walls, salts can migrate throught the adhesive dabs and spoil the decor. Backed, insulated boards should be used in these situations.
 
Failed DPCs?

Sometimes a reasonably dry wall lies beneath inadequate plaster, which may not have been replaced for a variety of reasons, and your mortgage surveyor has picked up moisture meter readings because the plaster contains salts which cause it to retain moisture.

Problems can occur when the damp-proofing contractor does the ‘drill-and-inject’ part of the job, and the home-owner gets another contractor to re-plaster. Frequently, the correct re-plastering specification is not used, or the job not undertaken at all! The problem is that when the wall was damp, the plaster absorbed salts. As the wall dries out following the installation of a damp-course, these salts can become more concentrate, and the plaster can look worse that it did before the damp-course was put in.

If, in this case, you check the wall with a pin or pad type moisture metre, you will get high readings. This is because the plaster, being salt contaminated, will absorb atmospheric moisture. The actual wall itself could be completely dry. When this type of contention occurs, it is necessary for various tests to be carried out.

A sample of the plaster can be sent a laboratory for analysis, to check if it is consistent with a post-dampcourse specification. A core sample can be drilled out from the masonry, to test the actual moisture content within the wall. Often this is well below acceptable limits, thus indicating that the problem lies with the plaster. Also, the plaster can be tested to see if ground salts are present.

Contractors, naturally, are unhappy with having to organise these tests, due to the costs involved, and the use of their own time, only to prove that there is nothing wrong with their work! Home-owners, though, require these to determine what the problem is, and whether to order re-injection, or re-plastering. It would be fair then, that the home-owner pays the contractor for these tests when the problem does not lie with their work. Maybe the payment could be lodged with a third party? If any guarantees are held, can the guarantee company act as go-between? As if moving home was not bad enough already!

Correct replastering, symathetic to the masonry, is very important.

Types of DPC