Rising damp normally occurs in older properties where the brickwork or the mortar between the brick work is more porous.

Mortar in older properties looses its adhesive properties, becomes soft  and crumbles easily, almost reverting back to a sandy base, this becomes absorbent and can pass water through it.

Rising damp hardly ever occurs in newer properties where the bricks and cement mortar courses are sound, dampness in these building are generally caused by either penetrating damp or condensation.

How does rising damp occur?
Water moves up and through the porous mortar lifting from the ground salts (Chloride & Nitrates), capillary action. Once the rising moisture is above floor level, the water evaporates and leaves the salt deposits behind. The evaporation often draws more water up and in turn leaves more salts behind.
The longer the moisture is allowed to rise the more the salts will become evident. This process will damage internal decoration, damage internal plastering and will, if the moisture content is high enough, allow timbers attached to the problem walls to rot.
The chloride & nitrate salts are hygroscopic. Hygroscopic salts attract moisture from either the walls or the atmosphere  (condensation).

What is penetrating damp.
Penetrating damp is any damp which comes through the outer walls or roof.
Main causes of penetrating damp are:- poor fitting or rotten door/window frames, broken gutters/down pipes, broken render or damaged pointing, blocked or bridged cavities.
Penetrating damp often washes sulphate salts through the walls which are left behind when the moisture evaporates, these salts are efflorescent and do not absorb moisture from the atmosphere.
Stopping the water ingress generally clears up any problems.

Testing for dampness.
Meters used during our inspection are basically conductivity meters, an electrical current is passed through a medium i.e. decoration, plaster, wood or brickwork to see if the medium conducts electricity, generally these items do not conduct electricity if dry.

A recent television program made fun of this type of testing, the programs front man went around testing his head and the camera. Both of these item conducted electricity and showed a reading on the meter he came up with the conclusion that they were both wet and needed treatment. Common sense will tell you that the human body is made up of water and that the camera body is made of metal, but both will conduct electricity.

Testing only with use of a meter is open to personnel interpretation,  a visual inspection of the property is as important as any meter tests. A few things to look for are broken gutters/down pipes, built up ground behind damp walls, a lack of sub floor ventilation, broken water pipes to name but a few.

The only guaranteed test is a  carbide meter or commonly known as a 'speedy meter' test, this is very disruptive and is carried out by making a hole internally in the plaster every few feet around the entire property to test the plaster and in turn the mortar between the brickwork to check for moisture content.
A vender of a property is very unlikely to allow a 'speedy' test to be carried out as the internal decoration be ruined, the test will also take several days and will be very expensive.

Cob Buildings
Some older properties are naturally damp and are best left this way, cob built buildings are constructed of earth, aggregates and straw, the inner and out areas of the main walls are covered in a lime plaster/render and finished of with a lime wash.
The use of lime allows the walls to breath and for the moisture which is absorbed to evaporate out naturally, modern finishes do not allow the walls to 'breath' and therefore moisture will build up and eventually lead to problems.
External renders, timbers and roofing should be kept in good condition, ant damage to these areas will lead to water ingress which may lead to further problems.

Further information on Cob buildings can be obtained from

Devon Earth Building Association 50 Blackboy Road, Exeter, Devon.
Or email  pchild@env-dept.devon-cc.gov.uk