Damp history.

Nine times out of 10 a building with damp problems will date to before the 1920s when the vast majority of buildings didn't have a damp-proof course (DPC). There are exceptions: some decent builders in the past installed damp-proof courses, for example, in Georgian buildings, out of slate. You also sometimes come across new buildings that don't have a DPC, although building regulations require them.

Sources of damp

There are three sources of damp: rising damp, in which the moisture comes up from the ground (you can see the tide mark up to a certain height) and external penetration, in which the damp comes through faults in the building, such as poor pointing and missing roof slates. The third type is internal moisture which can't escape from the building and forms condensation and creates mould on walls and clothes. This is more common in modern buildings.

Rising damp

The traditional way of treating rising damp is to take plaster off to a certain height - around four or five feet. You drill and inject a chemical DPC or insert electric wires that stop the damp rising above them. There are pros and cons to the various methods of controlling rising damp but we've found that the most successful is combination of drilling and injection of a DPC and applying a vertical membrane such as Thoroseal. Rising damp may not be apparent as some solutions cover up the problem without solving it. If you put in foil-backed paper or bitumen you will hold the moisture in the wall and chase it up higher. Where moisture comes in through faults in the building it causes paint and plaster to flake off. If it's left for too long you risk getting dry rot, which is a real problem in older buildings.

Dry rot

Dry rot affects timber and in older buildings the timber is structural (in modern buildings most structural members are made of steel or concrete). The rot is a parasitic mushroom that thrives on damp wood and once it gets a hold it can do an awful lot of damage. It can travel through mortar in brickwork. August and September is the growing season for dry rot because it is warm and humid. There is an aura of mystique surrounding dry rot and I've heard of buildings being condemned because of it. As far as I'm concerned, every building can be saved.

Cure the rot

Dry rot isn't the end of the world and can be dealt with. It is caused by moisture, so you need to remove the moisture source and then treat the rot. Ideally, if you had time, you should isolate the rot, let it dry out and monitor what happens. If there is no time for that, take out the rotting timber and treat the masonry with a fungicide. You also need to compromise. Text books will say you have to cut the timber back to a certain depth but you need to work with the building. If cutting it back too far means wrecking original ornate plaster you can find another solution.

Let it breathe

The key to ensuring a property is free of damp and its related problems is to keep the building watertight but allow it to breathe. People find it very hard to believe that a black-stained ceiling and green mould on clothing is caused by moisture in the air. Condensation is a particular problem in some new apartment blocks. The apartments comprise minute boxes which are sealed up like Fort Knox during the day. A couple will be out at work and then return to their small-roomed apartment, have a shower and cook a meal. The moisture is sealed into the building and sits in the air. When the temperature drops it hits the cold surfaces as condensation. Black mould thrives in damp warm stagnant air. The mould germinates and lives off minute dust particles - it thrives on natural fabrics.

Discrete damp No two buildings' damp problems are exactly the same even if they are identical and next door to each other. What matters is how the building has been maintained and how it is used, how many people live there, whether there are three teenagers all taking showers every day, if there are fireplaces and what types of decorations there are. Heavy duty vinyl wallpaper will stop water from evaporating, for instance.

Condensation

Condensation has developed as result of new construction, the way people live now and the fact that more houses are left during the day. In the past there was nearly always someone at home: the heat was on and the house was ventilated as doors were opened and closed. Open fireplaces and draughty windows also ensure a change of air. Air is the best thing for timber - wooden floors that aren't ventilated will rot. All the treatment in the world is a waste if you don't have fresh air.

Emma Cullinan is the editor of Select magazine