Damp is one of those common terms we all know and often one of home buyers biggest fears but what do Surveyors really mean when they talk about damp?
Firstly damp is a relative term, some construction materials are drier then others, some will be wet when they are first used and dry out over time, some are even intended to stay wet and you have a serious problem if they dry out.
As a Surveyor, I like to define damp as being an unusually high level of moisture within a construction material AND the moisture is having a detrimental affect on the building element or is likely to do so in the near future.
The next question is how do you know if there are high levels of moisture? Aside from the obvious clues such as big damp patches, the way a Surveyor will determine the moisture level, in say a wall of piece of timber, is by using a moisture meter. Despite the name, a moisture meter does not measure moisture, it actually measures the electrical conductivity of a material. Water is a good conductor of electricity, certainly much better then wood, plaster or bricks. So the higher the moisture level the higher the conductivity. As a serious home-buyer you may wish to invest in one yourself, simple models can be bought for £100 with professional models ranging from £300 upwards.
If you do buy a damp meter is important to know how it is calibrated. The conductivity of timber is not the same as plaster or brick, so a meter designed for timber will not be very helpful when it comes to a wall. The other important thing to understand is what the readings are telling you; just because a wall may be reading as damp does not necessarily mean it is damp. There are plenty of other reasons you may get a high reading, for example the salts that can build up in a chimney over years.
So you have established that part of building is damp, be it a wall or some timber, the next question to ask is “is this actually a problem?” Many traditional buildings were designed to allow for a certain amount of damp, mainly because in those days it was almost impossible to keep it out! Builders allowed plenty of air to flow through a building to ensure that things could dry out. They also incorporated ideas such as dado rails, these hide the join between the bottom half of the plaster and the top half when the bottom section had to be replaced due to damp.
This was fine at the time but these days we expect our buildings to perform better and be warmer. Often this means blocking up vents, new windows and doors and increased insulation, all of which can reduce air flow. A wall may have been fine for 100 years with a certain level of damp but now problems can arise.
If a wall is damp you need to see if the plaster is damaged, is the paint flaking or powdered? If the timbers are damp, are there any signs of rot? Are there other elements nearby which may be affected if the damp spreads? Do you have any plans that may affect the air flow through the building?
Unless you are very confident in answering these questions then the best advice is always get a Surveyor in to have a look. It is even better to get them to take a look as part of a full Survey so you can find out if there are any other hidden problems lurking.