What is a damp proof course, and what does it do?
A damp proof course (or DPC) is a layer of material between the foundation and the rest of the building which prevents water from getting into the building fabric. In a new building it's standard and cheap to fit a DPC during construction. Traditionally these were two overlapped layers of slate or lead, although some old buildings do not have them at all. Modern buildings tend to use bitumen or polythene based materials (you can get recycled plastic DPC). Land drainage around a building is also important.
Do I need a new one?
Timber frame buildings with minimal foundations will only need small amounts of DPC material. It is important to fit a DPC here as timber can soak up moisture and will rot if not able to dry out easily. However whether masonry or brick buildings do the same is open to debate. There is a growing school of thought that ‘rising damp’ is an urban myth (or a complete con) and that retrofitting chemical DPCs can be a waste of money, quite apart from the environmental impact of the chemicals used.
Famously, Mike Parrett of Lewisham Council in London has investigated over 5000 buildings and has never found a sign of rising damp. In every case, damp was attributable to something else. To accurately test for moisture a company would need to do proper testing, such as a drill test (possibly time consuming and costly), rather than just using a damp meter. If you’re convinced you have rising damp then get a proper drill test done by an independent surveyor.
If you have an expensive DPC fitted and it doesn’t solve the problem you may find that you're not covered by the guarantee. A subsequent drill test which shows that there’s no ‘rising damp’ could be taken to indicate that the DPC worked and you just have some other problem - despite this other problem potentially being the case all along!
So what should I do?
First check your roof, gutters, pointing and ground levels (or get a building expert to do it). Dig out anywhere where the ground level is higher outside than in. In some cases, digging down 200-300mm and putting in stones or gravel will help drainage around the site. Fixing these problems will cure most damp problems.
The other possible cause of damp is condensation, which generally occurs lower down walls (where it’s coldest) again looking like ‘rising damp’. In this case you also need to look at your heating and ventilation systems. See our page How can I reduce condensation in my house?
If walls have been affected by long term damp even when the problem has been solved it will usually involve re-plastering internally as the original plasterwork will be affected by hygroscopic salts which could result in continued damp problems. Bear in mind that the building can take up to 12 months to dry out.
There can be a problem if the DPC has been bridged - for example with render. Water can potentially move up the render where it can’t move up the brickwork or masonry. This could also cause problems with frost damage. DPCs must extend through the full thickness of the wall, including pointing, applied rendering or other facing materials.
A side effect of retro fitting DPCs is that often in older properties, original lime based plaster (breathable so allowing moisture to move through it, and flexible) is replaced with strong cement renders. This can cause serious cracking problems at the point where the old plaster meets the new. Make sure you replaster with lime or a weak cement mix.
But I’ve been told I must fit a DPC
There is a concern that you have to fit DPCs to satisfy mortgage companies, but many will now not insist on the installation of a modern DPC in older properties if there is no evidence of a damp problem. Many more people are now aware of the many possible causes of damp, and if you can identify the cause of any problem (such as gutters/roofs etc) and solve it, perhaps with some feedback from a professional (such as through the AECB), then this is often fine.
Article on the misdiagnosis of rising damp, and the control of damp and its effects. Written by Tim Hutton of Hutton And Rostron.
Abbey Independent Surveys - http://www.abbeyis.com - 01400 273359
Offer independent surveys to identify the correct cause and remedial action for rising and penetrating dampness, wet and dry rot decay, wood-boring insects (e.g. woodworm), etc.
Independent Surveyors and Valuers Association (ISVA) - http://surveyorsweb.co.uk/ - 0800 970 8521.
Website lists independent surveyors
Dampbuster (Mike Parrett's website) - http://www.dampbuster.com
Includes a small photo gallery of damp problems, and details of books and DVDs Mike has published on the subject.
AECB: the Sustainable Building Association - http://www.aecb.net - 0845 456 9773.
Network of builders, architects, manufacturers, and organisations. Aims to develop, share and promote best practice in environmentally sustainable building. There is a list of members on the website.