A Guide to Avoiding the DPC When Laying a Driveway
The damp-proof course (DPC) of a house is of paramount importance in keeping damp at bay. If your new driveway bridges the DPC, then you could be letting yourself in for a whole host of problems.
What is the Damp-Proof Course?
The idea of a DPC is to prevent the water in the ground from being drawn up into the walls of the house by a process known as capillary action. The bricks act like sponges, although they aren’t that efficient, but over time water can get pretty high up a wall. This leads to damp problems, with plaster damage and mould forming on walls, which can cause respiratory problems for people in the house.
Putting a waterproof barrier (the DPC) a few courses of bricks above ground level prevents the water rising past that level. Modern houses will have a layer of black polyethylene which is flexible and comes on a roll. This can usually be seen outside, just above ground level. Older houses might have slate or a row of clay bricks, both of which are denser and more resistant to capillary action.
Dangers of Bridging the Damp-Proof Course
Provided that the DPC isn’t broken or bridged then it will probably do its job for as long as the house stands. Bridging is more likely to be a problem with driveways, although it’s not exclusive to them. Bridging occurs when the ground level outside the house rises up, for whatever reason, so that it is above the DPC.
This means that water can get at parts of the wall that are unprotected, allowing damp to start entering the house higher up the walls. This often happens as a result of soil building up on flower beds adjacent to the house, but you need to be aware of it with driveways, too.
Driveway Installation
Bridging is most often a problem when renewing or constructing a driveway or path means raising the ground level to a point where it gets too close, or even above, the level of the DPC. This must of course be avoided, although it still happens today, usually when a householder has been tempted into using an unqualified or unscrupulous driveway contractor.
The current minimum distance from the exterior ground level to the DPC is 15cm (about 6in) as specified in the England and Wales building regulations. But older DPC’s may be lower than this and many will be a lot higher, particular with houses built on sloping ground. In fact, a DPC may be stepped to cope with the change in ground level around a house.
So, if the height of the driveway brings the ground level up to less than 15cm (6in), then the driveway design will have to change to cater for it. If excavating to lower the whole of the driveway is impractical or too expensive, the solution is to create a gap between the driveway and the wall of the house. But the gap must have Good Drainage.
Drainage Channel
How you achieve that good drainage depends on the lie of the site, the access to existing storm drains and gullies, and the likelihood of heavy rain in the area. If there is good access to the storm drains and the fall of the channel can be engineered so that there is a good slope to the drains, the gap can be left empty. Consider lining the bottom with a dished paving slab to encourage water into the centre.
If drainage isn’t as good, you’ll need to slow the water down in the gap itself, so consider placing a perforated drainage pipe in the channel then filling it with gravel. A raised lip on the edge of the driveway adjacent to the channel, perhaps using a slightly proud kerb stone, will help steer water away from the channel and ease the drainage problem.
Levels are Important
Pay particular attention to levels when planning the rest of the driveway, putting in falls where you can get away with it that will divert rain water away from the problem area.
Get Ready for Rubbish
But finally, be aware that the gap tends to be attractive to all sorts of windblown rubbish. So, keep it as narrow as possible, so that it’s not a danger to pedestrians, but wide enough to get hands or brushes in to sweep it out.
By: Chris Hogan