How do I find my damp proof course and check it is working?

Almost every UK house built since the 1870s has a horizontal barrier low down in its walls to stop groundwater climbing the masonry. Houses before that often have none, and were designed around it with thick walls, lime mortar and lime plaster that let moisture evaporate away. Knowing which you have, and where the line sits, changes every subsequent decision about damp.

What you'll need

• Torch
• Tape measure
• Small mirror or a phone camera on a stick (optional)

Step by step

1. Walk the outside of the house at ground level.Crouch down and look along the mortar joints on each elevation. You are looking for one joint that reads differently from the ones above and below it — thicker, darker, or with a slightly different texture.
2. Measure up from the ground.The line should be about 150mm above the finished outside ground, which is two brick courses. If you cannot find a line at that height, the ground may have been built up over it, which is the single commonest problem.
3. Identify what it is made of.Pre-1920s houses often used two courses of slate, which shows as a fine grey seam. Between the wars, bitumen felt, which looks like a thin black line. Post-1970s, black or blue polythene, which sometimes shows a small ragged edge sticking out.
4. Follow it round the whole building.It should be continuous. Where it steps up or down — at a change in ground level, at a bay, at an extension — the step is a detail that can be missed or badly built, and steps are a common leak point.
5. Check where it passes the door thresholds.A doorstep built over the line, or a threshold replaced without a new tray under it, bridges the barrier at exactly the point where rain lands hardest.
6. Look for anything crossing the line.Render carried down past it, a rendered plinth, paint, a path, decking, a planter, a step, a pipe collar, mortar splashes. Anything continuous over the line gives water a route around it.
7. Go inside and find the floor level.If the internal solid floor or its screed sits higher than the outside line, the wall is bridged internally, which no external inspection will show you.
8. Judge failure last, not first.Slate can crack on settlement and bitumen can be broken by later work, so genuine failure exists. But a barrier that is bridged behaves exactly like one that has failed, and bridging is far more common and vastly cheaper to fix.

Tips

• Photograph the line at each corner with a tape measure in shot. It gives you a record of the ground level, which creeps up over the years every time anything is landscaped.
• In a cavity wall there are usually two barriers, one in each leaf, and they may not be at the same height.
• A house with no barrier at all is not automatically a problem. Solid-wall houses built before they were required were designed to handle moisture by evaporation, and often do it well until somebody seals them with cement and plastic paint.

Common mistakes

• Assuming a damp patch low on a wall means the damp proof course has failed — The overwhelming majority of low-level damp in UK houses turns out to be a bridge, a leak, condensation, or salts left over from an old and already-cured problem. Failure of the barrier itself is uncommon.
• Not checking the internal floor level — A screed or slab poured above the external line bridges the wall from the inside where nobody looks, and produces textbook 'rising damp' symptoms that no amount of external work will cure.

Questions people ask