What is a bridged damp proof course and how do I tell if mine is bridged?

The barrier in the wall is only as good as the fact that nothing goes round it. It is a horizontal line, and water is perfectly happy to travel up the outside of a render coat, across a doorstep or through a lump of mortar and arrive above the line without ever passing through it. This is why an injected chemical course so often fails to fix anything: the chemical goes into the wall at the level of a barrier that was already working.

What you'll need

• Torch
• Tape measure
• Trowel or small spade (optional)
• Cavity inspection mirror or an endoscope camera (optional)

Step by step

1. Check the external ground level first.Measure from the barrier down to whatever is against the wall — soil, paving, tarmac, gravel, decking. Anything within 150mm is a bridge, and anything level with or above it is a certainty.
2. Look for render or a plinth carried down past the line.A cement plinth painted black at the bottom of a wall is common and is very often taken straight past the barrier onto the ground. Water tracks down the back of it and enters the wall above the line.
3. Check whether the render has a bell cast or a stop bead.Render should stop above the barrier with a throated bead so water drips clear. Render run down to the ground with no break is a continuous wick.
4. Go inside and compare floor levels.Stand at the door and sight the internal floor against the external line. A solid floor, a screed, a self-levelling compound or a tiled build-up higher than the outside barrier bridges the wall internally.
5. Check the cavity if you have one.Lift or drill an inspection point only if you know what you are doing, or ask a surveyor. Mortar droppings sitting on wall ties or piled at the bottom of the cavity carry water straight from the wet outer leaf to the dry inner one.
6. Look at every planter, bed and step.A raised bed against a wall is a bridge that is deliberately watered all summer. A step or a doorstep built over the line is the same thing at the point of heaviest rain.
7. Check the internal plaster.Gypsum plaster carried down behind the skirting to the floor, or a cement backing coat applied by a previous damp treatment, can carry moisture up from the floor edge regardless of what the wall itself is doing.
8. Remove the bridge and then wait.This is the part people skip. A wall that has been wet for years contains a great deal of water and holds salts that attract more. Allow a full heating season after removing a bridge before judging whether anything else is needed.

Tips

• Removing a bridge is usually a day with a spade and a skip, against several thousand pounds for injection and replastering. Do it first and see what happens.
• If a damp survey does not mention ground levels, render terminations or internal floor levels, it is not a diagnosis — it is a quote.
• Take dated photographs before you dig. If you later need to argue with a vendor, a landlord or an insurer, the ground level before and after is the evidence.

Common mistakes

• Injecting a wall without checking for bridges — The chemical creates a barrier at one level in the masonry. If water is arriving above that level by travelling round the outside, through a render coat or across a floor, the injection cannot possibly intercept it and the wall stays wet.
• Judging the result a month after clearing the ground — Masonry dries at roughly a month per 25mm of thickness in good conditions. A 225mm solid wall that has been saturated for a decade takes a season or more, and it will look no better at all for the first few weeks.

If it doesn't work

Questions people ask