A bridged damp proof course is one that's still there and working, but moisture is getting around it, through high ground, a raised path or patio, render carried down over it, or debris in the cavity. Removing the bridge is usually the fix, rather than injecting a new DPC.
This is one of the most common causes of low-level damp I find, and one of the most commonly misdiagnosed as rising damp. The DPC is there, it hasn’t failed, but water has found a way past it.
How a DPC gets bridged
A damp proof course is a horizontal barrier in the wall, near the bottom, to stop ground moisture travelling up. It only works if nothing gives the moisture a route around it. Common bridges include:
- High ground levels. Soil, flowerbeds, gravel or a lawn piled up against the wall above the DPC.
- Paths and patios. A new patio or driveway laid at or above DPC level, often sloping towards the house.
- Render and pebbledash carried down over the DPC to the ground.
- Cavity debris. Mortar droppings and rubble at the base of the cavity, sitting above DPC level, which lets moisture cross from the outer to the inner leaf.
- Internal bridges. Plaster carried down onto a solid floor, a new floor screed laid higher than the old one, or a concrete floor with its membrane not lapped to the wall DPC.
- Leaning garden walls, steps, or extensions built against the house without the DPC detail being continued.
What it looks like
Bridging often looks like rising damp: low-level damp, peeling paint and wallpaper, and blown plaster. The clue is usually that it’s localised. It matches where the patio is, or where the flowerbed sits, or a particular section of cavity. On one job in Royston the damp was highest in the middle of one wall and tapered off towards each end. When we opened it up, the cavity was packed with debris sitting on the DPC.
Fixing it
- Lower the ground, or cut back a path or patio, so there’s a clear gap below the DPC. Where that isn’t possible, a gravel-filled channel or a drainage channel can help, but get advice so you don’t undermine foundations.
- Cut render back to above the DPC with a proper bellcast or stop bead.
- Remove cavity debris through small openings in the outer leaf.
- Inside, finish plaster above the floor rather than down onto it.
Once the bridge is gone, the wall needs time to dry, and salty plaster may need replacing. Clear the bridge first, then decide whether anything else is needed.
When to call a specialist
If you have low-level damp and suspect a bridged DPC, a survey will find the bridge and tell you whether clearing it is enough. Book a survey with All Dry on 01226 287181 or email info@alldrydampproofing.com.
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Frequently asked questions
Can a bridged DPC cause rising damp symptoms?
Yes, it produces low-level damp that looks very similar, even though the DPC itself hasn't failed.
How do I fix a bridged DPC?
Remove the bridge, such as high soil, render or cavity debris, then let the wall dry.
Does a bridged DPC need injecting?
Usually not once the bridge is removed, though each case should be assessed.