Updated
Ground Level Above The Damp Proof Course
Paving, decking or soil has been built up the outside wall, burying the black line of the damp proof course.
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What's happening?
The damp proof course is a horizontal barrier low in the wall that stops ground moisture rising into the masonry above. It only works if it is above the wet ground. Approved Document C is specific: on an external wall the damp proof course should be at least 150mm above the level of the adjoining ground. Once a patio, a raised bed, a new driveway or a timber deck is built up to or over that line, water in the ground can enter the wall above the barrier and the DPC is bridged — the barrier is still there and still working, it has simply been outflanked. The symptoms show inside as damp low on the wall, blown plaster and skirtings that decay.
What usually causes it?
- New patio, path or driveway laid over the original level without lowering it or forming a drained channel
- Raised planting bed or border built directly against the wall, holding wet soil above the DPC
- Timber decking built up against the wall, trapping debris and moisture at the junction
- Render or a plinth carried down over the damp proof course, providing a continuous path across it
- Ground falling towards the house rather than away, so surface water runs to the wall
- Air bricks buried or partly buried by the raised level, so the sub-floor void stops ventilating
- Debris or mortar in the cavity base, which Approved Document C addresses by requiring the cavity to run at least 225mm below the lowest DPC or a tray instead
Is it dangerous?
Not dangerous to be around, but it is one of the most reliable ways to rot a house slowly. Sustained damp in the base of a wall wets skirtings, joist ends and wall plates, and the Property Care Association is clear that timber allowed to stay excessively wet will decay. Buried air bricks make it worse by shutting off the ventilation a suspended timber floor depends on, which is also how woodworm gets the humidity it needs. Damp plaster picks up ground salts and will not dry until the bridging is removed.
Can I fix it myself?
Some of it, and it is worth doing. Clear soil, mulch, leaves and debris away from the wall and find the DPC — usually a visible line of bituminous felt or engineering bricks two or three courses above the original ground level. Dig out a raised border, cut back overgrown planting, and clear any buried air brick. Lifting and relaying a patio, forming a drained channel along the wall, or altering levels and drainage is a bigger job, and hacking off a render plinth without knowing what is behind it is not a good idea.
When should you call a builder?
Call a builder where paving, decking or a driveway has to be lifted and relaid, where a drained channel or French drain along the wall is the answer, or where ground levels and surface water drainage need altering. Call one sooner if there is already damp showing inside, if skirtings are soft, or if air bricks are buried — the ventilation matters as much as the damp barrier.
What will the builder actually do?
- Locate the damp proof course and measure the actual shortfall against the 150mm Approved Document C expects
- Lower external levels, or form a drained channel or French drain along the wall where levels cannot be reduced
- Lift and relay paving to fall away from the house and connect surface water to a proper outlet
- Cut back render or a plinth that bridges the DPC and reinstate it clear of the barrier
- Clear and, where needed, extend or re-form air bricks so the sub-floor void ventilates properly
- Replace decayed skirtings, joist ends and plaster once the wall has been allowed to dry
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