Why the level matters
A damp-proof course only works if it is the lowest continuous path into the wall. It is normally set at least 150mm above finished external ground level, so that splashing rain and surface water cannot reach the masonry above it. Every time an external surface is raised, that margin is eaten into. A new patio laid over the old one, a driveway resurfaced in tarmac, a border built up with topsoil, gravel piled against the wall, a raised planter or a levelling screed for decking can all take the external level above the course. Once that happens, ground moisture and surface water enter the wall above the barrier and it is bypassed entirely, however sound it may be.
What the internal damp looks like
The pattern closely resembles rising damp, which is why bridging is so often treated as rising damp. Damp appears at the base of the wall, up to perhaps half a metre, with blown plaster, perished skirtings and sometimes a tide mark. It is present for much of the year and worsens in wet weather because surface water adds to it. The clue is usually outside rather than inside: if the internal floor level is at or below the level of the paving outside, or if a rendered plinth runs down into the ground, moisture has a route in regardless of what has been injected into the masonry. Comparing internal and external levels with a straight edge and a spirit level takes a few minutes.
Other ways the course gets bridged
Ground level is the most common bridge but not the only one. Render or a cement fillet carried down over the damp-proof course provides a continuous path around it. A solid floor screed laid tight to the wall can link floor moisture into the masonry. Mortar droppings in a cavity, debris built up at the base of a cavity, or cavity insulation that has slumped can all carry water across. Steps, ramps, thresholds and conservatory floors built against the wall have the same effect. So do air bricks that have been buried or blocked, because a sub-floor void that cannot dry keeps the surrounding masonry and timber damp.
Why the wall stays damp even after treatment
If a chemical damp-proof course is injected into a wall that is being bridged, the injected line may perform perfectly while the wall above it remains wet, because the water never passes through the treated band. Owners are then told the treatment has failed, or that a second course is needed. Occasionally a second course is installed, higher up, which conceals the mechanism further. The only reliable way through this is to identify the bridge and remove it, then allow the wall to dry and deal with any salt-contaminated plaster. That sequence usually costs less than the treatment that did not work.
How the correction is done
Where the ground can be lowered, the usual approach is to excavate a margin against the wall to at least 150mm below the damp-proof course, with the surface falling away from the building, and to maintain a clear gap so that splashing cannot reach the masonry above the course. Where the level cannot be reduced, because of a shared access, a threshold or a change of use, the alternatives are a drainage channel with a linear drain or a French drain connected to a working outlet, or in restricted situations a vertical damp-proof membrane against the buried face. Render should be stopped short of the ground with a bell mouth or drip, not carried into it.
What to check around your own building
Walk the perimeter and note, at each elevation, where the ground surface sits relative to the internal floor and to any visible damp-proof course line in the brickwork, which often shows as a slate or bitumen joint or a change of mortar. Check whether paving falls towards or away from the wall. Look for air bricks that are buried, blocked or below the paving. Look at where downpipes discharge and whether gullies are clear. Note any planter, border or store built against the wall. Photographs of each elevation, with a tape measure held against the wall, are exactly what a surveyor needs and often show the cause before anyone arrives.
When to have it assessed
If low-level damp persists after a damp-proof course has been installed, if a quotation for injection has been given without any reference to external levels, or if you are about to lay a new patio or drive against the house, it is worth having the levels and the drainage assessed first. Our inspections examine the external condition and levels as carefully as the internal readings, and our reports state plainly where bridging is the mechanism. In many cases the recommendation is groundwork and drainage rather than damp proofing, which is a different trade and a different cost.
Getting it right when new paving is being laid
The cheapest time to deal with ground levels is before the paving goes down. If you are having a patio, path or drive laid, ask the contractor to keep the finished surface at least 150mm below the damp-proof course, to fall away from the building, and to leave a clear margin against the wall rather than butting the paving up to it. Where the level cannot be dropped, specify a linear drainage channel discharging to a working outlet, not a soakaway against the wall. Make sure air bricks remain clear, fitting periscope vents if the level rises. Keep render stopped above the ground with a drip. None of this adds significantly to the cost of the job, and it avoids a low-level damp problem that is expensive and disruptive to correct afterwards.
Limitations of general guidance
This article is general guidance. It cannot confirm the cause of dampness in a specific property. Recommendations should follow an inspection of the building, its external environment and its internal conditions.
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