Outside ground levels are easy to change
New paving is often laid over old rather than excavated to the original level. Flower beds are built up and driveways resurfaced. Over decades the finished ground can creep upwards until it sits close to or above the DPC.
Render can carry water past the barrier
External render taken continuously down across the DPC can create a bridge, particularly if the lower render is wet from splashback or contact with the ground.
Cavity debris is hidden
Mortar droppings and rubble at the bottom of a cavity can connect a wet outer leaf to the inner wall. This can produce low-level internal damp even though the DPC is present in both leaves.
Internal finishes matter too
Plaster carried down into contact with a damp solid floor or masonry below DPC level can create an internal bridge. The visible defect may therefore sit entirely on the room side.
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