What causes damp patches on internal walls?

Damp patches are usually caused by one of three things: penetrating damp (water coming through the wall from outside defects), rising damp (groundwater wicking up due to a failed DPC), or condensation (internal moisture hitting cold surfaces). In basements, it's often hydrostatic pressure forcing water through the wall. Correct diagnosis is vital; injecting a DPC won't fix penetrating damp from a leaking gutter. MPS conducts thorough surveys to identify the root cause.