Mortar droppings bridging the cavity
What it looks like
Damp patches on internal wall faces at low level or below window openings, often appearing weeks after the wall is closed up and worst after driving rain.
Why it happens
Mortar squeezed from bed joints falls down the cavity and collects on wall ties or cavity trays, forming a solid path for water to cross from the outer to the inner leaf.
What it costs if it is missed
Persistent internal damp, spoiled plaster and decoration, and in bad cases mould growth. Remedial work means opening up finished walls.
How to inspect for it
Inspect the cavity before it is closed: use a cavity mirror and torch at every tie course, check the base of the cavity and above every tray and lintel. Photograph the clear cavity as evidence.
How to prevent it
Bevel the bed joint towards the outer leaf, keep a cavity batten on the ties and lift it every course, and clear the cavity base before the DPC course is covered.
Basis in regulation
Last updated 23 August 2026. This is our editorial summary of publicly available regulation and site practice, not building control approval or a warranty determination.