Rain commonly enters at joints above and beside openings
Failed sealant, cracked render and open mortar around a frame can admit water. At the head, defective trays or lintel details can move moisture sideways before it appears.
Sills must throw water clear
A blocked or ineffective drip on a sill allows rain to curl back towards the wall below. End joints and cracks can also direct water into the reveal.
Cold reveals favour mould
A thin or poorly insulated reveal can be much colder than the main wall. Speckled black mould close to the frame, especially in winter, may therefore be condensation rather than rain penetration.
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