Start where roof meets wall
Flashings and upstands have to turn rain away from the original wall. Cracked render or open masonry joints above the flashing can let water behind it.
Check the pitch
Some tiles are unsuitable below a certain roof angle. If a shallow extension has leaked since it was built, measure the pitch rather than repeatedly replacing individual tiles.
Movement at the junction
New and old parts of a building can move slightly differently. Rigid mortar repairs at the junction can crack again if the detail needed flexibility.
Water can arrive from above
Windowsills, pipe penetrations and wall cracks above the extension may feed water to the same internal stain. Include them in the inspection.
Want local roofers to look at the job?
Describe what you have noticed and, if you can, include when it happens and which part of the roof seems involved.
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