Water on an upstairs ceiling
Note whether the mark sits below a chimney, valley, roof window or open roof slope. Check the loft from safe boarding after rain. If one rafter or patch of underlay is freshly wet, trace it uphill. If the whole loft is covered in fine droplets on a cold morning, read the condensation guide before assuming the covering has failed.
A slate or tile is out of line
Photograph it from the ground. One isolated piece may be a local fixing failure. Several displaced pieces after wind or repeated slips over time justify a wider fixing check. Do not try to push a slate back from a ladder.
Damp down an external wall
Watch the gutter in rain and inspect the downpipe outlet. Water behind the gutter can wet fascia and masonry without producing a dramatic front overflow. If the damp starts below a roof-to-wall junction, flashings and pointing also need attention.
Staining beside a chimney
Look at the leadwork, mortar, flaunching and the stack itself. If the flue is unused, internal condensation remains a possibility. Chimney leaks are rarely diagnosed well from one photograph taken from the pavement.
A leak after previous repair
Mark and photograph the new stain, compare the weather conditions and find out exactly what was repaired last time. Do not automatically repeat the same patch. The original route may have been missed or a second defect may exist nearby.