Watch the gutter in proper rain
The useful question is where the flow stops behaving normally. Water may surge over one section because the outlet is choked with moss, the gutter has dropped between brackets or the downpipe cannot clear the volume.
Look behind it as well
A gutter can leak at its back edge and wet the fascia without a spectacular front overflow. Old roofing felt may also sag behind the fascia and collect water arriving from higher on the roof. That hidden route is easy to miss.
Cleaning may not solve the fall
Once debris has gone, water should travel towards the outlet. Long puddles show where levels are wrong. Brackets can loosen over time or be refitted badly after fascia work.
Repeated wall damp
If brickwork below the gutter is green, washed or persistently damp, deal with the drainage first and allow it to dry. Decorating the internal wall before the outside wetting is corrected usually brings the stain back.
If you are ready to get the roof inspected
Give the contractor the symptom rather than your diagnosis. The weather pattern and a few safe photographs can be useful.
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