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There is the rear guttering that drains into a water butt that obviously overflows in heavy rain right by the rear wall of the house, and cellar. That's the worst affected area so hopefully an easy fix - dig a soak away further down the garden or route runoff into existing drain that kitchen waste goes into.
It's an old house, maybe 150 years, what else should we be looking at as the culprit if not just "underground"? We have a copy of a survey which highlights the damp but doesn't identify any causes.
Arducius-
It's an old house, maybe 150 years
There is the rear guttering that drains into a water butt that obviously overflows in heavy rain right by the rear wall of the house, and cellar.
Jesus wept
What @TW said, I'd be eliminating all potential culprits; cracked drains, shoddy brick / pointing, water getting in via the chimneys (if present), fucked guttering, stupid water butt arrangements, leaking pipework...you need to get to a point where you know everything else just works so that the damp in the cellar is 'just' underground damp.
Howard
Even if it’s that basic they don’t have to be damp; that the sub floor is getting fucked by it suggests something else is wrong. Investigate.