Clod spotting and decay to plasterboard wall
Damp build up behind a plasterboard stud wall (basic ‘tanking’ solution), causing plasterboard to decay.
Half hight external render (prior attempt at damp proofing, that had made the whole thing worse – moister unable to escape to outside, so all going inside, to plasterboard)
Root cause 1, high ground levels (patio) above DPC causing breach external
Root cause 2, cement slurry added to flooring and allowed to run to walls edge and breach DPC internal