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They frequently enter domestic roof spaces. Once inside they chew woodwork, strip insulation from electrical wiring and water pipes, tear up fibreglass insulation and, occasionally, drown in water tanks.
Treatments
Physically blocking gaps and entry holes with wire mesh is the best answer once the squirrels have gone. Various types of trap are available as is a poison based on warfarin, but only to professional technicians. Release of grey squirrels caught in cage traps is illegal.