When a plumber was moving my washing machine from one side of a room to the other he drilled a channel along the concrete floor, about 2 to 3" deep, to sink the water pipes, and clear plastic, the DPC, was visible. He then filled the channel with concrete. When removing the rubble I noticed a few small broken pieces of this plastic, as if the drilling broke it. The house, a dormer, was built in 1978, this room was a garage with a bedroom overhead, part of the main house. The garage was converted a few years later, when the floor was built up, and I think this DPC was laid then. I'm concerned there will be a problem because the plastic was torn, but also wonder if there would already have been a lower DPC when the house was built, even though it was a garage, i.e. should a DPC have been laid under a garage in 1978? Thanks.