I have repositioned a down stairs toilet and during the excavation i had to got through this 20 mm black material. I am not sure of its purpose other than it could be to match up different floor levels as this is in an extended part of the house. i don't think it is for damp as there is a membrane beneath the 50 mm of concrete floor. The house was built in the 50's and i believe extended in the 60's.
Thank you
Thank you