Minimum thickness of a polythene damp-proof membrane
0.3 mm (1200 gauge)
Polythene sheet used as a damp-proof membrane below or above a ground-bearing slab.
Every figure for this topic is on one page: Cavity trays and DPC: where are they needed and how high?
Read the full answerA polythene damp-proof membrane under a ground floor should be at least 0.3 mm thick — 1200 gauge — and laid on a blinded, smooth base so it is not punctured.
Basis in regulation
How to check it on site
Walk the membrane before the pour and look for punctures over hardcore high spots; patch with the same material and tape.
Common failure
Thin polythene laid straight over sharp hardcore with no sand blinding, torn through in dozens of places by the time concrete arrives.
Last updated 23 August 2026. This is our editorial summary of publicly available regulation, not building control approval or a warranty determination.