What does it mean?
A damp proof membrane is a sheet or other specified barrier used to control moisture passage through an assembly, often within a ground floor build-up. It needs continuity at joints and interfaces. Estimating should distinguish net floor area from laps, upstands, penetrations and damage allowance. The correct specification depends on the complete detail and site conditions.
In practice
A 6 m × 4 m floor has a net area of 24 m². The order quantity can be larger because roll widths, joint laps and perimeter upstands consume additional membrane beyond that simple plan area.
Keep in mind
When preparing an estimate, record the specification reference, units and source of each assumption. Separate the net measured quantity from purchase allowances so changes remain easy to trace. Confirm product dimensions, yields and installation requirements with the supplier. Quantities do not establish structural adequacy or material suitability: use the project’s approved design and details, and seek the responsible designer’s clarification where the specification is incomplete.