Membranes are normally 1200 gauge (300 micron) plastic. Slip membranes do not compensate for abrupt variations in level of the sub-base, which should be flat and smooth.
Any foundation or pile head that abuts to the slab soffit should be provided with an additional layer of membrane.
It is important to lay the membrane without creases and that it is overlapped at the edges by at least 300mm. Care must be taken to ensure that it is not damaged during the construction process.
The plastic sheet will inhabit the loss of water and fines from the concrete to the sub-base. However, in some circumstances, a polythene slip membrane may not provide sufficient resistance to water vapour.
Gas membranes and venting systems have become commonplace as more construction is carried out on contaminated land.