Vapour barriers
A moisture barrier is required in all strip footing excavations on Class H sites that exceed 700 mm in depth. The barrier must be double layer polythene and 0.2 mm thick.
The polythene barrier provides a separation between the concrete strip footing and the ground. It prevents 'down drag' (or upheaval) from the ground to the footing.
Soil suction at depth in cohesive reactive clay can adversely affect the footings. In turn, this affects the building being supported by the footings. The polythene allows sideslip between the footing and the soil along the vertical sides of the footing.
