The obvious place to use a continuous damp-proof membrane is under the oversite concrete. The membrane is spread on a layer of comparatively dry concrete, clinker or ash which is spread and levelled over the hardcore as illustrated in Fig. 26. The edges of the membrane are turned up the face of external and internal walls ready for concrete laying so that it may unite and overlap the dpc in walls.
The membrane should be spread with some care to ensure that thin membranes are not punctured by sharp, upstanding particles in the blinding and that the edge upstands are kept in place as the concrete is laid.
The advantage of a damp-proof membrane under the site concrete is that it will be protected from damage during subsequent building operations. A disadvantage is that the membrane will delay the drying out of the oversite concrete that can only lose moisture by upwards evaporation to air.
Where underfioor heating is used the membrane should be under the concrete.
Fig. 26 Below concrete damp-proof membrane.