Polythene or polyethylene sheet is commonly used as a damp-proof membrane with oversite concrete for all however severe situations of dampness. It is recommended that the sheet needs to be not less than 0.25 mm thick (1200 gauge). The sheet is supplied in rolls 4 m large by 25 m long. When used beneath concrete oversite the sheet must be laid on a blinding layer of sand or compacted fuel ash spread over the hardcore.
The sheets are spread over the blinding and lapped a hundred and fifty mm at joints and continued across surrounding partitions, underneath the dpc for the thickness of the wall.
The place site circumstances are reasonably dry and clean, the overlap joints between the sheets are sealed with mastic or mastic tape between the overlapping sheets and the joint completed with a
polythene jointing tape as illustrated in Fig. 29.
For this lapped joint to be successful the sheets should be dry and clean else the jointing tape won't adright here to the surface of the sheets and the joint will depend on the weight of the concrete or screed pressing the joint sufficiently heavily to make a watertight joint. As clear and dry situations on a building site are rare, this type of joint needs to be solely used where there's unlikely to be heavy absorption of ground moisture.
The place site conditions are too wet to make use of mastic and tape, the joint is made by welting the overlapping sheets with a double welted fold as illustrated in Fig. 30, and this fold is kept in place by weighing it down with bricks or securing it with tape until the screed or concrete has been placed. The double welt is shaped by folding the perimeters of sheets together after which making a welt which is flattened.
The plastic sheet is successfully inconceivable to fold and so stiff and elastic that it will always are inclined to unfold in order that it requires a deal of endurance to fold, hold in place and then contrive to fold along the joint. By using the maximum dimension of sheet available it is doable to minimise the number of joints.
The sheet ought to be used in order that there are only joints one way as it's impractical to kind a welt at junctions of joints.
The place the level of the damp-proof membrane is under that of the dpc in walls it is obligatory to show it up in opposition to partitions in order that it can overlap the dpc or be. turned over as dpc as illustrated in Fig. 31. To keep the sheet in place as an upstand to walls it is necessary to hold it in place with bricks or blocks laid on the sheet towards walls till the concrete has been positioned and the bricks or visqueen roll
blocks removed as the concrete is run up the wall.
At the inside angle of walls a reduce is made within the upstand sheet to facilitate making an overlap of sheet at corners. These sheets which are generally used as a damp-proof membrane will serve as an efficient barrier to rising damp, providing they are not punctured or displaced during subsequent building operations.