Polythene And Polyethylene Sheet

Polythene or polyethylene sheet is often used as a damp-proof membrane with oversite concrete for all however extreme circumstances of dampness. It is recommended that the sheet must be at the least 0.25 mm thick (1200 gauge). The sheet is provided in rolls four m extensive 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 150 mm at joints and continued throughout surrounding walls, beneath the dpc for the thickness of the wall.

Where site situations are reasonably dry and clear, 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 achieve success the sheets should be dry and clear else the jointing tape will not adhere to the surface of the sheets and the joint will depend on the burden of the concrete or screed pressing the joint sufficiently closely to make a watertight joint. As clean and dry conditions on a building site are rare, this type of joint needs to be only used where there may be unlikely to be heavy absorption of ground moisture.
Where site conditions are too wet to use 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 saved in place by weighing it down with bricks or securing it with tape till the screed or concrete has been placed. The double welt is formed by folding the sides of sheets together after which making a welt which is flattened.

The plastic sheet is effectively unattainable to fold and so stiff and elastic that it'll all the time are likely to unfold so that it requires a deal of persistence to fold, hold in place after which contrive to fold along the joint. By using the utmost size of sheet available it is potential to minimise the number of joints.

The sheet ought to be used in order that there are solely joints one way as it's impractical to form a welt at junctions of joints.

Where the extent of the damp-proof membrane is below that of the dpc in partitions it's necessary to turn it up in opposition to partitions in order that it will possibly overlap the dpc or be. turned over as dpc as illustrated in Fig. 31. To maintain the sheet in place as an upstand to walls it's necessary to keep it in place with bricks or blocks laid on the sheet in opposition to partitions till the concrete has been placed and the bricks or visqueen sheeting blocks removed because the concrete is run up the wall.

On the internal angle of walls a reduce is made in 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 effective barrier to rising damp, offering they aren't punctured or displaced throughout subsequent building operations.