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PolyDrainage Help & Info


Polythene (DPM)

What is Polythene? (Damp Proof Membrane)

DPM stands for damp-proof membrane. A damp proof membrane (DPM) is used to create a barrier between a concrete (or screed) subfloor and the floor covering that is laid on top such as wood, ceramic or quartz tiles, laminate, or carpet. This barrier is intended to stop moisture from passing from one to the other.

The Purpose of DPM

DPM is a ‘thick’ plastic sheet mostly 1000 – 1200 gauge polythene material.  The installation of the DPM is extremely important as this acts as a barrier to prevent damp rising up through the substrate, from the ground below.  Care must be taken to ensure:

a) there are no holes or tears in the sheet that could allow water penetration.

b) there is sufficient overlap of sheets by at least 150mm should more than one sheet be required.

The joint should then be taped to ensure a water-tight seal is established.

We recommend that you ensure the material to be used conforms to the relevant British Standards.  To use a thin, lower grade material and then find you have water ingress at a later stage, could prove to be a VERY expensive mistake.

It is normal that the DPM overlaps with the DPC (Damp Proof Course).  The DPC is normally laid within the bed joint of the brickwork or blockwork, primarily to stop ground damp from rising up the walls by capillary attraction.  The height at which the DPC is installed would normally be a minimum of 150mm above the external ground level.    The DPM should then be folded up the inside wall and lap over onto the DPC thereby forming a continuous water barrier.

Should damp be rising up through the floor, this will should that either there is a defect with the DPM and/or there is no DPM there at all – which of course is most likely with old buildings in particular.

How to apply DPM