More Info on DPM & DPC
DPC and DPM are both materials to ward off moisture but differ in how and where they are applied. When there is a chance of water seepage through capillary action, DPCs are used vertically at ground construction or horizontally at the base wall to stop the damp from penetrating further. DPCs may comprise of metal, plastic, felt paper, slate and are now also made with modern technology in the form of bricks. A mason typically rolls out a layer of DPC over a bed of cement, cutting it as per the size required and adds another layer on top. This DPC is applied to a height of 150mm above the ground level. This way, if it rains the wall will not soak or let the damp within. A DPC may also be installed flat all along under the inner skin of a wall or floor, so as to combine it with a DPM.
DPMs are usually a thick plastic or polythene or polypropylene sheet made in different grammages and are bulky. These are generally spread out in a single sheet along the concrete slab to stop groundwater seepage into the floor above. DPMs have to be carefully extended at joints to reach the DPC applied at walls.
The basic difference between DPC and DPM is that the former is used in the walls and the latter in incorporated into the flooring. An “Oversite” is when the mason first installs a hard layer, then a soft layer of sand, with a layer of DPM and concrete on top.
Sometimes old houses may not have any damp proofing and will then require a “Retro” DPC fitting, which may be tedious, but goes a long way in keeping away damp while retaining the vintage house. This will need the wall to be cut into 1-metre sections with a heavy chainsaw. The DPC is then inserted into the gap, with slate used to fill the remaining gap. The entire house has to be covered section-wise, taking care to ensure that each DPC overlaps the next one.