The Self Build Self Help Site Keeping Water Out
Keeping Water Out

Water has an uncanny knack of getting in to where it is not wanted, and when in, can either be just a nuisance to be mopped up, or can wreak a lot of damage - for example in timbers. The following text outlines some of the key areas to focus on, and for some of these, we have seen where things have actually gone wrong.


The damp proof membrane, on which the slab concrete is poured, often has to be joined. These joins must be watertight. Although the DPM is quite well separated from any underlying water - the sub-base material may be several inches deep and it may well be lying directly on expanded polystyrene sheeting - this is still one area where damp may get in. Overlap the DPM sheets at the join by at least 300mm and stick with the appropriate jointing tape.

If this isn't done properly then the concrete may get underneath some of the DPM as it is poured and destroy its effectiveness.

Door thresholds

Rain has a nasty habit of getting driven under door thresholds and coming through to the inside. Even though a sealant should be used under the door frame, put an extra layer of DPC under the door frame and turn it up on the inside - holding it with the wooden flooring or screeding as in the diagram. Any rain that penetrates gets thrown back from whence it came.

Diagram of design for door threshold

Click on the diagram for a larger view