building process

Education Corner: All About Damp Proofing

  • Creation Date: Tuesday, 14 May 2019

  How Do I..?

13 March 2019

Once water or moisture gets into your home it is both difficult and expensive to stop. It is therefore important that damp proofing is done correctly and in the right places by your builder first time. I have only highlighted the basic areas where every home needs damp proofing. There may well be other areas depending on the design of the building that may need particular attention with respect to damp proofing and your architect should make your builder aware of such areas and provide solutions for them.

The first item of damp proofing is under your floor slab where a DPC (Damp Proof Course) plastic sheet is laid on the compacted sand filling on which the surface bed (floor slab) is cast. If the edges of the plastic sheet, where the floor slab meets the wall are not correctly done, then you will experience rising damp along the bottom of the walls inside your home. These days, there is a secondary damp proof layer in that a polystyrene sheet is now inserted under the floor slab and along the vertical edge of the foundation walls to satisfy the requirements of the energy efficiency regulations.

The second item of damp proofing is at the base of all your external walls at the level of the floor slab. Where the outside walls are built as cavity walls this DPC layer is stepped down by one brick towards the outside and where “weep holes” (where the mortar is left out of vertical joints between the bricks) are constructed at 1m centres to facilitate air flow into the cavity. In the case of solid walls, the DPC is laid flat.

The damp proofing here should be linked to the damp proof sheet from under the floor slab if it is to be effective. Internal walls that have individual foundations (that is, not built on a thickened floor slab) also require a damp proof course at the level of the floor slab.

The third and often neglected area is at window sills and also above and next to windows and external door frames. Some builders prefer to make use of silicon to seal window and door frames because it is easier but this is not recommended. Given a few years and the incorrect product, and this will lead to damp and maintenance issues. If your DPC is properly in place then you will not have any difficulties.

Ask questions of your builder regarding the correct positioning of damp proofing which you can check on while your builder is constructing your house. If you have any difficulty with what you observe, contact your architect and ensure that it is correctly done. It will save you many headaches later.

Edu Damp Proofing

Compiled by Les Abbott. PrArch