Last edited 11 Sep 2019



Waterproofing is the act of making something waterproof, ie, ensuring it is able to keep out 100% of any water that comes into contact with it. So, we talk about a waterproof raincoat or waterproof shoes.

In construction, although it is not common to refer to a building as being 'waterproof' (although it would not be wrong and it would be understood by most) it is more common to use the term ‘watertight’ or 'weatherproof'. So, a building will eventually reach a stage in construction where it has been made watertight – able to prevent water from penetrating into internal areas. Alternatively, an existing building may undergo remedial works to make its basement watertight, to prevent penetrating damp or rising damp and so on. For more information see: Damp proofing.

However, the term waterproof can be applied to particular elements of the building fabric which are designed so they are able to completely keep out water. For example, although high-grade concrete can be highly impervious to moisture this is difficult to achieve in practice due to a number of construction reasons. But, when PVC or copper water bars are incorporated at joints (the weak points) and a waterproofing agent is added to the mix, a waterproof construction can be achieved.

Similarly, a synthetic roofing membrane may be termed waterproof not only because water is unable to penetrate the molecular structure of the material but also because it can form a continuous, seamless roof surface that can be waterproof when properly installed.

In contrast, a slate roof may be termed watertight as it will generally do an excellent job of keeping water out. However, it may not be waterproof as in severe conditions wind driven rain may cause water ingress through joints. Similarly, cladding systems such as rainscreen cladding, may allow water to penetrate through a first layer of defence, but this is then drained away before reaching the interior.

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