What Is Damp Proofing?

By Author: Katherine Bowers
Total Articles: 19
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All modern British buildings have a damp proof course (DPC). In buildings that are less than 30 years old, this consists of a damp proof membrane (DPM), which is an impermeable layer of material (normally a high-density polyethylene) that's laid in the bedding joint between two courses of bricks. Its function is to prevent moisture in the earth from rising up through the brickwork via capillary action ('rising damp'), which can leave walls damp.

Why do you need it?

Not only can the absence of a DPM lead to severe damp problems; it can also result in mould and poor indoor air quality, which is problematic for those with respiratory conditions. Surface mould can cause wood rot that will eventually threaten a building's structural integrity. Wood-boring insects are massive fans of damp basements and they can significantly weaken floor joists and beams.

Types of Damp Proof Membranes

Membranes are vertical or horizontal, depending on how the moisture is spreading. Categories and names of DPMs will vary amongst manufacturers, but they're generally used in these ways:

Flooring membranes to prevent ...
... moisture from rising through the ground (horizontal)
Damp proof course to prevent rising damp between layers of brickwork (horizontal)
Tanking membranes to prevent water from penetrating walls below ground (vertical)
Meshed membranes to treat walls already affected by damp (vertical)

How Do Damp Proof Membranes Work?

Most DPMs are made out of high-density polyethylene which is waterproof, resilient and unlikely to corrode. Membranes designed for floors and walls usually have a 'studded' feel which elevates the sheet slightly away from the source of water. This lets water flow across the membrane in order to drain, helping to avoid pressure build-up in any part of the wall. DPMs for remedial work are generally purchased in a mesh form which can be plastered over. This damp-proofing strategy lets the affected area breathe and dry out once the moisture source has been blocked.

Risks

Traditional membrane-based waterproofing systems can be risky due to the threat of pressure build-up, which has the potential to cause a major leak. Here's the science bit: by containing water in the soil surrounding foundations, membrane treatments cause hydrostatic (water) pressure to significantly increase. Since masonry can't bend or stretch under pressure in the same way as wood, it cracks in response to this pressure. When combined with high water pressure, the cracks cause leaks in the membrane. It only takes a few tiny leaks in a membrane or coat of paint to release a large volume of water into a basement. There can also be issues with the consistency of rolls of membrane which can contribute to the development of weak spots.

Do make sure that you also get quotes from companies that take a different approach to traditional damp proofing treatments before making any decisions.

Basement Systems GB specializes in damp proofing in Oxford and other surrouding ares. Visit them online for a free basement waterproofing estimate.