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Damp Proofing

Are you aware of damp on walls above the skirting boards? This is almost certainly rising damp, which means that damp proofing is urgently required to stop the problem getting worse. Even if you currently have no signs of damp, a missing or faulty damp proof course is likely to lead to rising damp in the future.

A damp proof course is a physical, chemical or electro osmosis barrier designed to prevent rising damp – the diffusion of water from the ground and up into the walls of a property.

Which damp proofing methods can Devon Damp offer?

Physical damp proofing:

A physical damp proof course is usually the most effective method as it allows the damp proof course to be linked up with impervious membranes under the floors. However, this method is time-consuming and hence expensive.

The traditional ‘wet pack’ method involves removing the bricks at a certain height and laying an impervious material such as slate, felt or plastic on a fresh mortar bed. In this method, work tends to be done on one metre sections at a time.

The ‘dry pack’ method can be applied to a continuous section of wall as it uses a masonry chainsaw or handsaw with tungsten carbide tips to cut a 5-7mm slot extending a metre into the external wall. Into the slot is pushed the damp proof course which is pre-loaded with mortar.

Electro osmosis damp proofing:

Electro osmosis damp proof courses work in one of two ways. One works by inserting a copper strip two-thirds of the way into the wall and earthing it using copper rods placed at 50cm intervals; this is the passive method.

An active method works by passing a DC mains current into an anode set into the wall. In both cases, the osmosis of water through the walls is halted.

Rising damp and re-plastering:

Ideally, any remedial plastering work that follows the laying of a damp proof course should be done by the same contractor. This is because the formula for any plasters or renders need to block the passage of soluble salts, particularly a group called hygroscopic salts (e.g. chlorides and nitrates). These become concentrated in walls following rising damp, and continue to create problems by absorbing moisture from the atmosphere.

The plaster, which contains salt-retardant chemicals, needs to be mixed in the exact proportions specified by the manufacturer.

Chemical damp proofing:

A chemical injected damp proof course can be applied manually, as a cream or mortar injection, or can use a pressure system (or, for aqueous solutions, a gravity transfusion).

First, the wall’s structure is exposed by removing rendering, plastering, skirting and any obstructions (furniture, radiators, etc.) up to a metre from ground level or 300mm above any evidence of damp; the walls also need to be checked for pipes and cables before any drilling . Second, holes are drilled along the wall; the spacing, size and depth of the holes depend on the method used. Next, the chemical is introduced into the holes.

For the pressure and gravity methods, holes are spaced at intervals of as close to 110mm as possible, with the size of the holes 10mm or 22mm respectively.

Cream chemical damp proofing is a revolutionary new technique that saves labour and mess. There are various types of cream on the market, but they are all introduced in the same way. 12mm holes are drilled at 110mm intervals along a suitable mortar line, so that each hole is at the top or bottom of a vertical joint. The depths of the holes need to be to within 15 to 25 mm of the other side. Next, an application gun is used to introduce the cream into each hole before the holes are back-filled and pointed over.

Manual mortar injection is slower and messier and involves adding water to a dry mix of cement, quartz and damp-proofing chemicals (a BBA accredited third party is recommended to carry out this procedure). First, a course is selected that is no lower than 150mm above external ground level and, ideally, below timber flooring on the inside. 19mm or 22mm holes need to be drilled at 225mm intervals, inside and out, so that they are staggered like interlocking fingers. The holes should be at a 30° downward angle and extend four-fifths of the way into the wall.

Once bore dust has been flushed out with water, the chemical mix should be turned into a cream by adding three parts of cold water to five parts of mix. This is then injected into the holes.