Rising damp & damp course systems
Rising Damp is the upward movement of water against the force of gravity due to forces acting at the surface of the water (the surface tension). This is able to occur in materials containing continuous small pores or capillaries and the effect is often referred to as capillarity or capillary attraction.
Bricks and mortar contain small pores and therefore have the capacity to soak up water in much the same way as a wick. Ground water in contact with the base of a wall will therefore tend to rise up and into the wall unless an impervious barrier such as a damp proof course prevents it.
Outlined below is the method employed by our company to control rising damp.
Chemical damp proof course (DPC)
The injection of a chemical damp proof course is the most popular method of introducing a new damp proof course into an existing building. The process works by modifying the surface tension as a result of coating the capillaries within the brickwork with a water repellent material. The water repellent material is injected as a liquid or cream into holes drilled at the appropriate position and centres at the base of the wall to be treated. Subject to the type of material being used the curing process occurs either by reacting with the water held in the brickwork, or with air borne carbon dioxide. The curing process may take some time to achieve. This method of rising damp control is covered by our long-term guarantee.
An additional complication brought about by rising damp is that ground water invariably contains dissolved chloride and nitrate salts. As evaporation from a wall takes place these salts become concentrated at the wall’s surface, i.e. in the wall plaster. The presence of these salts in the plaster means that even if the further rise of moisture is controlled there is still a strong possibility of decorations being spoiled because these salts are what is termed hygroscopic. This means they have the ability to absorb moisture from the atmosphere especially during inclement weather or humid conditions. A salt contaminated wall surface will therefore remain visibly damp giving the impression that the damp proof course installation has not been successful.
As part of the treatment process it is therefore essential to remove the contaminated plaster and for it to be replaced in strict accordance with the specification outlined below in order to achieve and maintain the desired dry decorative surface. The re-plastering specification must be adhered to in order to prevent the migration of hygroscopic salts that will still present in the wall structure from contaminating the new decorative surface. Please note that whilst every care will be taken to identify hidden cables and pipes, we cannot be held responsible for damage to such services where they are concealed in such a manner as to make detection unreasonable. We expect that services follows traditional runs ie. Vertical. Should damage occur, you will be notified immediately and advised as to the course of remedial action necessary. You will be given the option of employing your own contractor or we will appoint a relevant tradesman and add this to your account at cost.
From walls schedule for re-plastering remove skirting boards, architrave’s etc. and hack off all perished plaster up to a minimum height of one metre or 300 mm beyond the last visible/recorded signs of damp. Rake out the exposed joints to a depth of 15 mm (½’’) to help resist the natural shrinkage forces of new cement renders and reduce cracking.
This coat should comprise of 3 parts sharp washed sand to 1 part Portland Cement, and must contain a water proof/salt retardant in the gauging water at the appropriate dilution ratio. Compact this coat well into the raked out joints and apply to an overall thickness of 12 mm. Once this coat has been applied the surface should be keyed (scratched). To ensure good adhesion the following floating coat should be applied before the initial set begins.
The mix is as for the first coat except that no additive is used in the gauging water (clean water only). Apply a further 12-mm thickness or leave 2 – 3 mm behind the level of the original finished plaster. Scratch to form a key for the setting coat.
Apply a setting coat of multi-finish or similar and leave as level as practically possible with original finish plaster. This coat should be porous and must not be over troweled or polished.
As an alternative to applying a rendering coat and a floating coat we may install a cavity drain membrane,or tanking to the walls with either a plasterboard and skim finish or a wet plaster finish.
- Variation from this specification will invalidate your guarantee.
- It is possible that some shrinkage cracks may occur during drying, but this should not be of concern.
- Re-plastering must not be taken down to contact with solid floors.
- On no account must the re-plastering be force dried by any means whatsoever. To do so will result in a weak render that may crumble or fall from the wall.
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