Damp proofing methods

What is a DPC?

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What is rising damp?

What actually causes natural rising damp?

It is a strange part of our physical world - the way water climbs up through capillaries - the fine tubes inside building materials.

Remember, there are many other causes of damp, salt or discoloration, so check those first - see our Project Page - Damp: Diagnosing and Analysing.

Any masonry structure, unprotected by a properly installed damp proof course, is susceptible to both natural and man made rising damp. Moisture will rise by capillary action (a suction phenomenon arising at the boundary between moisture and a small tube, crack or other tiny, enclosed passageway known as a capillary) through the pores of the masonry, seeking a means of evaporation. The moisture will continue to rise until it reaches a height where, unless no evaporation is possible, gravity takes over and pulls it down again. This height is seldom more than 1.20m, although deposits of 'salts' may be found higher in the wall, often driven there by the use of non-breathing plasters, renders, 'tanking', paints or vinyl wall papers.

For help with finding out what is causing your damp call our experts on 01626 331351.


What is damp proofing?

Damp proofing at ground level is the provision of a barrier across the whole width and length of a wall, which cuts off the capillary supply of moisture naturally rising from the ground. In external walls, to avoid rain water splashing up above the damp proof course, the DPC (damp proof course) has to be positioned at least 6 inches (150mm) above the outside ground (Building Regulations and British Standard 6576).

For help with damp proofing Regulation call our experts on 01626 331351.


What types of barrier can be used?

Historically, a variety of materials have been installed during the building process - stone, slate, lead, zinc and stainless steel and more recently (and currently) plastic.

However, once a building has been erected it is more difficult to insert a physical DPC (although stainless steel sheet can be vibrated into walls, via saw cut slots), so other methods have been developed for remedial action;

1. Water repellent - liquid, paste, gel, or cream by injection - amateur and professional

2. Pore blocking - cement based mortar, by injection - mainly professional

3. Osmotic - passive or active, via embedded wires - mainly professional

For help with types of damp proof course call our experts on 01626 331351.


How these are installed?

1. Water repellent - holes, usually between 10-12mm in diameter, are drilled either into the mortar joints or via the bricks or stones and the chemical is injected using a high pressure pump (liquids) or low pressure hand pump or skeleton gun (creams, pastes and gels). The chemicals spread through the damp masonry, over a period of several months, to join up and form a continuous water repellent layer. This is NOT a solid vapour barrier - it prevents further liquid water from passing through, thus allowing the wall to dry out.

2. Pore blocking - the cement based injection mortar is mixed with water to form a 'slurry' and injected from a re-usable, plastic bodied, heavy duty 'gun'. It rapidly sets, giving off great heat, to form a solid plug in the wall. The holes are generally 18-20mm in diameter and this method is only really suitable for thick, stable stone walls. The injected mortar then slowly generates impermeable 'salts', which over several months block the pores in the wall and prevent moisture from rising. This method requires large drilling machines, expensive drill bits and mortar guns, making it uneconomic for amateur use.

3. Osmotic - the old, 'passive' system used copper wires, which unfortunately corroded, but the latest Lectros Active System utilises a titanium wire, connected to a special mains powered control box, which is run around the walls rather like a ring main. Bent at regular intervals to form anodes, the wire loops are inserted into large holes drilled into the wall (internally, externally or both if necessary). The wire is hidden in formed "chases" or existing, raked out brickwork joints and the whole system is earthed. The electricity from the mains supply passes through a transformer, through the anodes and to earth, setting up an electrical field, which repels the damp. The holes are filled with a special mortar, to ensure good conductivity. This method requires large drilling machines, expensive drill bits and special mortars, making it uneconomic for amateur use.

For help with methods of damp proofing call our experts on 01626 331351.


What if my walls are not brick?

It does not matter - you can still use DPC Cream.

The modern injection cream products perform well in all types of wall and unlike the older liquid injection methods the slow release of a uniform amount of cream per hole ensures good spread between holes and a reliable result.

For help with damp, thick stone walls call our experts on 01626 331351.


How "painful" is installation?

Not at all.

Installation using DPC cream based products is neat and very quick. A series of small holes (10-12mm diameter) are drilled at least 6 inches (150mm) above ground level (exterior walls), the chemical is injected, the holes filled and the result almost invisible. In some cases, the plaster can even stay on the wall, if it can be seen to be sound. It must be remembered however, that rising damp will probably have brought water soluble salts into the plasterwork and these hygroscopic salts will still attract moisture from the atmosphere, even after the wall itself has dried out. Care must be taken to minimise the effect of the residual salts. Sometimes Salt Neutraliser liquid can deal with minor patches of 'salt', if the plaster is otherwise solid.

For help with installation of a damp proof course call our experts on 01626 331351.


For more technical information, product costings or the name of your nearest qualified Contractor go to Property Repair Systems - call us on 01626 331351.

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Site written by: David Moore

David Moore, B.A. (Hons.), C.T.I.S., C.R.D.S. Technical Author

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