The second main type of damp proof course is a remedial chemical DPC.  The damp proofing chemical is normally injected along the base of walls that are suffering from the effects of rising damp.  Contrary to common belief a chemical DPC injection does not form an impervious physical barrier along the wall.  Chemical damp-proofing is an effective method of lining the pores along a stretch of masonry wall with a silicone resin water repellent to combat capillary action.  For the purists, silicone rising damp treatments work by neutralising the charge attraction of the masonry pores to the water molecules to inhibit capillarity.

DAMP PROOFING
The damp-proofing of rubble filled walls and walls having a thickness greater than 320mm should be left to a trained professional, who may need to use more intricate electro-osmotic damp proofing systems or cementitious pore blocking systems to ensure effectiveness throughout the depth of such wall constructions.
Damp proofing in conventional cavity wall and solid wall domestic housing is however a relatively low-skill task that can now be undertaken by a competent builder or DIY enthusiast equally as well as by the ‘specialist’, though at a fraction of the cost.
Modern chemical damp proofing systems are eco friendly and solvent free.  The silane / siloxine creams are designed to be introduced into the mortar joints, as the mortar presents the only continuous passage ascending up a wall through which water is able to rise.  Attempts to introduce a DPC into each brick or block unit along a stretch of wall will not stop the ground water from rising through the joints between.  The DPC injection is sited along the base of the wall.

12mm holes are drilled into the mortar at 100-120mm intervals at a level that is at least 150mm above external ground level (BS6576: 2005 ) and internally beneath vulnerable timbers, or close to the floor if solid.  The depth of the hole is 90% of the wall thickness and, in order not to burst through the wall, the use of a lightweight drill fitted with a depth gauge is recommended.

DPC injection cream is quickly and easily dispensed to fill each hole.  The effectiveness of DPC cream formulation relies upon having the correct blend of ingredients to ensure that the silicone migrates fully along the mortar bed to form a continuous water repellent layer.  Damp proofing creams that have been independently assessed and accredited by the British Board of Agrement are the only creams that are certified for the effective treatment of rising damp. 

An essential requirement of BS6576: 2005 Code of practice for installation of chemical damp-proof courses is to replace wall plaster that has been contaminated by the ground salts deposited by the rising dampness. These ground salts will migrate to the wall surface during the drying out period that follows DPC injection.  As the salts are 'hygroscopic' they have a capacity to absorb moisture from the air, especially under humid conditions.

Best practice, though perhaps not always the most practical, is to leave the plaster above the new damp proof course on the wall for several months following the DPC treatment.  This allows the salts to surface during the drying out process and the extent of plaster replacement to be properly ascertained.

For free assistance in matters relating to rising damp in domestic housing phone the Twistfix technical help and advice line on 0845 123 6006 or visit www.twistfix.co.uk  

Further Reading: BRE Digest 245: 2007