There are essentially two ways of introducing a new damp-proof course into an existing wall, but only one of these methods – inserting a new physical membrane DPC into the wall is certain to cure rising damp in all circumstances. This is often referred to as the traditional method. If a new physical membrane is installed properly, there is no way that moisture can penetrate across it. The other method is to inject a chemical barrier into the wall.
Putting in a physical barrier
Any physical membrane introduced into the wall will have to be let into a course of mortar. So, the first problem is deciding which mortar course to use. In older houses suffering from rising damp which have suspended wooden floors, the new DPC should always be installed below floor level and at least 150mm above outside ground level. The DPC should be inserted below the level of any wooden wallplates or joists which are in direct contact with the outside walls. If the structure of the house makes this impossible, apply liberal quantities of wood preservative where necessary. Apply wood preservatives as well to any timbers which have become damp.
In houses with solid floors which have a damp-proof floor membrane, the DPC should be inserted as near to the top level of the floor as possible, and any remaining gap on the inside wall should be painted with a bituminous-type material. This will protect any wooden skirting boards placed over the gap which are in direct contact with the wall.
Once you have chosen the mortar course in which to insert the damp-proof membrane, the next step is to clear the course on both sides of the wall. Remove any obstructions such as skirting boards, electrical connections or pipes. Then cut through the entire thickness of the mortar course using an angle grinder or a tungsten-carbide tipped chainsaw specially designed for the purpose. Insert the damp-proof membrane – copper sheet, bituminous felt or thick polythene – and drive wedges into the gap between the brickwork and the membrane until this gap can be back-filled with mortar. Do not tackle any more than half to one metre of wall at a time to prevent settlement and structural damage as the thickness of the mortar course is removed. Separate lengths of the membrane material should be overlapped.
The equipment needed for inserting a physical membrane can usually be hired from a tool-hire firm, but you should take great care when using it – large angle grinders and chainsaws are potentially dangerous tools and there is always the possibility of cutting through electrical cables or gas or water pipes you did not know about.
There are many firms who specialise in this sort of installation, and it may be wiser to call one in rather than to attempt the job yourself. The technique does have its limitations and is the most expensive. It cannot be used on very thick walls or on walls with a loose rubble infill. It can be used only on walls with regular mortar courses such as brickwork or coursed stone blockwork not on the random coursed flint walls that are found in various parts of the country.