Injecting a damp-proof course

Rising damp is an unpleasant and destructive problem which must be tackled as soon as it is discovered. However, by adapting a common garden pressure spray you can instal a chemical DPC quickly, cheaply and efficiently

Rising damp is one of the most serious problems affecting older houses. It ruins decorations, creates an unpleasant atmosphere, causes musty smells, and threatens any adjoining woodwork. And the continued presence of damp in walls is likely to lead to either wet or dry rot in skirting boards, door and window frames, and structural joists. Most houses built in the UK before 1875 were never given a damp-proof course (DPC). And many newer properties have DPCs which have failed through being breached by settlement cracks.

Traditional DPCs consisted of a layer of either slate—which can crack—or bituminous felt—which can become brittle and crack after a time. Slightly less common was two or three courses of hard engineering bricks (which are impervious to water).

There are several ways of installing new DPCs in existing buildings:

dpc walls

• It is possible to cut out a layer of mortar and insert bituminous felt or plastic DPC.
• Ceramic respirators can be inserted into the brickwork to carry damp away by evaporation.
• Copper wires can be plugged into the wall to drain off the tiny electric currents which encourage water to rise up the wall.
• The bottom courses of the brickwork can be injected with a siliconebased fluid. For the DIY enthusiast, the last method is by far the easiest and most certain way of dealing with rising damp. The first method, although by far the most effective, is not practical for the average home owner.

The silicone injection system

Silicone is basically a wax with powerful water-repellent qualities and in DPC fluid, it is dissolved in a spirit base together with anti-fungal additives. To treat brickwork, holes are drilled into the walls, interior and exterior, all around the house at a certain level. Then the DPC fluid is pumped in until it displaces the water in the brickwork. Finally the spirit base evaporates, leaving a waxy gel inside the pores of the brickwork and mortar. This cuts off and reverses the wall’s natural capillary action, forcing damp downwards and preventing the passage of external moisture.

Silicone DPCs are best installed in brickwork but they can be used also in some types of stone; they are effective for walls up to 450mm thick. A variant on the standard fluid contains a metallic additive which is specially formulated to provide a DPC through the rubble infill found between the two skins of brickwork in older, ‘puddle’ walls (though these are rare). Identifying rising damp If your house either has no DPC or appears to have a faulty DPC you must remedy the situation as soon as possible to prevent damp spreading. To start with you should try and discover the age of your property.

If it was built before 1875 there is almost certainly no DPC but of course, newer houses may also lack one. Look for a dark layer of slate between two courses of bricks near ground level. The presence of rising damp is indicated by a number of possible symptoms: rotten wood at ground level; decorations stained by a water tide-mark, spreading upwards; soft, decaying plaster; mould or fungus growth on wall surfaces; wallpaper lifting from the wall or an unpleasant musty smell. But it may be that the damp is caused by something relatively trivial, which can be remedied without recourse to a silicone installation. One possible cause is a pile of earth that has bridged the DPC.

Another is a crack in the brickwork which runs through the DPC. In this case the remedy is simply to repair the crack and the DPC locally. One common cause of damp is the DPC being bridged by external rendering which has ‘blown’—worked loose— allowing moisture to travel up the wall by capillary action.

Pre-installation treatments

The way that you tackle rising damp depends to a large extent on the construction of the lower floors, and on the type and thickness of the walls.

Solid floors: These may be wholly solid, concrete surfaced with tiles, or stone. They must incorporate a dampproof membrane (usually a sheet of heavy gauge PVC or a layer of black bitumen-based compound) which meets up with the DPC.

solid floor DPM membrane

To test for rising damp in solid floors make a ring of putty on the floor and then press a sheet of glass firmly on to the ring in order to form an airtight seal.

If there is no adequate damp-proof membrane, condensation will form beneath the glass within a few days. A cheap and unreliable way to fix floors which lack a damp-proof membrane must first be levelled with a self-levelling compound and then painted with two coats of damp-proofing fluid.

If there is a damp-proof membrane, make sure that it continues some way up the wall; if it does not, the margin between the floor and the wall must be treated before any replastering and after the DPC installation. Having cut away the damp wall plaster—to a height of at least 200mm above floor level—fill any gaps that are left in the brickwork with mortar containing a waterproofing additive. Then paint at least three coats of bituminous fluid along the outer edge of the floor, over the filled gaps, and at least 150mm up the wall. Take care to leave no gaps: damp can rise through the smallest gaps and thinnest layers.

Suspended floors:

suspended floor damp

There are two main types of suspended floor. In type 1 the joists rest on a sleeper wall and in type 2 they are set into an exterior wall. Check which kind you have simply by lifting a couple of floorboards near to the edge of the floor and looking for the presence or absence of sleeper walls with a torch. Type 1 floors are much less threatened by rising damp than type 2 because usually there is a thin bituminous layer between the brickwork of the sleeper wall and the timber itself. Push a knife into the timber on top of the wall which supports the floor joists (the wall plate): if it resists the blade you can be sure that it is free of rot. In the case of type B floors you must test the joists themselves and use a torch to search for any signs of fungus and rot. In the case of wet rot in type 1 floors you must completely replace all affected timber, then instal a DPC in both the sleeper and the outside walls.

If you have dry rot then you should call in a specialist firm to carry out the treatment. Make sure that the DPC is at least 150mm above the ground level in order to protect the walls, skirtings and decorations. If the timber shows no signs of rot, simply spray the timber with a preservative. In the case of a sound type 2 floor with no DPC you must instal a DPC in the outside wall from below the joists to 150mm above ground level. This may involve lowering the ground level around the walls a little. But where rotten timber is encountered you must replace all the affected floorboards and joists, and spray the rest of the timber with a preservative. If you are in any doubt about the presence of dry rot, spray both the timber and the masonry with dry-rot fluid.

Walls below ground level:

It is not possible to instal an effective DPC in any walls below ground level. However, in cases where there is a different ground level against one side of the house, there are two possible alternatives. One method is to instal a continuous DPC following the ground, with ‘verticals’ linking the different levels. In this case any walls which cannot be protected by the DPC interior wal must be tanked internally against the damp. This is done with waterproof cement or bitumen waterproofer and is covered further on in the Repairs and renovations course. The better method where the walls make direct contact with high banks of earth is to dig these away and form a trench around the house. Also, if possible, construct a small retaining wall to hold back the earth. You can then instal a continuously level DPC, which is less prone to being breached.

Installing the DPC

Professional installers pump the silicone fluid into the wall but although the pumps can be hired they are messy and tend to waste much of the fluid. Conventional DIY practice is to decant the fluid into the wall from inverted bottles and although this method involves little effort, penetration may not be thorough. Probably the best method for allround ease and efficiency is to inject the fluid from an ordinary garden-type compression spray. With this you simply remove the nozzle and insert the hose into a home brewing bung matched to the size of your predrilled holes. The DPC fluid itself is generally available in 25, 5 and 2.5 litre drums.

You will need about one litre of fluid per 300mm length on an average, solid (225mm) wall. Because you will need to drill a great many holes in the brickwork, it makes sense to hire a medium sized rotary hammer drill with pneumatic action and a clutch to save fatigue and jams. Buy a carbidetipped masonry bit to match the thickness of the bung you are using, and make sure that it is long enough to penetrate the wall at an angle. Use bungs between 16mm and 20mm in diameter.

Preparing the walls

In order to ensure the success of the treatment you must remove any old or unsound plaster, the first 150-200mm of rendering, and repair any cracks in the wall. Pour a thin mixture of concrete grout into any cracks and allow it to set before injecting the DPC. Similarly, rake out and replace any crumbling mortar, and replace any badly damaged brickwork. Note that old plaster on the other side of a party wall can bridge your DPC. In this case, you must tank the party wall up to at least lm from the ground and if the damp is particularly bad, extend your DPC over three courses of brickwork.