Damp trouble is more likely in solid Bin. (or 22 cm) walls. ‘Dampness can attack the wall in three ways: from the top downwards; from the bottom upwards; from the outside face to the inside. There are, therefore, two main types of damp — rising and penetrating.
To prevent rising damp you can insert a damp-proof membrane at a minimum of 6 in. (or 15 cm) above ground level. The damp-proof membrane (DPM) may be in various forms. A layer of 4-6 lbs (1 -80 kg-2.70 kg) lead may be inserted into the mortar joint, working to a length of about three bricks. Insert the lead to overlap its adjacent piece by at least 3 in. (or 7.5 cm). Prepare a stiff 1:3 mortar mix; ram the mortar firmly into the joint. Fibrous asphalt felt is an example of a plastic DPM.
Another method is to insert a couple of courses of engineering bricks. Work to about three bricks at a time. The advantage of inserting the bricks is that it gives you a bigger working space. Use a 1 : 3 mortar mix. Other methods used include the insertion of “impervious” materials, such as slate. This is normally inserted in a depth of three layers, with the joints overlapping half way. The method avoids a direct vertical joint. Sprayed silicone is a further alternative suitable in some cases. Where brickwork on an external wall has become porous, the walls may be sprayed with a silicone fluid to form a moisture barrier. Spraying equipment can be hired from the firm supplying the fluid.
Moisture running down from the top of the wall, a problem most likely to occur on parapet walls, may be cured by capping the top of the wall with a coping. This may consist of concrete or brick. Before fitting a concrete coping, first insert a cavity tray. This consists of a similar material to the house DPC — bituminous DPC felt. The device checks damp which gets beyond the coping. The tray should be stepped so that there is a slope from the inside to the outside of the wall.
To prevent damp passing through the wall horizontally, you can render the external surface. This is, however, a less effective method of keeping water out. The render, applied with a plasterer’s trowel, should be a weak mix of 1:6. If the mix is stronger it may crack when it rains; water runs down the wall until it meets a crack and simply soaks into the wall. Once the backing wall gets wet, the water cannot escape and eventually the render flakes off.
There are two types of silicone injection damp course. In both, holes at 9 in. (or 22 cm) intervals are drilled out at 45 deg angles 4 in. (or 10 cm) above floor level. Silicone is then injected under pressure or allowed to drip from 570 ml bottles set into the holes. The surface tension of the silicone layer formed reverses capillary action, forcing rising dampness down.
Electro-osmosis is a method of damp prevention carried out by specialist companies. A ribbon of copper or copper wiring is inserted into the wall at DPC level, connected to copper rods set in the earth. Correct placing of ribboning and earth rods creates a low-resistance circuit between the walls and earth-moisture cannot rise above this charge.