How to install a damp proof course

Rising damp is indicated by a tide mark just above floor level on internal walls. It does not usually rise more than about 1 m up the wall. Its presence means either there is no damp proof course. which is common in houses built before 1875, or that an existing dpc has failed. A dpc that is not functioning should be regarded as non-existent.

Most methods of installing a dpc should be employed only by specialists. A method which you can use yourself, however, is the infusion into a wall of a special dampcoursing solution which gels within the wall and creates a continuous chemica barrier to prevent rising damp. The system is widely used by specialists, but you should be able to achieve equally good results provided you are very thorough. If the job is skimped, the dpc will break down in places and the rising damp will return. One specialist company provides detailed working sheets and a technical advisory service.

When selecting a DIY damp proofing process, check the company supplying the materials and instructions offers a guarantee which would be acceptable for mortgage purposes. Warning The infusion system cannot be used for walls of more than 450mm thickness, or walls constructed of materials other than brick or soft sandstone.

Inserting the dpc

The job involves three processes: drilling the holes at a selected level, carrying out a preliminary run with water to test for damaged brickwork and the infusion of the fluid.

Selecting position The level at which the dpc is to be inserted depends on whether the ground floor of the house is of suspended timber construction or solid concrete. If it is a timber floor, the dpc should be immediately below the floor level; with a concrete floor it should be adjacent to the floor.

If it is impracticable to insert a dpc below a suspended timber floor – for example, where there is a patio – it will have to be inserted at floor level. In such a case, spray the floor joists and floorboards liberally with a good timber preservative to prevent attack by rot.

Where a house is on sloping ground, step the dpc to follow the line of the ground floor. In a semi-basement install the dpc above ground level to a height of about 150mm. Internal rendering, carried out after insertion of the dpc, will prevent damp rising above this level. Drilling Use a lightweight rotary hammer drill. obtainable from a hire shop, and fit it with a 16 mm diameter drill bit. The length of the bit needed will depend on the depth of the holes to be drilled, which in turn will depend on the thickness of the wall. Fix a piece of adhesive tape to the drill bit at the required depth to guide you.

In all cases holes should be drilled at an angle of 30 degrees downwards from the outside face of the brickwork and at 75mm intervals horizontally; start drilling just above a mortar joint. If working from one side only, use the side most accessible and convenient. Remove the skirting board before drilling on the internal wall. If you are drilling through a party wall, let your neighbours know what you are doing.

For 112.5mm brickwork Drill to a depth of 100mm, working from one side only.

For 225mm brickwork Drill to a depth of 200mm, working from one side only.

For 337.5mm brickwork Treat as for 225mm brickwork up to a depth of 330mm. You may treat 337.5mm brickwork from both sides if wished. In this case, drill 200mm holes one side and 100mm holes the other side.

For 450mm brickwork Treat from both sides, drilling 200mm deep holes.

For 275mm cavity brickwork Drill 100mm deep holes from both sides.

Water test If you inserted damp course liquid into a wall which was cracked or otherwise damaged. it would seep out of the cracks and be wasted. To prevent this, make a trial run with water to find out if there is any damage. Fill each hole to the brim, using a funnel. If the water disappears quickly, this indicates there are cracks. Fill the holes with a mix of two parts sand and one part cement to seal the cracks, ramming the mortar. firmly into the holes with a length of dowel the same diameter as the holes; let the mortar dry before redrilling the holes in the same places.

Walls vary in porosity, so use the trial run to assess the absorption rate. This will enable you to judge how many holes you can fill at a lime when you use the fluid.

INTRODUCING THE FLUID Don’t introduce the damp-proofing solution until at least six hours after the water test. You can hire special irrigation bottles for pouring in the fluid or use a jug and offset funnel. As a guide to the amount of fluid needed. a hole drilled in a 225mm wall will lake about 190ml Opt, which is about the same as filling each hole four and a half times. So use a jug holding 570ml AND allocate ONE jugful to three consecutive holes.

It is important not to allow a hole to empty or the liquid will start to cure at the mouth of the hole and prevent further saturation. If the absorption is particularly speedy, do not tackle too many holes at a time.

Fill the first three holes to the brim and keep the remaining liquid for topping up, doing this progressively and methodically until all the holes have been filled.

Leave the holes for a few weeks while the solution is drying out. Then fill with a mix of three parts sand to one part cement.

FINISHING TREATMENT

When the dpc has been installed, rising damp will cease but there will still be dampness in the walls due to the action of hygroscopic salts, general condensation or water penetration. The finishing process stops the damp and eliminates the effects.

First remove the skirting boards so the walls can be treated down to skirting level. Where dampness has existed for a few years, hack off the plaster to bare brick; work to a point about 450mm above the highest damp patch. POROUS BRICKWORK Where the brickwork is porous, you need to treat it to prevent water penetration. Before treating it, cut out and replace any damaged bricks and repoint defective mortar joints. Then brush or spray on two coats of a silicone-based waterproofer and sealer to the outside wall.

REMOVING MINERAL SALTS Rising damp will dissolve some mineral salts from the brickwork, which will diffuse through the wall and plasterwork in damp conditions and, on drying out on the surface, will show as a whitish chalky substance. These salts are hygroscopic and if sufficient water is absorbed, they will dissolve in it forming ‘condensation’.

Use a chemical neutralizer to treat the affected area. Apply the solution to the brickwork with a 100mm paint brush. Work quickly and make sure you treat all areas where hygroscopic salt and efflorescence action was evident on the plaster. The solution penetrates up to 150mm into the brickwork and forms a barrier to the soluble salts while the wall is drying out. Rendering After at least 48 hours, render with a sand and cement mix containing an integral waterproofer diluted with ten parts water. Add a mortar plasticizer to make the mix more workable. Apply the render at least 10mm thick and finish with a lightweight plaster skim. Leave a gap of at least 38mm between the render and plaster and the floor; the gap will be concealed when you replace the skirting.

Where a high ground level was involved and the dpc was installed 150mm above ground level, a slightly different treatment is required at the base of the wall. From a point 300mm above ground level down to floor level apply three coats of rendering. When refixing the skirting, use impact adhesive so there is no danger of the rendering being punctured by nails.

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