No_3_October_1963 — Page 131

wall and the ground and a low voltage is maintained between them. The Station has not been able to establish the effectiveness of this procedure.

DRAINAGE AND DRYING

There are some types of walling where it may be impracticable to insert or inject a d.p.c.. for instance natural stone walls having a rubble core. Or. in other cases, the provi. sion of a new d.p.c. may not be feasible economically. În these cases there are several measures which can be taken to reduce the amount of ground water reaching the wall or to increase evaporation from it. These may be effective alone or they may be decided on as preliminaries to a lining treatment.

One such measure is to lower the ground water level near the wall by site drainage: another is to expose an area of wall below ground or floor level SO as to encourage evaporation there. for example by laying corrugated asbestos cement sheeting against the wall. A dif ferent approach is to provide a moisture barrier. for instance by providing impermeable surrounds such as a pathway, draining away from the wall.

Increased heating in the building will. of course. help to dry out a wall but its effectiveness greatly de- pends on how bad the rising damp is and what air circulation is available in the rooms affected by it. Neither heating nor ventilation is likely to be adequate where the decorative finish is impermeable, e.g. a gloss painted surface, and restricts the evapora. tion of water from the inner surface of the wall. Whatever form of heating is used needs to be main- tained if the effects of rising damp are to be avoided permanently, and this measure will therefore involve a fairly high and continuing expense.

According to a proprietary system designed to increase evaporation, porous tubes are inserted into the wall to encourage evaporation at the outer surface. Removing some bricks from the wall

may have similar effect, though in both these methods it is difficult to see how the increase in evaporation is sufficient

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to be of real benefit. Better ven- tilation of the external wall surface

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will also increase evaporation, and clearing the nearby trees and shrubs may assist in some cases.

METHODS TO CONCEAL RISING DAMP

Wall linings

Various treatments are available that will ensure that the decorations are kept out of contact with the moisture and salts in a damp wall, None of these does anything to les sen the dampness in the wall and as already mentioned they may, if they impede evaporation from the wall, actually drive it higher. It is there- fore recommended that the wall sur- face up to 18 in. above the highest affected parts should be treated, and suitable precautions must be taken if there is any risk of the dampness reaching the first floor timbers. To limit this rise it is best to use por- ous decorations, such as wallpaper. distemper or emulsion paint.

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The cheapest and in some ways the best method, which also provides a measure of thermal insulation, is to line with plasterboard on timber battens. To prevent rotting, the battens should have been pressure- impregnated with non-staining inodorous preservative. To prevent mould growth in the cavity, the wall surface and back of the lining board should be treated with a fungicide. Lining techniques are dealt with in greater detail in Digest 9 (second series).

Two other methods introduce an impervious layer between the wall and the new plaster and decoration. They both involve stripping off the old plaster first. One uses a backing of corrugated pitch or bitumen lathing which is mechanically fixed to the wall and plastered over. This is an effective method, provided that the fastenings are made efficiently. In the other method the wall is coated with a rubber/tar or bitu- men preparation. The old plaster is first stripped, and the wall is coated with the preparation. This also is an effective treatment but the pre- paration may lose adhesion after about ten years. In both these cases plastering with an undercoat that does not shrink on drying is neces- sary; a mixture of one part brown- ing plaster to one-and-a-half parts clean sand is usually recommended.

As a temporary expedient only, a cheaper alternative exists where the old surface is fairly sound; it may be possible to fix a barrier layer on the old plaster and to decorate over that. Suitable layers are pitch paper fixed with bitumen, lead foil fixed with a mixture of red lead and gold size, or with old fat varnish, or aluminium foil backed with a heat- sensitive adhesive which can be iron- ed on to the old surface.

Plastering

Apart from the proprietary treat- ments described above that provide an impervious backing to the new plaster and decoration. there are several plastering methods that may effectively impede damp penetration.

Where a d.p.c. has been inserted in a wall. dampness, as already men- tioned, will persist as long as hy- groscopic salts remain in the wall surface, replastering will therefore be necessary to a height of, say, 18 in. above the affected area. This, however, should be delayed as long as possible to allow soluble salts to move from the masonry into the plaster.

Replastering may also be effective, for a while, where no d.p.c. has been inserted. in preventing the dampness from reaching the wall surface. For this purpose the old plaster is strip- ped, the wall surface is keyed by raking out the mortar joints square- ly to a depth of in, and an under. coat of 1:3 cement/sand. possibly with an integral waterproofer, or an undercoat of aerated 1:6 cement/- sand, is applied, followed by a plas ter finish coat. As a further precau- tion, decorations should be porous as mentioned above.

Many old walls of soft brick or porous stone are not sufficiently strong to restrain the shrinkage of a dense 1:3 cement/sand rendering. and in these circumstances the use of a weaker mix of 1:6 cement/sand plasticized by means of an air-en- training agent is preferred to a cement/lime/sand undercoat of similar strength. The aerated mix is likely to be the more effective impeding the passage and salts.

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of moisture

Prepared at the Building Research Station, Garston, Herts. Crown copy right.

THE HONG KONG & FAR EAST BUILDER-VOLUME 18, NUMBER 3