No_3_October_1963 — Page 123

Rising Damp in Walls

THIS Digest examines the circumstances in which rising damp is likely to

occur and suggests possible remedies. The measures considered include providing a complete moisture barrier by inserting a damp-proof course, redu- cing the amount of water in the wall by drainage and evaporation, and concea- ling the dampness by an impermeable wall lining. Some recent developments in the technique of damp-proof course insertion are described. The treatment of rising dampness in floors is not considered here.

RISING DAMP

Building materials are porous and therefore have the capacity to soak up water in much the same way as a wick. If ground water is allowed to reach the foot of the wall it will tend to rise and, unless stopped by a barrier such as a damp-proof course, will reach a height depend- ing on a variety of factors. such as the supply of water. the pore struc- ture of the wall materials, and the rate of evaporation from the wall surfaces. The picture is further complicated by the fact that ground water almost invariably contains dis- solved salts which tend to concen- trate at the wall surfaces where the evaporates (Fig. 1). Precisely how these salts affect the ultimate height of water in the wall. or its rate of rise, is at present not clear, but their presence in the wall surface means that, even if the further rise of water is prevented. the decorations are still likely to be spoiled. This is be cause some of the salts originally drawn up into the wall are

118

FORTEER

C-1

1-5

-2-0

Undercoat plaster} Finishing-coat plaster}

-16-0 【 Wallpaper)

-0-2 (Undercoat plaster),

2-7 (Finishing-coat plaster) '9+5 ( Wallpaper)

Fig. 1. Concentration of salts in a party wall in which rising damp has persisted for 80 years; the figures show the percentages, by weight, of chloride plus nitrate. The shaded area is heavily con taminated.

hygroscopic, that is, they can absorb moisture from the air. so that the surface tends to become damp whenever the air in the building is humid. Ways of dealing with plaster-work affected by such salts are described later in this Digest. It should not be thought that the pro- vision of a damp-proof course is likely by itself to be a complete remedy. Replacement of salt-con- taminated plasterwork is nearly always necessary as well.

Identification and occurrence

Apart from water used for construction purposes, which causes only temporary inconvenience whilst drying out. dampness can result from rain penetration and condensa- tion, in addition to rising damp. It may be noted here that the typical effect of rising damp is to produce a roughly horizontal 'tidemark on the wall, above which there is no damage but below which the plaster may be disrupted and the decoration destroyed by efflorescence. or bleached or caused to lift and peel under the damp conditions. Where hygroscopic salts have accumulated the surface appears damp and mould growth may be present; it is liable to occur on any persistently damp surfaces. Pictures and furniture placed against the wall, and clothes in wall closets. may suffer from the damp and become mouldy.

Rising damp commonly affects old houses having no damp-proof courses or ones which are faulty; it was not until the Public Health Act of 1875 that the incorporation of a d.p.c. became compulsory in Britain. Since then, damp-proof courses in many old houses have given trouble because they

were either inefficient when laid or have since become SO

Dur-

with age. Nowadays B.S. 743 speci- fies materials acceptable for damp- proof courses, including asphalt, bitumen sheet, lead, copper, slate and dense burnt clay brick. ing the next few years the use of other materials may be permitted, such as sheet polythene and other plastics. There is a good case also for including dense quality-control- led concrete, provided that means are found for ensuring the high standard that is needed.

The use

of these materials in buildings constructed nowadays can easily prevent rising dampness. but, unfortunately, it is still seen in many modern houses; this is be- cause the d.p.c. has been bridged in some way, thus providing an alter- native path along along which mois- ture can rise up the wall. Bridging can occur in a number of ways, for instance by solid fuel or garden earth being piled against the wall

(Fig. 2a), or a path raised above d.p.c. level (Fig. 26). It is not at all un- common. with buildings butting on the street for pavements to be placed above the level of damp-proof courses and they are likely to be bedded in ash or clinker containing soluble salts. Co-operation between architects and local authorities is needed to ensure that the pavement is below d.p.c. level. Bridging of the d.p.c. can also be caused by a porous rendering (Fig. 2c), by covering the exposed edge of the d.p.c. with pointing mortar (Fig. 2d), or by mortar that has been al- lowed to drop between the two leaves of a cavity wall (Fig. 2g).

A common from of bridging that is not easily recognised is that caus- Al- ed by a porous floor screed. though British Byelaw regulations require the inclusion of a d.p.c. in

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