This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
The dampness of external walls above-ground is often a more annoying evil than that of damp basements. It may be due to the rising of moisture from wet ground, to the splashing up of rain-water dropping from faulty eaves, to leaky lead flashings in parapet-gutters, flats, roots of chimneys, etc, to leaky coping on parapets and gables, or to rain being driven or drawn through the materials of which the wall is built.
It is not a difficult matter to fix eaves-troughs where none have been before or to repair or renew existing troughs. In exposed situations, the roof should overhang the walls a foot or 18 inches at all caves and gables; this is an almost tain preventative of damp in at least the upper parts of the walls, but it is a method which cannot be applied to existing houses without much trouble and
Lead flashings may admit rain in consequence of being inadequately pointed. Careful pointing with oil mastic may remedy the defect. Sometimes, however, damp walls are caused by the gutter being filled with snow above the top of the flashing, and when the snow melts, the water soaks into the wall or ceiling, provided that it cannot escape very rapidly through the outlet.
The leadwork around chimney-stacks may be considered at fault, when really the blame rests with the bricklayer or mason. A half-brick wall, built probably with ordinary mortar, often forms the external structure of the chimney-stack; it is no wonder, therefore, if, in the exposed positions which chimney-stacks usually occupy, it is little or no protection against driving rain, not only becoming thoroughly soaked with moisture, but even allowing the rain to pass through it and run down the flues. This, and not the lead-flashing, is often the reason why damp spots appear on the plaster below chimney-stacks. The defect may sometimes be merely a question of pointing; in which case, the joints must be raked out and well pointed with mastic or with Portland-cement mortar (not containing more than two parts of sand to one of cement). If the bricks themselves are too porous, the stack must be taken down, and rebuilt (9 inches thick if possible) with dense bricks laid in cement mortar, all the joints being thoroughly Hushed; a damp-course of lead, inserted immediately above the lead flashings at the root of the stack, will assist in preventing moisture soaking into the walls below.
The dampness of main external walls is often a more serious matter. Some building-stones and bricks are so porous that, in continued wet and boisterous weather, moisture penetrates throughout their substance, and rapidly discolours or destroys the decorations on the inner face of the wall. The radical cure suggested for the chimney-stack will scarcely be considered applicable in this case by the average householder, who will usually try every method under the sun rather than rebuild the house or the offending side of it. The method which is usually first attempted, consists in pointing all the joints with cement or mastic, after raking out the old mortar. If this fails, a so-called damp-proof wash or covering is generally applied to the inside surface of the wall, then the wall is papered again in the ordinary way.
Damp-resisting washes arc undoubtedly of service. I have recommended their use in several cases with good results, but my experience is that they require periodical renewal. Among successful solutions of this sort are Szerelmey's Stone Liquid, Orr's Petrifying Liquid, Carbolineum Avenarius, and Morse's Damp-resisting Solution. The principal merit of this method of treatment is its economy; it has also the advantage of not altering in any way the appearance of the building outside, as is the case when the solutions are applied to the external tare, but it has the great defect of allowing the rain still to penetrate the wall, which entails a slow but sure decay of the materials of which this is built, as well as coldness within the house. The proper method of procedure is to apply the water-proof coat to the outside of the wall, and so prevent the moisture entering the wall at all. Most, if not all, of the washes in the market unfortunately discolour to some extent the surfaces to which they are applied; the effect is more noticeable in the case of new stonework than of brickwork. These solutions should only be applied when the walls are perfectly dry, and care should be observed before applying them to remove all dirt, lichen, paint, and other matter, which might prevent the solution soaking thoroughly into the pores of the structure.
Oil paint, or some preparation containing linseed-oil, is also applied to damp walls externally, but must be renewed at intervals.
Portland-cement stucco often cures a damp wall, but its appearance is so devoid of charm that one hesitates to recommend its adoption, until other menus have been tried. It is less objectionable when covered with "roughcast ".
Covering the offending wall with blue slates is another unlovely expedient, which undoubtedly is almost invariably successful, but is certainly costly.
Weather-tiling serves the purpose equally well at a slightly additional cost, and has a much more pleasing effect It may sometimes suffice to cover only the upper part of the walls; in such a case, the tiles may have an overhang at the bottom, which, if large, must be formed by "sprocket-pieces", as shown in Fig. 682. The tiles must be nailed into the joints of the wall, or, where this is not possible, to horizontal and vertical wood laths secured to the wall at the required distances, as shown in the illustration.
Where the damp rises from the ground into the wall, it will usually be found that a damp-proof course has not been inserted. The radical remedy would be to cut out the walling below the ground-floor and above the external ground, a little at a time, and to insert a suitable damp-course - as shown at a in Fig. 679, - and make good the walling with stone or brick, as the case may be, bedded in good cement mortar. This is, of course, a very expensive operation, and house-owners are not always willing to sanction it. Drainage of the subsoil and excavation of the external ground to as great a depth as possible will do something to improve matters, and an external rendering of cement mortar from the foundation to the ground-floor will also tend to prevent the ground-water entering the wall. Sometimes the base of a wall damp in consequence of rain-water pouring from the roof and splashing up from the ground on to the wall; when this is the case, an eaves-trough provides a simple remedy.
Fig. 682. - Section showing a Method of Covering an Old Wall with Wealther-tiles.