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.
Damp may enter a house from above, below, and from all sides. It may conic through the foundations, floors, walls, roof, windows. It may be the result of ground-moisture or of moisture in the air, or may be caused by the back-flow of drains or by defective water-pipes and fittings. It is the cause of decay in wood, of rotting carpets, soiled wall-papers, swollen doors and drawers, dank smells, mildewed pictures and books, and even of smoky chimneys. Truly there are disadvantages in a damp house other than its unhealthiness.
To secure a dry house is sometimes difficult, but it is not usually impossible, provided funds do not run short. A few general rules may be laid down. They may be divided into two groups: the first consisting of those for the exclusion of ground-moisture, and the second for the exclusion of atmospheric moisture.
I. To Exclude Ground-moisture.
1. Drain the subsoil.
3. Construct cellars under the whole building.
4. Let the ground-floor (i.e. the principal floor) be well above the external ground.1
5. Let every wall have a horizontal damp - proof course above the external ground and below the ground-floor. If the basement is for habitation, let there be another horizontal damp-proof course just under the floor-level, and connect the two with a vertical damp-proof course outside the wall; or form an open area to a depth of about six inches below the basement floor.1,2 So-called "dry areas" are useful, but not always satisfactory.
Fig 18 - concrete Ground-layer. Open Area, &c.
A. foundation of concrete alone; B. concrete ground-layer; C, horizontal asphalt damp-course; D, vertical asphalt damp-course; E, asphalt fillet; F. area-wall; G, house-wall; h. blue-brick plinth: I. fanatic for wood-blocks; K. wood-block flooring; l. stone window-till throated beneath. and grooved on top to recive weather-tongue 1 See Plate II See Fig. 18.
7. Pave and drain the yards and paths around the house, so that the rainwater may be removed quickly and effectually.1
II. To Exclude Atmospheric Moisture.
1. Build the walls with materials as impervious as possible, taking particular care about the mortar and the flushing of the joints; in special cases, hollow walls may be used, or a small vertical cavity may be formed and filled with impervious material.
2. Lay an asphalt damp-proof course under all parapets and gutters.1
3. Let all window-sills project and be throated under,2 and all cornices and projections be weathered on the top, so that the moisture will not be conducted into the wall but thrown clear of it.
4. Let the eaves and gables of the roofs project a foot or more beyond the walls:1 this is one of the best preventatives of damp bedroom walls; but if ordinary parapets or eaves-troughs, and stone-coped or brick-coped gables are desired, let not the plumber be stinted in his lead.
5. All slates and tiles should be of good quality and should have sufficient lap; where the money can be spared, they should be laid on boards covered with waterproof felt.
6. If lead gutters must be used, they must not be laid level and the drips omitted or reduced to an inch or so; let the fall be as much as possible and the drips at least two inches, and do not forget snow-boards.
8. Birds' nests are often beautiful, but a sparrow's feather lined hayrick is out of place in the head of a rain-water pipe. It is wise to take away the temptation by preventing access to the pipe.
9. Windows and doors and other exposed woodwork should be strong, of good material and workmanship. Sash-windows are more easily made waterproof than casements, especially if the latter open inward; leaded lights, in exposed situations, almost invariably admit rain.
1See. Plate II
2 See Fig. 18.
10. Proper warming and ventilation are helps to dryness, because they prevent the condensation of moisture on walls. When the walls are built or faced with non-absorbent materials - whether cement, glazed ware, paint, or varnish - adequate warming and ventilation are indispensable for comfort. Ordinary walls, it is well known, "breathe", i.e. air passes to and fro through them, and a certain amount of moisture is in this way slowly removed from occupied rooms. That this is the case, may be easily seen in any house where some of the walls are painted and varnished, and some covered with ordinary unglazed paper pasted to common porous plaster; when the walls have been cooled by frost, or in cold weather by lark of fires for some days, a warm day covers the impervious wall with moisture, sufficient, perhaps, to trickle down in drops, while the absorbent wall shows little or no sign of dampness. The moisture contained in the warm air is condensed on the cold walls, and if it cannot find entrance, must remain till warming and ventilation remove it.
Damp, however, may be caused not only by moisture in the ground and air, but also by defective water-pipes and fittings. All water-pipes should he amply strong mough to resist the pressure to which they will be subjected, and must be carefully protected from frost. Safes and gutters should be provided where nessary, to take away the water in case of accident. It is surprising how long a small leak in a service-pipe or waste-pipe may pass undetected or unremedied.