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 word "damp-course" is usually applied to a horizontal damp-resisting course forming part of the structure of a wall. The building-regulations of most large towns and cities require such a course to be laid in all walls of buildings. The London County Council's by-laws state that it must be "at a level of not less than six inches below the level of the lowest floor". Opinions may differ as to the necessity of placing the damp-course six inches below the floor-level, where a solid floor is adopted as in figs. 18, 23, and 27, but where an ordinary joisted and hoarded wood floor is desired, the depth of six inches will probably be exceeded, as in figs. 21 and 25.
For walls in contact with the ground, however, one damp-course is not sufficient. Reference to Fig. 25 will make it clear that the outer skin of the hollow wall there shown will absorb moisture from the ground; and this moisture will rise by capillarity into the solid wall above the cavity and probably make itself visible on the plaster and wall-paper, unless its upward progress is stopped by a damp-course at B. This upper damp-course may be laid over the outer portion only of the wall, but in most cases it is a cheese-paring economy not to cover the inner portion as well. For the necessity of two damp-courses, see also figs. 23 and 24.
Many different kinds of materials and contrivances have been used for damp-courses. Good asphalt is one of the best, and has the great advantage of forming a continuous sheet with the asphalt ground-layer. Nothing further need be said about the ordinary methods of using natural and artificial asphalts, as they have already been described at some length with reference to ground-layers and vertical damp-courses, but attention may be drawn to one variety of asphalt damp-course, known as Callender's Pure Bitumen Damp-course, which coinsists of sheets of bitumen supplied in lengths of 24 feet, and in various widths. To lay tin- damp-course, the sheets are simply unrolled on the wall and the several lengths joined together by means of a hot iron. The material is not a felt and is guaranteed free from coal-tar and pitch.
Somewhat akin to asphalt is bituminous felt, or, as it is sometimes called, fibrous asphalt, which can be obtained in sheets of various widths from 4½ inches upwards. The Bheets are laid to overlap about 2 inches at the joints, or two thickness are used, breaking joint. These sheets are very convenient and economical, especially for works in the country, and have the advantage of not cracking when any settlement of the building takes place. Doubts about the durability of the material are sometimes expressed, but with what reason I cannot Bay.
A layer of good Portland-cement mortar (1 cement + 1 sand) is sometimes need, but cannot be recommended, as it is not entirely impervious at the best, and cracks with any settlement of the building
Two courses of strong slates, thoroughly bedded in cement-mortar and laid to break joint, are a letter remedy, although they also are liable to fracture, and leave a thick unsightly joint in the face of the stonework or brickwork, unless a bed is sunk to receive them.
Sheet-lead was formerly much used for damp-courses and answered the purpose admirably, but the introduction of other satisfactory but less costly materials has led to its partial abandonment.
Two or three courses of blue Staffordshire bricks with the vertical joints left openareeconomical,and,aboveground,effective.
Where the lowest floor of a house is of wood and above the ground or area outside, the best material is the stoneware ventilating damp-course, shown in Figs. 29 and 30, and also in Fig. 21, p. 72. The slabs are perforated, and may be obtained in thicknesses from 1½ to 3 inches, and in widths from 4½ to 18 inches and upwards; the length of the slabs is usually 9 inches. Besides being proof against damp, they afford continuous and constant ventilation to the space below the floor, and so help to prevent the decay of the wood. Special slabs are made for salient angles.
Fig 29 -"Broomhall" Vitrified Stoneware Ventilating Damp-course, 1½, 2½, and 3 inches thick, with Open Joins.
Fig. 30. - Doulton's Vitrified Stoneware Ven-tilating Damp-course, 1½, 2½, and 3 inches thick, with Tongne-and-groove Joints.