Types of Damp Proof Courses
Damp Courses. As before mentioned, the principal consideration in the construction of houses is to ensure equable temperature and to keep out cold and damp, which latter may be effected by means of (a) horizontal and (b) vertical damp courses.
(a) Horizontal Damp Courses. The horizontal damp course is required to prevent damp being drawn up into the walls by capillary attraction, much in the same way as moisture is drawn up by a sponge. It should be formed of some non-absorbent material, fixed not less than six inches above the surface of the ground, and all wall plates and flooring must be placed above it in order to be protected from the damp, picture 25 shows a horizontal damp course with air-bricks inserted at intervals to ventilate the space underneath the floor, and thus to guard against dry-rot.
Picture 25. A Horizontal Damp Proof Course.
There are several materials which are used as damp courses, of which the following are the principal ones in use :
1. Two courses of slates, breaking joint, and set in cement, form a usual and effective material, but the objection frequently raised against it is that the slates are liable to crack with the slightest settlement, although the fracture will not always materially affect their efficiency.
2. Asphalt, which should be one of the varieties of rock asphalt, forms a very good damp course and should be applied in two layers of ¾ in. each.
3. Lead is sometimes used to form a damp course, but owing to its cost is not often employed.
4. Patent bituminous felt compositions laid with lapped joints are used, and being elastic in character, remain effective in the event of a slight settlement in the building.
5. A course of vitrified fire-clay air-bricks built in cement prevents the rising of damp and at the same time ventilates the space under the ground floor, thus preventing any tendency to dry-rot in the timbers. In putting damp courses into old buildings where they have been omitted, this is often the best and cheapest method, as a course of bricks can be cut out in sections all round the building and the air-bricks inserted.
6 Vertical Damp Courses. Vertical damp courses are provided to prevent damp entering a wall from adjacent wet ground; hollow walls (already dealt with) and areas can also be used for this purpose. An asphalt lining in two thicknesses is shown in picture 26.
Pictures 26 and 27, A Covered Dry Area and A Vertical Damp Proof Course.
Dry Areas. A dry area, placed outside the wall and left open, is shown in picture 28, and a dry area, drained, arched over and ventilated, is shown in picture 27. This dry area may be from 2 ½ in. to 12 in. wide, and the cavity should be carried up 6 in. above the surface of external ground, and a damp course inserted at its base, as seen in picture 27.
Picture 28. An Open Dry Area.
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