Author:Cowin, Douglas M
In:The Flat Roof in Domestic Architecture

ALTHOUGH this article is intended to deal primarily with the technical side of the construction Of flat roofs in domestic architecture, in view of the opposition offered to their adoption by architects of the older or traditional school, it would be as well to answer a few of their objections first, from the aesthetic point of view.

In truth there is nothing modern about the flat roof, for if we turn back to the time of the ancient Egyptian empire, we find the flat roof extensively used, as it has been for generations in the south of Italy and Spain.

The pitched roof, as we know it to-day, is the product of Northern Europe, where heavy snowstorms are experienced during the winter months, and was put into use there, as the most logical method of preventing the heavy weight lodging on the roof. It seems natural, then, that here in South Africa, where we seldom have snowstorms of any severity, we should follow the example of the hot, snow-free countries such as Egypt and Italy.

Apart from the advantage the flat roof has over its rival the pitched roof, in its use as a roof garden or elevated summer house, it simplifies the architect's problem of planning to a considerable degree, for the shape of the house and size of rooms therein, are no longer governed by the pitched roof above them.

In spite of its obvious, practical advantages, the flat roof as used in South Africa at the moment, has the drawback that it costs more than any other type used.

The reinforced concrete slab is in most general use on account of its strength and durability, but the younger architects are experimenting in various materials, and there is no doubt that in the very near future, some type of flat roof will be evolved, which will cost as much, or less, than the tiled and slate roofs in general use to-day.

The main factors in the cost of concrete slab roof are the necessity for insulating it against heat, and rendering it waterproof.

The simplest way to eliminate the transmission of heat through the concrete to the rooms below, is by means of a ventilated air space immediately below it. This can be achieved by the rather costly use of a false ceiling, but a firm in Johannesburg is at present experimenting with a type of construction in which light wooden frames, about 9 ins. (23cm) deep and 3 ft. (1m) wide, are covered with reeds, and used as permanent shuttering. When the concrete is poured on the spaces left between, the frames form a series of small beams, and the thickness of the slab can be reduced to as little as 2 ins. (5cm). The reeding is plastered on the underside, and the result is a light, economical, heat-and sound-proof roof.

The final results of the experiments should be of the utmost value to architects in the Union, and if successful, will go a long way to solving the problem of insulating concrete flats against heat.

If circumstances do not permit of the introduction of the air space, an alternative is the use of one of the many fibre boards, either above or below the concrete. The results of extensive tests with multiple thermometers on the roof of the new Library of the University of the Witwatersrand, where this method of insulation has been used, will prove how far it is successful.

A fact which is generally overlooked when a flat roof is being finished, is that black absorbs approximately twice as much heat as white, and it is useful to remember that if the final coating or layer of waterproofing used is of a light colour, considerably less heat will be absorbed by the exposed surface.

A composition of bitumen and aluminium powder painted on to the flat, has been proved very successful as a finishing coat, as the aluminium powder gradually works up to the surface, and keeps it white, or nearly so.

There are several brands of liquids and powders on the market, which when mixed with concrete, render it waterproof, but however good they may be, once a crack occurs in the concrete, their value is lost.

One method of overcoming this difficulty of cracking concrete is to reduce the sizes of the slabs by having the ceilings of the rooms in the house at different heights. The adoption of this principle will generally be found of dual advantage, in that it enhances the appearance of the house at the same time.

This article was published in Cumming-George, L, 1934:101.