Besides the three methods of covering flat roofs already described, there yet remains another method, that of covering them with asphalt.

The advantages possessed by this material are economy, toughness, and durability; it is also non-absorbent, dries quickly after rain or being washed down, and is sanitary, non-flammable, vermin-proof, and easily cleansed, and is laid without joints or seams.

The concrete surface to be covered is finished with 1/4 inch of cement rendering, finished rough with a wooden float or straight edge, for the reception of the asphalt.

Mastic asphalt is used; this is a bituminous lime stone prepared in the following manner: -

After being taken from the quarry it is ground to a fine powder and mixed with a proportion of fine grit, uniform in size and free from dust. After a thorough admixture it is placed in large fixed cauldrons and heated by strong fires. In order that the asphalt shall not calcine during this process, mineral tar is added as required, the whole mass being kept moving by machinery until the whole is reduced to a mastic. It is then run into moulds containing about 125 lbs., where it is allowed to cool, and the blocks are then removed.

There are three qualities of mastic: -

(1) Fine (containing no grit).

(2) Fine gritted.

(3) Coarse.

Qualities Nos. 1 and 2 are used for flats.

To prepare the blocks for laying, they are broken up and melted in cauldrons, from which the molten asphalt is taken in large ladles and spread over the cement screed (prepared for it) in two 1/2-inch thicknesses, each thickness screeded with a wooden screed, and the top layer finished with wooden hand floats to a smooth surface.

Where the flat adjoins a brick wall a vertical skirting 6 inches high and 1/4 inch thick should be formed. The top of the skirting is turned 1 inch into a raglet cut into the brickwork to receive it. The top edge of the skirting should be slightly rounded, and angle fillets about 1 1/2 inch wide should be formed to all internal angles of skirting and between the skirting and paving.

Channels can be formed in the asphalt as required, whilst the material is hot, leading to outlets as may be convenient.

End Of Vol. I.

Printed by Morrison & Gibb Limited, Edinburgh