In waterproofing floors, roofs, subways, tunnels, etc., alternate layers of paper or felt are laid with asphalt, bitumen, or tar. These materials range from ordinary tar paper laid with coal-tar pitch, to asbestos or asphalt felt laid in asphalt. Coal-tar products deteriorate when exposed to moisture. Some asphalts are more suitable than others for waterproofing purposes; therefore the properties of any asphalt intended for waterproofing should be thoroughly investigated.

In using these materials for rendering concrete water-tight, usually a layer of concrete or brick is first laid. On this is mopped a layer of hot asphalt; felt or paper is then laid on the asphalt, the latter being lapped from 6 to 12 inches. After the first layer of felt is placed, it is mopped over with hot asphalt compound, and another layer of felt or paper is laid, the operation being repeated until the desired thickness is secured, which is usually from 2 to 10 layers - or, in other words, the waterproofing varies from 2-ply to 10-ply. A waterproofing course of this kind, or a course as described in the paragraph on asphalt waterproofing, forms a di tinct joint, and the strength in bending of the concrete on the two sides of the layer must be considered independently.

When asphalt, or asphalt laid with felt paper, is used for waterproofing the interiors of the walls of tanks, a 4-inch course of brick is required to protect and hold in place the waterproofing materials. Fig. 11 shows a wall section of a reservoir (Engineering Record, Sept. 21, 1907) constructed for the New York, New Haven & Hartford Railroad, which illustrates the methods described above. The waterproofing materials for this reservoir consist of 4-ply "Hydrex"

Fig. 11. Method of Waterproofing Reservoir.

Fig. 11. Method of Waterproofing Reservoir.

felt, and "Hydrex" compound was used to cement the layers together.

Fig. 12 is an illustration of the method used by the Barrett

Manufacturing Company in applying their 5-ply coal-tar pitch and felt roofing material, and it shows in a general way the method of laying asphalt and felt for waterproofing purposes. The company's instructions for applying this roofing are as follows:

"First coat the concrete (A) with hot pitch (B) mopped on uniformly. Over the above coating of pitch, lay two thicknesses of tarred felt (C), lapping each sheet seventeen (17) inches over the preceding one, and mopping back with pitch (D) the full width of each lap.

"Over the felt thus laid, spread a uniform coating of pitch (E) mopped on. Then lay three (3) full thicknesses of tarred felt (F), lapping each sheet twenty-two (22) inches over the preceding one.

"When the felt is thus laid, mop back with pitch (G) the full width of twenty-two (22) inches under each lap. Then spread over the entire surface of the roof a uniform coating of pitch, into which, while hot, imbed slag or gravel (H)."

In applying asphalt and felt for general waterproofing purposes, the felt, as already stated, would be in a continuous roll, and not in sheets as shown for roofing purposes.