Among the many patented waterproofing materials on the market is the "Medusa." This compound is claimed to prevent efflorescence, as well as to make concrete waterproof. In using it, the following directions, given by the manufacturing company, are to be observed:

"To render cement work impervious to water, a small quantity of the compound is thoroughly mixed with the dry cement, before the addition of sand and water. For most purposes, from 1 to 2 per cent of the weight of cement used will be found sufficient. This is equivalent to from four to eight pounds of the compound to one barrel of cement. The precise amount to be used must be left to the experience of the user, and depends upon the proportion of sand, etc., employed, and on the kind of work to be done Our own experience has been that the use of 1 percent - 4 pounds to the barrel, or 1 pound to the sack of cement - is enough to make hollow concrete building blooks water-tight. For cistern and reservoir Linings and other work which must be absolutely impervious, a somewhat larger amount should be used. Thorough mixing is of the utmost importance."

Fig. 13. Five Ply Gravel Roofing.

Fig. 13. Five-Ply Gravel Roofing.

In the operation of waterproofing, a very common mistake is made in applying the waterproofing materials on the wrong side of the wall to be made water-tight. That is, if water finds its way through a cellar wall, it is generally useless to apply a waterproofing coat on the inside surface of the wall, as the pressure of the water will push it off. If, however, there is no great pressure behind it, a waterproofing coat applied on the inside of the wall is usually successful in keeping moisture out of the cellar. To be successful in waterproofing a cellar wall, the waterproofing material should be applied on the outside surface of the wall; and if properly applied, the wall, as well as the cellar, will be entirely free of water.

In tank or reservoir construction, the conditions are different; in that it is desired to prevent the escape of water. In these cases, therefore, the waterproofing is applied on the inside surface, and is supported by the materials used in constructing the tank or reservoir. The structure should always be designed so that it can be properly waterproofed, and no asphaltic waterproofing should be laid at a temperature below 25° F.

The above-described methods of waterproofing are applicable to stone and brick masonry as well as to concrete.