The original cost of water-proofing is a function of a number of different factors. The cost of the material itself delivered at site, that of the direct labor involved in placing it, the cost of such provisions as are made for flashing along the lines of termination of the water-proofing, the cost of the protective or armor coat, the additional depth of flooring and the greater strength that must be provided in steel bridges to carry the dead load of the water-proofing and its protection, and the accessibility of the site of the work, are all considerations that influence the solution of this question of economics. It is readily seen that the cost of water-proofing must be a fundamentally variable factor, as well as one which fluctuates with the building-material market. Moreover, the cost per square foot is usually greater in the case of steel bridges than when the protection is applied to concrete viaducts; because, in the latter, the increase in the dead load does not materially affect the design, and also because in concrete bridges the protection coat is usually a part of the paving-floor system (except in bridges of the solid-spandrel, earth-filled type), and hence involves no additional cost. At present-day prices, however, a cost of 40 cents per square foot for water-proofing steel bridges or 35 cents per square foot for water-proofing concrete bridges is a fair average.

The relative cost, i.e., the cost of the water-proofing as compared with the total cost of the structure, is also variable. In the case of unique steel bridges, such as the Hell Gate Arch or the Quebec Cantilever Bridge over the St. Lawrence River, or in unusual concrete bridges, such as the Tunkhannock Creek Viaduct on the D. L. & W. R. R., this percentage cost is abnormally low, because of the extremely high cost of those structures per square foot of floor area. But such structures as those are so unusual that we may ignore them in our cost considerations, and base our results on the more common types of bridges.

Data gathered from several eastern railroads show that, of the total cost of a steel bridge, four per cent is properly chargeable to water-proofing. In computing this figure various standard types of bridges were considered i.e., concrete-filled trough-floors, plate-girders with transverse I-beam floors, longitudinal I-beam floors with steel-plate, and deck-girder bridges. Cost data were also gathered for different periods-before, during, and after the war-and the results were averaged. It might be remarked in passing that the advance in water-proofing costs has not kept pace with the advance in other elements of construction. For example, in 1914 the steel in a certain bridge cost $73 per ton erected and the water-proofing 28 cents per square foot. In 1917-1918 the steel in a bridge of exactly the same type cost $143 per ton erected, while the water-proofing cost but 35 1/2 cents per square foot. Thus, although the steel increased in cost ninety-six per cent, the water-proofing increased only twenty-seven per cent. When it is stated, therefore, that  the  cost  of  water-proofing  averages  four  per  cent  of  the  cost  of  the structure,  we  are  taking  a  figure  which  is  really  extreme  and  which  we  may  safely  assume  will  not  be  exceeded,  except  in  isolated  and  very