The cheapest good type of ballasted floor is that in which the ballast rests on a solid base of creosoted planks; and a high authority on railroad bridge building and operation claims that, considering the cost of maintenance, this deck is cheaper than the open-timber one for spans of ordinary length. It would be very uneconomical to omit the treatment of the timber; because, in replacing the base, it is necessary to remove and store the ballast and later to put it back.

Ballast resting directly on steel plate is not much used, although it produces a shallow deck, which is sometimes a sine qua non. It is expensive, and the steel plate is liable to rust.

Ballast resting on a reinforced-concrete slab represents the highest grade of construction, but it is too heavy for long spans. In short spans, weight is desirable for high speed so as to check vibration, notwithstanding the fact that the cost of the steelwork is increased thereby. It is the best type for overhead crossings for the following reasons-the noise is reduced to a minimum, it can be built waterproof to exclude drippings, the depth can be made comparatively small, and the maintenance cost is the least practicable. This style of deck requires a specially-good drainage system; and, if the steel is encased, the deck must be waterproofed, which adds to the expense. True economy calls for a two-ply, three-ply, or even four- ply membrane of cotton drilling or burlap covered with asphalt of the proper consistency. The cotton has proved to be more durable than the burlap; and, therefore, it is the more economic. It is advisable for the sake of durability (and, consequently, for that of economy) to cover the waterproofing with either protection-bricks or thin concrete, the latter being the lighter and cheaper. An important portion of the work is the joint of the flashing with the web of the girder. This requires very careful attention in both design and construction, if leakage is to be prevented.

Rails laid directly on the steelwork give a minimum depth for the floor, but they require a rather expensive floor-system; and there being no cushion between rails and steel supports, the riding is not easy and the noise involved is excessive. For these reasons the type is not desirable; and the only excuse for using it is a compulsory call for an exceedingly shallow floor.

Spans without Floor-Systems

The floor-system proper may be omitted in I-beam spans, most deck plate-girder spans, those half-through plate-girder spans in which the wooden ties rest on either the bottom flanges of the main girders or on special shelf-angles, and short deck truss-spans in which the ties are supported directly by the top chords. This omission of the floor-system is entirely proper from the points of view of both economy and expediency, as far as I-beam and deck plate-girder spans are concerned, and even in the case of deck truss-spans when the truss-spacing is not too wide. Beyond the limit of ten  feet  from center  to  center  of  trusses  the  size  required  for  ties