There are several methods of laying the floor plank upon these beams, which are placed from eight to ten feet apart, according to the dimensions of the machinery to be placed in the mill. The first floor of three-inch plank, planed on one side and grooved on both edges, is laid planed side down, and the hardwood splines are inserted into the grooves before the planks are pressed up and spiked to the beams. An agreeable finish is sometimes arranged underneath by plowing a rabbet in each of the corners, and inserting a bead in the groove thus formed, which is secured by nails driven diagonally into the plank on one side only, because if the nails were driven into both sides, the bead would be split by the contraction of the plank.
These planks should be cut to sufficient length to cover two bays of the mill; and their transverse resistance is that of a beam fixed at one end and supported at the other, or one and three-fifths as much as a plank of the same size but half the length would support; but it should be remembered in this connection that, if evenly distributed on the floor, five-eighths of the load would be carried by every alternate beam unless the planks are so laid to break joints at convenient intervals of about three feet.
The top flooring is generally laid directly upon the floor plank, with one or two thicknesses of roofing paper interposed; but the preferable method, which deadens the sound and vibration, and also greatly increases the fire-resisting qualities of the structure, is to lay a coat of mortar on the floor plank, preserving the uniform thickness by means of furring placed about sixteen inches apart, and then to lay the upper floor upon this.
For these upper floors hardwood plank, one and one-fourth inches thick, and not over four inches wide, is used. The black birch is considered by many to possess the greater resistance to wear; and Southern pine is ranked next, although the latter wood gives trouble by stringing, especially when trucks are rolled over it. White maple forms an excellent top floor, although not so hard as others, especially where the floor is likely to be exposed to water, as in paper mills and bleacheries.
ROOFS.
Benjamin Franklin once said that next to a good foundation a good roof was the most important feature of a building. Although the constructive features of mill roofs are well defined, yet with regard to roof covering there is a wide diversity of experience and opinion.
The present form of factory roofing resembles a floor in its construction, being made, in a similar manner, of plank laid upon beams which project through the walls, where they act as a bracket to the cornice, the ends being sawed after any suitable ornamentation. The inclination for such roofs is about three-fourths of an inch to the foot. Where a mill is narrow enough for a single beam to reach from the wall to the ridge, they form cantilevers, the second point of support from the wall being by the columns one-third of the distance across the mill, and the ends of the beams are further secured together by means of iron dogs. For mills of greater width, the beam would reach only to the row of columns, and over the middle of the mill a beam is placed, usually horizontal on the under side, and hewn down from the middle to each end, so as to preserve the same slope on the upper side of the beam as for the roof.
In many instances mills are built with brick cornices, without any of the wood projection from the side; and in other buildings the walls are carried above the roof, which slopes toward the center, and all water falling on it or melted from the snow is conducted from it by pipes leading down through the middle of the mill.
It is not desirable to place gutters around the edge of the mill, as they serve no useful purpose, and are in continual need of repairs. By leaving the edge of the mill plank square and protecting it by sheet metal flashing, the rain falling from the roof can be received by a concave walk of coal tar concrete placed on the ground around the building. Suitable porches over doors, or some guard on the roof at these points, will prevent people who may be passing in at doors from being unduly wet by water from the roof.
There are numerous forms of roof coverings, the use of the different varieties being to a great extent local; that is, the sheet iron coverings used in the Middle States are almost unknown in New England; and in the latter place the ordinary tinned iron roofing is universally painted, while in the Dominion of Canada it is laid obliquely and never painted.