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.
It is conceded by all that sheet copper forms the most desirable method of covering a roof; and, if one could be assured of the permanence of the structure, irrespective of the necessity for making changes every half year in order to keep pace with the march of invention, it would doubtless be shown that under such conditions of permanency copper would form the cheapest roof.
The most widely used roofing materials for this class of buildings are the asphalt and the coal tar roof, the latter being the most widely used in New England. There are numerous varieties of these composition coverings, which are applied by various methods. Some of these are of the most satisfactory character, while others are poorly designed and unskillfully applied, and are a constant source of trouble and expense to the occupant of the building.
One of the leading manufacturers, the efficiency of whose work for many years over a large amount of mill property I can vouch for by personal knowledge, uses the following method of applying the roofing. Three layers of roofing felt are placed on the plank parallel to the eaves, and continued by lapping each additional layer two thirds of its width upon the preceding one,
and in this manner covering the roof with three thicknesses of the felt, breaking joints. This is secured to the roof by nails through tin washers and coated with a melted composition, and then two additional layers of felt are placed over the whole. Another coat of composition is then applied and gravel is placed over the whole while soft.
This maker does not approve of the practice of cementing each sheet of felt when it is laid, because it does not allow the felt freedom to yield from the expansion and contraction of the roof. When tin is applied to roofs, resin-sized building paper should first be laid on the roof plank, and the sheets of tin should be painted on the lower side before being laid.
Of late years cotton duck has been applied as a roof covering, and has been watched with a great deal of anticipation, although it has been used for similar purposes in covering ships' decks for many years. But the two uses are not strictly comparable, because the ship's deck is calked tight, and therefore the covering is free from the application of moisture underneath, while the roof is never tight, and the warm air underneath, heavily charged with moisture, which permeates the cracks between the planks, becomes chilled and condenses as it nears the top, carrying on a process of distillation.
As an example of the extent to which this can be carried on, I have known of instances where people presumed they were making a good roof by leaving slight air spaces by means of the furring laid between the roof plank and the top boarding. The circulation of air in these spaces deposited sufficient moisture to rot the boards.
A mill manager, wishing to have a roof over a very warm room, which should be both tight and a very perfect non-conductor, made a roof containing a space of about sixteen inches, which was filled with sawdust, and the roof boarding on top of this was covered with tar and gravel in the usual manner. In a few weeks the water began to drip through the ceiling as if the roof was leaking, although there was no snow on the top of the roof. Investigation showed that within that short time a sufficient amount of water had condensed with the sawdust to saturate the whole.
I would say in this connection that three inches of plank afford an ample protection against condensation over any ordinary process of manufacture, although four inches of plank have been used as a roof over paper machines in order to be safe beyond peradventure; but it is necessary that nails should not be driven into the bottom of this roof plank, because the point of a nail will reach to a lower temperature near the outside of the roof in the winter, and being a better conductor, it will cause moisture to condense upon the head of the nail.
Tin roofing is so general in use as not to require any allusion to methods of application, but the only course to reach economical and satisfactory results for a term of years, especially for locations near to the sea shore, is to use the best quality of dipped roofing plates of some brand which can be relied on as conforming to the standard and free from "wasters" or imperfect plates.
Duck roofing has been successfully applied by first laying and tacking down a covering of two-ply asphalt paper, and upon this was spread a covering of resin-sized sheathing paper, tacked in the usual manner. Upon this was laid a covering consisting of cotton duck, forty-four inches wide and weighing twenty-six ounces to the yard. Several methods of joining the edges of the duck together have been tried, resulting in the abandonment of the method of sewing used, for the preferable method of nailing the duck down, laying one strip over the other, and then opening the duck, a lock joint is formed without any jointure between the two sheets exposed to the weather. After the duck is stretched on the roof, it is securely fastened by means of round-headed woodscrews, one and one-fourth inches long, through a concave tin washer three-fourths of an inch in diameter, resting upon a seven-eighths of an inch washer made of roofing felt.
A coat of hot pine tar with a small quantity of linseed oil is laid upon the whole of the duck roofing, after being laid, for the purpose of filling the fiber and preserving the cotton fabric by means of the antiseptic principles of the pine tar. The surface is then covered with two coats of mineral paint.
Within a year, paper has been very successfully used as a roof covering. Sheets of wood pulp board about one-sixteenth of an inch in thickness are treated by a process which renders them hard and elastic, and secured upon the roof by means of tacks through concave tin washers. The edge of each sheet is grooved, in order to allow for the expansion and contraction of the roof. The whole roof is then covered with a heavy mineral paint. Experience with this during the past severe winter in Maine has been of the most satisfactory nature.
Shingles furnish a much better roof covering than slate, both in the matter of conduction of heat or cold in the extremes of summer and winter and also in resistance to fire. The heat of a slight fire underneath the roof will cause slates to crumble; and the same result will be obtained by heavy sparks falling and burning upon the roof. Some people treat shingles by boiling them under pressure in a solution of salt and chloride of lime, for the purpose of antiseptic treatment and also to render them fireproof.