As before stated, the ordinary mill roof is usually flat, i. e. with a pitch of about \ inch to the foot, and is framed in precisely the same way as the floors, the under side of the roof forming the ceiling of the upper story.
This form of roof may also be* used for warehouses, but for buildings that are to be constantly occupied, as office buildings, hotels, etc., it will be necessary to provide a ceiling below the roof, to prevent the rooms in the upper story becoming too hot. The space between the ceiling and the roof should not be less than 4 feet at the lowest point, and should be ventilated by openings in the side wall.
Pitch Roofs. - Mill construction may also be used to advantage in constructing pitch roofs over churches, and wherever the attic is to be finished. Fig. 500 shows a section through the eaves of a church roof constructed on this principle.
The rafters are made of heavy timbers spaced from 5 to 6 feet apart, and covered with 2-inch matched planks. On top of the planks should be spread a layer of lime mortar, about ¾ of an inch thick, which may be kept in place by horizontal strips, ¾x1½ inches, nailed to the planking about every 2 feet. Over the plaster, shingles, slate or tile may be laid in the usual manner. The layer of plaster would delay the roof catching fire from the outside, and also reduce the transmission of heat. The under side of the roof planking should he furred with 3/8-inch strips, and then lathed and plastered. Metal lath would of course be best. Sheathing lath may also be used. On this coat of plaster diagonal ceiling may then be placed, if a wood finish is desired, or it may be again furred, lathed and plastered. The two thicknesses of lath and plaster, with a ½-inch air space between, would undoubtedly make the room beneath much warmer in winter and cooler in summer. If the roof span is over 25 feet wide, every third pair of rafters must be trussed, and purlins hung under the middle of the other rafters.
Partitions. - When ■mill construction is used for the floors and roof the partitions, unless of brick, should also be made of solid timber, without air spaces, or else of 1 ½-inch I beams, and metal lathing and plaster. Fig. 497 shows a wooden partition, made of 2-inch planks, covered with the Byrkit-Hall Sheathing Lath. This lath adds much to the stiffness of the partition, and under fire would probably hold the plaster as long as the rest of the construction would stand.
Mill construction, when intelligently carried out, undoubtedly ranks next to steel construction in durability and fire resistance, and far surpasses unprotected iron or steel in withstanding a fire. It has been found, however, that where tall buildings are constructed on this principle, if a fire once gets under headway, it is almost impossible to save the building, the advantages of this method of building being more pronounced in buildings of not more than three stories.
The Chicago Building Ordinance permits the use of mill construction for office buildings, stores and warehouses between 60 and 100 feet in height, While the use of ordinary construction is limited to buildings under 60 feet in height.