A roof is essentially the same as a floor as regards the elements of construction. Its peculiar features are the pitch, openings for skylights, etc., support of pent houses, of tanks, etc.

The pitch in almost every case where a fireproof roof is used is very flat, generally a minimum of | inch per foot and varying from that according to the requirements of the roof lines in each particular case.

The beams and girders usually follow the pitches of the finished surface of the roof, so that no additional grading on top of roof is necessary, except locally in order to form cradles around skylights and other obstructions, from the down-spout to the wall immediately back of it, and in a few places where the pitch of the roof necessarily changes between the bearings of beams. In general, however, the pitch of roof changes only at the ends of beams and girders.

Fig. 25 BEAM

Fig. 25 BEAM.

Fig. 26 CHANNEL

Fig. 26 CHANNEL.

Fig. 27 ZEE

Fig. 27 ZEE.

The pitching of beams and girders makes it necessary to furr down the ceiling, if this is to be left level, as it generally is. This is done by hanging from the beams a ceiling made either of tile or plastered wire lath on small angles or channels. See "Fireproofing" for illustrations of this.

Tanks and pent houses require special framing for their support, and all roof houses generally are constructed with a frame of light angles and tees.