Buildings which are plastered, such as halls, schools, churches, etc., generally have a ceiling supported either directly by the tie beams of the trusses, as in Fig. 249, or by purlins, which are themselves supported at the lower joints, as in Figs. 256 and 160.
In figuring ceiling loads, however, no allowance is to be made for the truss itself, as that is included in the roof loads. The weights to be allowed for ordinary ceilings are given in Section as roof areas. Thus in Fig. 249, if the trusses are 12 ft. apart, the ceiling area supported at joint 6 will be 12 x c, and at joint 7, 105. The ceiling areas are computed in precisely the same manner 7' 8" + 8' 10" 12 x d. The distance c = = 8' 3", and d = 8' 10".
Therefore the roof area supported at joint 6 = 99 sq. ft., and at joint 7, 106 sq. ft.
For the weight of the ceiling joists, lath and plaster, we should allow 13 lbs. per sq. ft., and it would be wise to allow 7 lbs. additional for any odds and ends that might be stored in the attic. If the attic is floored over, an allowance of from 30 to 40 lbs. per sq. ft. should be made, in addition to the dead weight, according to the purpose for which the attic space is to be used.
Allowing 20 lbs. per sq. ft. for the weight of ceiling and light storage, we have for the load at joint 6, 1,980 lbs., and at joint 7, 2,120 lbs.
With the construction shown by Fig. 256, the ceiling joists are supported by purlins, but the ceiling areas are the same as if the ceiling joists rested directly on the trusses. Thus the area at joint 7 will equal c times the distance between trusses, and the area at joint 8 equals d times the distance between trusses.