In the early days of steel trusses, T sections for rafters and struts were usually employed, with rounds and flats for the tie bars. Modern methods of fabrication have gradually developed the use of the L (angle) section for nearly all structural members in roof trusses of moderate span, double angles being used for rafters and ties, single gusset plates riveted between the angles, and ties and struts of either single or double angle section riveted to the faces of the gussets.
The guiding principle is simplicity of form and of detail, and efficiency for service. Often more material is employed in a member than is strictly necessary--and a more economical use of material might be arranged—but usually economy of material means a loss in fabrication costs or the necessity of carrying stocks of many and variable sections of steel.
Double channels are very convenient and largely used for compression members in large span roofs and in unusual struc tures.
The types of trusses in modern use are given in fig. 9. These include the "queen rod," having vertical ties and inclined struts, the "trussed raf ter," in which the struts are at 9o° to the rafter, and variations of these forms. Perfect triangulation of steel frames is easy to arrange and the dimensions of the members can be suitably arranged by selection from a large number of standard sections of rolled steel.
Where large areas of uninterrupted floor space are required in textile sheds, garages, etc., a form of roof is often adopted in which main girders of the parallel type, up to 15o ft. span, sup kept down so as to give greater elegance to the eastern turrets. The apsidal termination of the choir in the east is covered in with a concrete vault surmounted by a timber roof, in striking contrast to the domes covering the other portions of the struc ture. Fig. i i is a section through the nave showing how the domes are buttressed, fig. 1 2 is a section through the sanctuary dome, and figs. 13 and 14 a section and part plan of the vaulting of the choir with its wood span roof above the concrete vault.
Covering Materials for Roofs.—There are many different roof-covering materials in common use, of which the principal characteristics are given. The nature of the outer covering affects the details of roof construction in many respects. A light cover ing such as felt or corrugated iron can be safely laid upon a much lighter timber framing than is necessary for a heavy covering of tiles or slates.
Roofing felt is an inexpensive fabric of animal or vegetable fibre treated with bituminous preparations to make it capable of resisting the weather. It is used as a roofing material for tem porary buildings. When exposed to the weather it should be treated with a compound of tar and slaked lime well boiled and applied hot, the surface being sprinkled with sand before it be comes hard. Bituminous felt is employed as a non-conducting
and safeguarding under-cover to slates and tiles, used only for the roofs of important buildings. The felt is supplied in rolls containing from 25 to 35 yd. by 3o in. wide. The sheets should be laid with a lap of at least 2 in. at the joints and secured to the boarding beneath by large-headed clout-nails driven in abOut 2 in. apart.
Corrugated iron is supplied either black or galvanized. It is especially suited for the roofs of out-buildings and buildings of a temporary or semi-permanent character. Being to a large extent self-supporting, it requires a specially designed roof framework of light construction. If, as is usually the case, the sheets are laid with the corrugations running with the slope of the roof, they can be fixed directly on purlins spaced 5 ft. to i o ft. apart according to the stiffness and length of the sheets. In pure air zinc coating of the galvanized sheets is durable for many years, but in large cities and manufacturing towns its life is short unless protected by painting. In such districts it has often been found that plain ungalvanized sheets well coated with paint will last, longer than those galvanized, for the latter are attacked by cor rosive influences through minute flaws in the zinc coating devel oped in the process of corrugation or transit or resulting from some defect in the coating. The stock sizes of corrugated sheets vary from 5 ft. to i o ft. long, and from 2 ft. to 2 ft. 9 in. wide with corrugations measuring 3 in. to 5 in. from centre to centre. For roofing purposes the sheets are supplied in several thick nesses ranging from No. 16 to No. 2 2 Standard Wire Gauge. No. 16 is for exceptionally strong work, No. 18 and No. 20 are used for good-class work, and No. 22 for the roofs of temporary buildings. The sheets when laid should lap one full corrugation at their sides and from 3 in. to 6 in. at the ends. Riveting is the best method of connecting the sheets, although galvanized bolts, which are not so satisfactory, are frequently employed. The joints should be made along the crowns of the corrugations to avoid leakage. Holes can be punched during the erection of the roof. For attachment to timber framework, galvanized screws or nails with domed washers are used. Fixing to a steel framework is effected by galvanized hook bolts which clip the purlins and pass through the sheet. Sheets corrugated in the Italian pattern have raised half-rounds every 15 in. or so, the portions between being flat. Such sheets have a very neat appearance and give a better effect in some positions than the ordinary corrugations.