THIS Course includes roofs of from 40 to 60 feet span. Such roofs come under the first head of the classification given in Part I., as they can easily be formed with straight rafters, and it will therefore be unnecessary to notice roofs with arched rafters or mixed roofs in these Notes.
Plate IV. shows various forms of trusses for iron roofs of spans up to 60 feet. As it has already been described in Part I. nothing need be said about it here. It will be noticed that the trusses are so arranged that the principal rafters are supported at intervals not greater than 8 feet.
In Part I. illustrations and descriptions were given of a roof in which each principal rafter was trussed by means of a strut supporting it in the centre (Figs. 264, 265), the strut itself being sustained by tension rods connecting it with the ends of the rafter.
Such an arrangement is best adapted for roofs up to 30 or 40 feet span; but when, as in larger roofs, the rafters become very long, they require support at more than one central point.
Figs. 266, 267, 268, Plate IV., show various forms in which a truss with two struts to support the principal rafter may be constructed. Details of such trusses are given in Plates IV. V. VI. Part I., also in Plates VII. VIII. of this Part.
In roofs of more than 40 feet span the rafters become so long as to require support at three intermediate points; the same principle of trussing may be continued as shown in Fig. 281 and in Plate IX.
Fig. 2 81 on page 155 is also an example of a trussed rafter roof with 3 struts. In this example the rafters are of T iron, the struts of double T iron riveted back to back. The tie rod and upper tension rod are of round iron, and the lower tension rod is of double flat bar iron.
The roof is surmounted by a skylight, supported by a cast-iron standard, and provided with wooden or iron louvres.
With regard to trussed rafter roofs Mr. Matheson says,1 the "forms just described are marked by an absence of vertical members, and for this reason the system is not a convenient one for hipped roofs, and for those roofs also where a longitudinal bracing between the principals is required in a vertical plane."
Fig. 281. Scale, 1/8" = l foot.
A form of queen-rod roof suitable for spans of from 30 to 40 feet was given in Part I.
Fig. 282. Scale, 1/8"=1 foot.
Fig. 282 shows an extension of the same principle adapted for use in roofs of from 40 to 60 feet span.
1 Works in Iron.
In this example the rafters and struts are of T iron, the tension and suspending rods of round iron. The covering is of Duchess slates laid on angle iron laths. The ridge lantern is of similar construction to that last described, covered with slates on angle iron laths, and supported by cast-iron standards.
Fig. 283. Scale, 1/10 = 1 foot.
Fig. 283 shows an extension of the form of truss just described, adapted for roofs of from 50 to 75 feet span.
In this case the struts may be of double T iron, riveted back to back, or of double bar or double angle, or double T irons kept asunder by cast-iron distance pieces. The rafters of double angle iron and the purlins of wood sustaining boarding, on which may be laid slates, zinc, or other roof covering.
The lantern is of simple construction, consisting merely of corrugated iron resting upon curved ribs of T iron, and supported by T iron side standards.
Fig. 284 shows a modification of the queen-rod roof often used in practice.1
In this form of roof the struts are at right angles to the rafter, and are therefore of minimum length.
In the example given the struts are of double bar iron of the construction described at page 158, the rafters of T iron, the tension and tie rods of round iron, the purlins of wood connected to the rafters by angle iron.
1 This example was taken from the roof of a drill shed.
Fig. 284. Scale, 1/8"=1 foot.
The skylight is shown on a larger scale in Fig. 307, page 166, and is there described.
The tie rod is attached to chairs formed upon the heads of the columns supporting the roof (see Figs. 304, 305), and is provided with a union joint in the centre by which it may be tightened.