244. In modern roofs the use of wrought iron in combination with wood has been more extensive than formerly. Instead of being confined to straps and screw bolts, it is now used for king and queen bolts, ties, and struts, and sometimes for principal rafters and purlins. But for common rafters, which require to be battened or boarded over, and for tie-beams, which have to carry a ceiling, wood has the advantage from the facility with which other timbers can be fixed to it.

When the roof is not required to support a ceiling, an iron tie-rod is preferable to a wooden beam.

For purlins, principal rafters, and struts, rolled iron can now be procured of almost any shape or size likely to be required.

As an inquiry into the principle of constructing roofs of iron would be out of place in a work on Carpentry, we shall confine our illustrations to those cases where the use of iron as forming a part of the truss is limited to ties and struts.

The simplest application of wrought iron is in such cases as shown by Plate XVIII., which is the same as the ordinary queen-post truss on Plate II., except that iron rods are substituted for the queen posts. The heads of these rods are fixed to the iron sockets, which take the ends of the straining beam and principal rafters. The lower ends pass through the cast-iron shoes, which receive the feet of the struts that support the principal rafters, and are continued through the tie-beam and secured by a nut, which enables the bolts to be screwed up tight.

Plate XVIII.

ROOFS.

Of the Forms of Roofs for different Spans 199Of the Forms of Roofs for different Spans 200

Plate XIX., Fig. 1, shows a form of roof suitable for a shed, where as much clear space as possible in height is required. The strain on the rafters, where connected by the collar-beam, is relieved by iron tie-rods, which are suspended at a considerable height by the king bolt, to which they are secured by a screwed end and nut. The lower ends of the ties are fixed to the cast-iron boxes (Fig. 2), by which the rafters are attached to the longitudinal bearers over the columns which support the structure.

Plate XIX.

ROOFS.

Of the Forms of Roofs for different Spans 201Of the Forms of Roofs for different Spans 202

A better arrangement, if it did not interfere with the space in the roof, would be to keep the tie-rods horizontal, or nearly so, and to continue the king bolt down to it, as there will always be a tendency to thrust out the sides when the ties are so much inclined as they are shown in Plate XIX.

Fig. 3 shows the arrangement by which the rafters and ridge-pieces are secured in a cast-iron socket.

A better arrangement for an open roof, with iron ties and struts, is shown by Plate XX. The tie-rods are made to pass through the feet of the rafters, and are secured to a continuous plate of wood, which rests on the walls. The struts shown in the drawing are supposed to be cast iron; but a piece of wrought T or angle iron would be preferable, and could be as readily secured to the ties and rafters.

Plate XX.

ROOFS.

Of the Forms of Roofs for different Spans 203Of the Forms of Roofs for different Spans 204

A very superior arrangement for a roof, which has to carry a ceiling, is shown by Plate XXI. In consequence of the suspension of the tie-beam, at so many points, the timber is not required to be of so large a scantling as in the ordinary queen-post truss.

Plate XXI.

ROOFS.

Of the Forms of Roofs for different Spans 205Of the Forms of Roofs for different Spans 206

In long spans, owing to the length required for some of the struts, wrought iron should be used in preference to wood.

There is no reason why the principle on which girders are used in the construction of bridges should not be applied to roofs.

Plates XXII. and XXIII. show a design by Mr. Penne-thorne for the roof of the Lecture Room at the London University.

Plate XXII.

ROOFS.

Of the Forms of Roofs for different Spans 207Of the Forms of Roofs for different Spans 208

Plate XXIII.

ROOFS.

Of the Forms of Roofs for different Spans 209Of the Forms of Roofs for different Spans 210

The purlins were also trussed frames. Indeed, the architect appears to have availed himself of the principles of practical science in a manner that indicates a tendency to improvement in this class of structure.

245. Cast iron has also been extensively used in combination with wrought iron and wood in the construction of roofs, but its adoption is not to be recommended where there is a liability to sudden strains, particularly cross strains.

Plate XXIV. shows a judicious combination of cast iron, in the form of struts, with wrought-iron ties and wood-rafters. For shoes and sockets cast iron is of course unobjectionable, and has been much used in securing the ends of the timber framing: the ease with which it can be moulded to any shape renders it for this purpose a valuable auxiliary in the practice of Carpentry.

Plate XXIV.

ROOFS.

Of the Forms of Roofs for different Spans 211Of the Forms of Roofs for different Spans 212

246. Plates XXV. and XXVI. are designs taken from Emy's and Demanet's works before mentioned.* The first is particularly adapted for a shipbuilder's shed, and the

Plate XXV.

ROOFS.

Of the Forms of Roofs for different Spans 213Of the Forms of Roofs for different Spans 214

Plate XXVI.

ROOFS.

Of the Forms of Roofs for different Spans 215Of the Forms of Roofs for different Spans 216

* ' Traite de Charpenterie,' and ' Cours de Construction.' second might also be used for the same purpose or to cover a wharf, and for a variety of other purposes. It has been used for covering some of the locomotive sheds on the Paris and Versailles Railway.

247. Plate XXVII. shows a design for a curb-roof, which the taste of the present period has rendered common in England. Roofs constructed on this principle, being so much exposed to the effects of the wind, require to be well braced, and if the truss can also be used as a partition, as shown, it will add considerably to the stiffness.

Plate XXVII.

ROOTS.

Of the Forms of Roofs for different Spans 217Of the Forms of Roofs for different Spans 218

The drawing shows the floor-beam of the truss, as unsupported throughout its whole length. Usually there are division walls to the rooms underneath, on which it may be allowed to rest, if they are sufficiently strong to carry the weight.

248. We shall conclude our description of roof trusses with one that was executed more than 400 years ago for the Basilica of St. Paul's at Rome (Plate XXVIII., Fig. 1). The truss is double, that is, consisting of two similar frames placed 14.9 inches apart, and one of these double trusses is placed at about every 10 1/2 feet apart.

Plate XXVIII.

ROOFS.

Of the Forms of Roofs for different Spans 219Of the Forms of Roofs for different Spans 220

The principal rafters abut against a short king post k. Between the trusses is placed a piece of timber s. Sustained by a strong key of wood passing through it, and through the short king posts, this piece sustains the tie-beams by means of another strong key at a. The tie-beams are in two lengths, and scarfed together, as shown by Fig. 2. The scarf is held by three iron straps.

Scantlings of the Timbers.

in.

in.

Tie-beams, t......

22.5

by

14.9

Principal rafters, p ............

2175

by

14.9

Auxiliary rafters, b ...........

13.8

by

13.3

Straining beam, C .............

14.9

by

12.8

Purlins, d ......

8.5

square, and 5 ft. 7 in. apart.

Common rafters ..............

5.3

by

4.25, and 8.5 in. apart.

The roof is made of fir, and the span is 78 . 4 feet. The common rafters are covered with strong tiles, about 12 inches by 7 inches, forming a kind of pavement set with mortar in the joints. On this pavement plain tiles, with ledges, are laid, and the joints covered with crooked tiles, as represented by Fig. 3. From this description some notion may be formed of the load supported by the roof.*

This roof is simple and strong, and the method of sustaining the middle of the tie-beam is ingenious: the covering though heavy is well calculated to exclude the heat and to preserve a uniform temperature within the building, which is a great advantage in a warm climate.

Note. - In former editions of this work was shown a design for a roof of 235 feet span, copied from Kraft's ' Recueil de Charpente,' and stated to have been used for the Riding House at Moscow, built in 1790. As no such roof was executed, according to the statement of M. de Betan-court, Chief Director of Public Roads in Russia, that actually used being of a different design, this roof of Kraft's has been omitted from the present edition. - Ed.