As the tensile strength of iron is much greater than that of timber, it is generally preferable to use the former for any member exposed to tensile stresses only.

Iron king rods would, as before mentioned, probably have come into general use, but that it is difficult to make a simple and good joint where the feet of the struts rest against them.

Iron tie rods would also be preferred to timber tie beams, if it were not that these often have to carry ceiling joists, which could not conveniently be fixed to iron rods.

In some roofs, however, the first difficulty above mentioned has been overcome; when no ceiling is required the second does not exist, and a judicious combination of iron and wood has been effected.

King-Rod, Or King-Bolt Roof (Wooden Tie)

In the roof, Fig. 329, a wooden tie beam is retained to carry a ceiling, but a king bolt is used, and the difficulty in forming a joint for the struts at its feet is avoided by the use of a straining piece (SP).

In roofs of a greater span than 24 feet,1 the tie beam may be supported at intermediate points by similar bolts hanging from the points where the struts meet the rafters.

1 See p. 162.

A further use of iron is here exemplified in the shape of a castiron socket to receive the upper ends of the rafters and the king bolt.

King Rod Or King Bolt Roof Wooden Tie 100294

It will be noticed that the principal rafter carries two purlins, an arrangement which produces a cross strain upon the principal rafters. The tie beam carries ceiling joists to support a lathed and plastered ceiling.

King-Bolt Roof (Iron Tie)

Fig. 330 shows a roof of which both the king bolt and tie rod are iron. The struts abut against one another, and rest upon a nut at the lower end of the king bolt, which can be screwed up so as to tighten the roof if necessary.

Fig. 330. King rod Roof (Iron Tie).

Fig. 330. King-rod Roof (Iron Tie).

Collar beam Roof.   In the roof shown in Fig. 331 an attempt

Collar-Beam Roof

In the roof shown in Fig. 331 an attempt is made to remedy the great defect of the ordinary collar-beam construction by providing tension rods to hold in the feet of the rafters. The more nearly horizontal these tension rods are made, the better is their object fulfilled.

When the feet of the rafters are firmly held in by a tie rod in a horizontal position, as dotted, the collar beam becomes permanently a strut, and the construction is a good one.

The ends of the tension rods and of the king bolt are furnished with screws and nuts, by which they can be tightened up when required.

Those of the tension rods pass through the lower extremities of the rafters, and through plates which abut against the feet of the rafters and extend the whole length of the wall. The upper end of the king bolt is received by a cast-iron socket.

Trussed-Rafter Roof

In the roof shown in Fig. 332 each principal rafter is trussed by means of two timber struts supported by tension rods. The connections are formed with cast-iron joints as shown in the details Figs. 333 to 336. The truss in Fig. 332 is for a 35-feet span. The table below shows the scantlings and number of struts used for other spans.

Fig. 332. Trussed rafter Hoof (Holt's).

Fig. 332. Trussed-rafter Hoof (Holt's).

Trussed Rafter Roof 100298

Fig. 333.

Fig. 334. Head.

Fig. 334. Head.

Such roofs are known as Holt's Roofs, and are especially suited for use in new countries. The iron members are obtainable from stock for spans of from 20 to 50 feet, the timber requires but little preparation, the cost is smaller, and the roofs can be easily erected by unskilled labour.1

Fig. 335. Foot of Principal Rafter.

Fig. 335. Foot of Principal Rafter.

Fig. 336. Foot of Strut.

Fig. 336. Foot of Strut.

Table of Tie Rods and Timbers for Holt's Roofs.

Form of Truss, see Plate III.

Span.

Wrought-iron Tie Rods.

Rafters.

Struts on each side.

A

B

c

D

E

Diam.

No.

Diam.

No.

Diam.

No.

Diam.

No.

Diam.

Size.

No.

Size.

Fig. 343

20

5/8

..

3/4

4

5/8

..

..

..

..

7x3

1

3x3

Fig. 343

25

5/8

..

3/4

4

5/8

..

..

..

..

,,

,,

,,

Fig. 345

30

3/4

1

13/16

4

5/8

..

..

..

..

,,

,,

,,

Fig. 346

35

3/4

2

7/8

4

5/8

2

5/8

4

3/8

,,

,,

,,

Fig. 346

40

13/16

2

7/8

4

5/8

2

3/4

4

3/8

9x3

1

3x3

1

3 1/2x3 1/2

Fig.

3 struts

45

7/8

2

1 1/8

4

5/8

2

3/4

4

5/8

,,

,,

,,

Fig.

50

7/8

2

1 1/4

4

5/8

2

3/4

4

5/8

9x4

2

4x4

1

3x3

In this country cast-iron struts are frequently used for such roofs, as in Fig. 337, which shows part of a Trussed Rafter with one strut.

Queen-Rod Or Queen-Bolt Roof

Fig. 338 gives the section of a roof of a shed at London Bridge Station, which has been referred to as a good combination of wood and iron. The feet of the suspending bolts and of the struts are here received by cast-iron shoes. A cast-iron socket carries the head of the king bolt, and a bracket of the same material supports the end of the tie beam.

Fig. 337. Cast iron Strut.

Fig. 337. Cast-iron Strut.

1 S.M.E. Course. The illustrations are from the price lists of the manufacturers, Messrs. Handyside of Derby.

It will be observed that the boarding of this roof is carried upon a number of horizontal common rafters or purlins.

Queen Rod Or Queen Bolt Roof 100303

Fig. 338.

The tie beam is supported at a great number of points, which renders it peculiarly adapted for carrying a ceiling.

This roof is shown for the sake of illustrating some of the parts, but such a truss would in these days, as a rule, be formed entirely of iron.

Trusses such as those shown in Figs. 331, 332, and 338 would, of course, be placed at intervals, as the trusses are in Fig. 322.