Duchess or other large slates are very often used for iron roofs, and they may be laid either upon boards, or upon angle iron laths, as described in Part I.

Tiling of all kinds may also be used, laid upon laths in the same manner as slating.

Corrugated Iron may be used in the shape of an arch to form the roof itself, without supporting trusses, as mentioned at p. 61 ; or it may be arched and supported by curved angle or T irons; or it may be laid in sheets upon regular trusses of any form.

The sheets may be laid with the corrugations running either way, either horizontally or down the slope of the roof; the latter arrangement is much to be preferred.

The sheets are of course strongest in the direction of the length of the corrugation; the strength depends upon the depth of the corrugation, thickness of iron, etc.; and in this direction they may be left to span spaces of from 8 to 15 feet without support.

If the corrugations run up and down the slope of the roof, the sheets are supported upon purlins; if the corrugations are horizontal, the sheets rest upon the principals themselves, or when these are widely spaced, upon secondary rafters.

Zinc may be laid upon boarding with wooden rolls, as described at p. 63; or on the Italian system, as described at p. 65.

Lead is now very seldom used in iron roofs, having been almost entirely superseded by zinc.

Glass is a good deal used in large skylights,1 which often form a considerable portion of the slope of the roof, and run nearly throughout its length.

1 For glazing without putty see p. 204.

It is also, in large roofs, extensively used upon the "ridge-and-furrow" system. This consists in forming small ^ roofs between the secondary rafters, which rest upon the purlins. The ridges of these small roofs generally run horizontally, and their slopes drain into gutters, lying along the upper flanges of the secondary rafters.

Designing Iron Boofs.1 - In designing an iron roof it should be borne in mind that as many of the braces as possible should be in tension, and the struts should be as short as possible.

When there are only a few purlins widely spaced on the principal rafters, they should be immediately over the joints of the bracing of the roof, so as to prevent bending strains as much as possible.

In such a case the principal rafter is in compression throughout its length.

When, however, the weight is distributed throughout the length of the rafter by means of a number of small purlins, the principal rafter is subjected also to a transverse strain.

In either case the struts should not be so far apart as to necessitate the rafter being of too large a section for economy.

Elaborate forgings should be avoided, and all joints should be as simple as possible. The cast-iron connections between struts and ties so common in old roofs should be avoided.

"For a tensile strain it is safest to have bolts instead of rivets, and sometimes if much depends upon their strength bolts with a nut at each end, so as to avoid the risk of a flaw in the forming of the bolt head."

"In the main tension rods of a roof screwed ends at all the points of connection are advantageous, welds are also so avoided, and there is an opportunity for adjustment." 1

Care should be taken in designing a roof to use such forms, sections, and scantlings of iron as can be readily found in the market. Sections of peculiar dimensions, though perhaps a little lighter than the nearest sections kept by manufacturers, will not only cause delay but cost more.

" In a roof which is rectangular in plan the distance apart of the principals should be from 1/8 to the span, and if these limits be overstepped there will be an unprofitable employment of material."2

It is sometimes economical to adopt the larger rather than the smaller interval, because, when the trusses are widely spaced, there is necessarily a large cross section given to the struts, but their length remains the same, they are, therefore, less liable to buckle under the thrust that comes upon them, and thus more resistance is obtained from an equal weight of metal.

1 From Part I. 2 Matheson.

A hipped roof is more expensive than one with gable ends, but the hipped end is a considerable support to the roof, and itself offers much less resistance to the wind than a gable.1

Trusses which do not contain vertical members are not so suitable for hipped roofs as those having such members.