General remarks - Development of roof construction in timber, cast iron, wrought iron, wrought iron and steel, mild steel - Classification of roof principals - Members of roof principals - Upper or compressive member or principal rafter - Sections for principal rafter - Shoes to rafter - Expansion apparatus - Main tie or lower tension member; in timber roofs ; in composite roofs; in wrought-iron roofs; in steel roofs - Risks of defective smith-work - Earlier steel tie-rods - Present-day practice - Flat bar ties - Link tie-rods - Occasional stiffening of main tie-rod in small roofs - Examples of tie-rods - Intermediate bracing - Struts - Ties - Purlins - Influence of nature of roof covering upon the arrangement of purlins - Details and sections of purlins - Distance apart of main trusses - Intermediate rafters - Roofing accessories - The collection and disposal of rainwater or melted snow - General arrangement of roof drainage - Roof guttering in cast iron or riveted steel; in lead - Experiment on rate of discharge in gutters and cesspools - Area of roof surface to be drained - Examples of guttering and down-pipes - Expansion joints - Stopped ends - Lanterns; skylights, and ventilators - General remarks - Lantern standards - Louvre blades - Roofs of flat pitch - Examples of roof construction of various types - Special type of roofing combined with vertical supports - Details - The testing of roof principals - Conditions of practical testing in the contractor's yard - Methods of measuring deformation and settlement - Remarks on cottering up - Setting out of roof principals - Scribing floor.

It is impossible, within the limits of a single chapter in a collection of "Notes" such as the present, adequately to deal even with the main features of roof construction in mild steel or iron.

All that the writer can hope to accomplish is to offer such suggestive remarks on the subject as may assist the student to a fuller consideration of this branch of practical construction, and lead him to a careful study of the numerous existing examples.

It will be assumed that the student has made himself acquainted with the usually accepted theories as to wind pressure, and the conditions of loading of a roof truss arising from dead load of structure, weight of snow, and wind pressure, whether the latter be considered as acting vertically, normal to the slope of the roof, horizontally, equally or unequally distributed; and that he is acquainted with the usual methods of calculations of stresses, such as graphic analysis and the method of sections.

Nor can the extensive subject of the nature and properties of the various kinds of roof coverings, such as slate, tiling, glass, zinc, copper, lead, corrugated iron, felt, and the like, be considered, except so far as they may influence the arrangement and detail of the metallic structure which is designed to support them.1

The history of the development of roof construction from its earlier forms in timber, and through the further stages of cast iron, wrought iron, with various cast-iron details, then wrought iron practically alone, wrought iron with steel tie-bars, and lastly, as at the present time, in mild steel, with occasionally some admixture in detail of the other two metals, would doubtless be both interesting and instructive, but practical consideration can only here be given to the final stage of mild steel.

Roof principals may be roughly divided into four main divisions, viz.:-

(a) Principals with straight upper rafters, of varying degrees of pitch.

(b) Principals with curved or polygonal upper rafters.

(c) Principals of special constructions, including the arch, arch with one, two, or three hinges, or of the "sickle "type indicated in Fig. 345, usually employed only in large spans.

(d) To the above may be added another class sometimes employed in covering large areas, in which lattice girders with parallel booms, and sometimes of large span, are placed side by side, and roofed over with intermediate principals, usually of the first type described above, but occasionally of a special class. In this class the upper boom of the main lattice girder supports a valley gutter for its entire length, or a series of ridges and valleys is arranged to cover the intermediate space. A further subdivision may also be taken to include a type of roof covering where the conditions require a cantilever method of construction, such as large verandahs, or the form frequently met with in railway station platforms, where the position of the supporting columns at some distance from the edge of the platform necessitates the continuation of the roof truss in the form of a cantilever. In all such cases the deflection of the cantilever portion must be carefully borne in mind, in order that the construction, guttering, etc., at the eaves may maintain their true and horizontal lines.

1 The student will find valuable assistance in the subjects of roofing materials in Vol. I. and in the Calculation of Stresses and the Theory of Roof Loading in Vol. IV. of "Notes on Building Construction."

Another arrangement of roof principals has been suggested, dividing them into two classes, intended to cover all roof structures, namely, the one in which the reactions of the supports of the principal are in a vertical direction, and the other in which the reactions are at an angle with the vertical, the first class being self-contained without horizontal thrust, the second those with a horizontal thrust, and dependent upon the resistance of the abutments for their stability.