This section is from the book "Notes On Construction In Mild Steel", by Henry Fidler. Also available from Amazon: Notes On Construction In Mild Steel.
The arrangement and construction of these important members of a roof structure must now be considered, and it will be found that the class of roof covering to be adopted will have considerable influence on their design.
Thus, if a covering of slates or tiles be used without boarding or battens, the purlins will take the form of angle laths, spaced at a distance corresponding with the gauge of the slates or tiles, which are wired to them. This is a common form of covering in gas-retort houses or similar structures.
Again, if slates, zinc, lead, or copper are laid on boarding, then the distance apart of the purlins will be regulated by the maximum span, which can be assigned to the boarding, allowing for dead and live loads, and with a proper amount of stiffness, or absence of undue deflection. Or if zinc be laid with Italian corrugations upon wood rafters, then the maximum span allowable for the rafters will determine the pitch of the purlins; while if one or other of the numerous patent forms of glazing be adopted, it will be found necessary to accommodate the spacing of the purlins to the details of the system employed, including also consideration of the maximum length of sheet-glass to be used, and the allowable span of the sash-bars.
A similar condition will be found to prevail when zinc is laid on boarding with drips, and the length from drip to drip will be ruled by the standard length of zinc sheet to be used, allowance being made for the length of sheet taken up in forming the drip, tucks, overlaps, etc.
Generally, conditions such as those outlined above will govern the pitch and setting out of the purlins, and following thereon the arrangement of the bays of intermediate bracing, and the subdivision of the main rafter.
The details and sections of the purlins themselves will be dependent upon the load to be carried, and their span, that is, the distance apart of the main trusses. This latter dimension usually varies with the span of the roof, although in many cases other considerations may govern the distance apart of main principals, such as the distance apart of column foundations (when ruled by local circumstances), the arrangement of the piers in supporting walls, and the like. Thus, for spans up to, say, 40 feet, a very usual distance centres of principals is from 6 to 10 feet. Principals of spans of 100 to 200 feet are commonly spaced 25 to 35 feet apart, while trusses of such exceptional spans as 300 feet or upwards may be from 50 to 70 feet apart.
The purlins in structures of such dimensions as the latter are lattice girders, of considerable depth and weight; those of, say, 25 to 35 feet in span may consist of trussed angles or tees, or occasionally rolled joists, while those of 6 to 10 feet span are usually either single angles, tees, channels, or rolled joists of light section.
Where the main principals or trusses are as much as 25 feet apart, intermediate rafters resting upon the purlins are frequently adopted, thus subdividing the spaces to be covered, and resulting in a span which can be met by one or other of the roof coverings above referred to.
Purlins of considerable span, such as 25 feet, when consisting of braced beams or lattice girders, and arranged so that the plane of the web of the girder is normal to the slope of the main rafter, which is frequently the case, are subject to a twisting moment, due to their centre of gravity having a lever arm about the point of support, which should not be overlooked.
This consideration points to the desirability of so arranging for heavy purlins that their webs lie in a vertical plane, and this will usually lead to the adoption of vertical members in the system of intermediate bracing in the main truss.
Such vertical members are also useful in the case of hipped roofs, and simplify attachments.
Such considerations will, however, only apply in the case of purlins of considerable span and weight.
Roofing Accessories - The collection and disposal of Rain-water or Melted Snow - Skylights and Ventilators. - The arrangement of the general scheme of roof drainage, and of the principal and secondary gutters, with their cesspools and downpipes or spouts, should always receive the very careful attention of the designer.
The material used most frequently for roof guttering and rainwater pipes in iron constructions is cast iron, although occasionally riveted steel gutters are used in special situations. In timber roofing for ordinary building construction, timber guttering, lined with lead or zinc, is commonly used, but to this latter form of construction further attention need not here be given.1
The disposal of rain-water, and the necessary dimensions to be given to gutters and down-spouts for the safe drainage of any given area of roof, constitute an important branch of the science of applied hydraulics, combined with due consideration of the meteorological conditions of the site as regards the maximum rainfall to be provided for, especially if the roof is to be designed for tropical climates, where excessive rainfalls, occurring over greater or lesser periods of time, are to be anticipated, and must be duly and efficiently met, with a sufficient margin of safety against overflow and flooding.
The purely hydraulic questions connected with the flow of water in channels, and through orifices which have to work under the widely varying conditions met with in practice, cannot here be dealt with, and it is probable that much of the design of this important branch of roof construction has been carried on by empirical, or more or less rule-of-thumb, methods, based, no doubt, on practical experience, but with which the ordinarily accepted rules and formulas of applied hydraulics have had little to do.
1 See "Notes on Building Construction," Vol. i.
Experimental evidence on many points connected with the design of guttering, cesspools, and drainpipes is, so far as the writer is aware, to some extent lacking, and it is much to be desired that correct information on these subjects should be extended.