The Viaduct Works' Handbook

Roofs of the arched form are sometimes preferred as presenting less surface to the wind, and as requiring less height of supporting walls or columns than the ordinary ridge roofs for the same height inside. When fine interior effect is desired, the arch may approach the semicircle, or assume the proportions of a bold ellipse, being in such cases formed to dispense with transverse ties or trussing. The following sketch is an example of this class of roofs recently made at Crumlin, of very substantial scantlings, calculated to resist the most severe hurricane of a tropical climate. The ribs are of the latticed girder principle, varying in depth according to the span ; covering is of very stout corrugated iron, and the top of the Louvre ventilator is covered with plate glass roughened on the inside for the purpose of moderating the effect of the sun’s rays. If it is desired, an inner lining can be added of corrugated iron of a thinner description than that outside, forming a double roof with air space between. This arrangement is often adopted in a tropical climate, as it admits of a current of air passing between the two coverings, which greatly assists in cooling the atmosphere of the building. Roofs of this class are made of very large spans. Some ex- amples are now in course of erection in this country upwards of 200 feet clear span, but it is desirable, if circumstances will admit, to introduce some kind of internal bracing, in the manner shown by the following sketch, particularly where the spans are very large, and in an exposed position, or subject to hurricanes. Large roofs of this class, however, are exceptional, and it is not proposed to do