25/04/2023
ROOF FRAMING
Roofs in all but a very small number of American houses are supported by wooden frameworks, for wood combines suitable strength and relatively light weight (as does metal, which is rarely used for this purpose in houses), thus making possible the long spanning members required for roof support. Two principal
roof-framing systems have been used in American houses. The earlier employed heavy principal rafters with hewn joints as the principal supporting members. Lighter members (either common rafters or common purlins) were placed between the principal rafters to provide a base for attaching the roofing material. This system is analogous to post-and-girt wall framing and, indeed, is found on most early post-andgirt—as well as masonry—houses. With the rise of lighter braced, balloon, and platform wall framing it was discovered that light, closely spaced common rafters, joined by nails, provided adequate roof support without the need for intervening heavy timbers. Such common rafter roof-framing systems are almost universal in American houses built after the mid-19th century.
Principal rafters of heavy timber require little additional bracing to support the weight of the overlying roof, particularly if the roof is steeply pitched and spans a relatively narrow space below. Rafters of lighter weight, lower pitch, or longer span require underlying supporting systems of joists or trusses.
Simple gabled roofs always require the least complex underlying framing; the additional roof planes of gambreled or hipped roofs demand additional framing members.
