Roof Rafter Options

Style and Loads Affect Options

• Key factors which affect rafter options are the design of the house, the locale and its weather loads and the size of the roof. Gable roofs use all common rafters, which slope one direction from a peak. Hip roofs also use hip and jack rafters, which slope at different angles. Style and width of a roof will affect its weight. Loads from accumulation of snow and ice or from force of strong winds such as tropical storms affect a rafter's ability to support a roof.

Influence of Lumber

• Lumber type also is an option. Most roofs are framed in pine or Douglas fir, but spruce and other lumbers also may be used. Each type of wood has different load strength, a maximum span or distance it can support without bracing. A pine 2-by-4-inch rafter with normal spacing on a low-slope roof with light loads will span 12 feet 8 inches, Douglas fir only 10 feet 10 inches. Lumber industry tables show the maximum span for each type of wood..

Spacing

• Rafter spacing is another option. Traditional rafters are spaced 24 inches apart, but altering the spacing can increase the maximum span. With 24-inch spacing, for instance, a pine rafter on a steep roof with a heavy load will span 8 feet 6 inches, but that increases to 10 feet 5 inches with 16-inch spacing and 11 feet 6 inches with 12-inch spacing. The closer the spacing, the stronger the roof, but the more material and labor required.

Installation

• There are basically three options for installing rafters: with a ridge board at the top, with rafter angles together at a peak but secured with cross braces or in trusses. A ridge board runs the length of a roof between the top angle cuts of rafters, which are nailed to the board and to walls on the sides. Rafters on smaller roofs often are installed without a ridge board; angled tops are nailed to each other and a cross brace, called a collar tie, is added across the rafters. Trusses are formed with rafters on each side, nailed to a bottom board and with vertical or diagonal braces added between the rafters and the bottom board.