Horizontal projection

Structure · Rafters, Roof Joists and Ceiling Joists · printed page 75 (PDF page 76)

Book index entries

Horizontal Projection of Rafters Determine Span Rafter – Span Determined by Horizontal Projection Roof – Horizontal Projection Determines Span Span of Rafter Determined by Horizontal Projections

Text in the illustration

20 feet
10' span = horizontal projection
roof rafter
ceiling joist
wall
5' span
collar tie
dwarf wall or strut

Generated explanation

Three schematic roof-framing cross sections illustrate rafter span measured by horizontal projection rather than along the sloping member. The upper diagram shows a 20-foot-wide gable roof with a 10-foot horizontal rafter span from the exterior wall to the ridge, and a ceiling joist connecting the exterior walls. The middle diagram includes a collar tie and an interior wall beneath the ceiling framing, with the horizontal projection divided into two 5-foot spans. The lower diagram shows a shallower roof with intermediate supports labeled “dwarf wall or strut,” a vertical support beneath the ridge, and an interior wall below. Its horizontal rafter projection is likewise divided into two 5-foot spans.

Components

roof rafters ceiling joists collar tie exterior walls interior wall dwarf wall or strut ridge support

Materials

wood