Methods of reducing rafter spans

Structure · Collar Ties, Knee Walls and Purlins · printed page 79 (PDF page 80)

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Book index entries

Collar Ties Rafter – Spans – Reducing Roof – Span Reduction Spans – Methods of Reducing for Rafters

Text in the illustration

collar ties
span
roof rafter
ceiling joist
wall
if the collar tie is more than about 8' long, it requires lateral support in the form of a 1x4 nailed perpendicular to the collar ties (prevents buckling)
12
4
collar ties can be used to reduce the span of a roof rafter when the roof slope is 4 in 12 or greater
collar ties are not effective when roof slope is less than 4 in 12
purlins
purlins - typically 2x6 minimum running perpendicular to roof rafters
must be greater than 45°
supporting struts should be 2x4 min., typically every 4'
struts longer than 8' should be braced to prevent buckling
bearing wall
knee walls
dwarf wall or strut
3
ceiling joists must be 1" deeper than normal (or even deeper if the roof slope is 3 in 12 or less) to carry the extra load
install solid blocking between joists when ceiling below is finished
cross section

Generated explanation

Three roof-framing cross sections illustrate methods of reducing unsupported rafter spans using collar ties, purlins with inclined struts, and knee walls. The upper diagram shows a horizontal collar tie between opposing rafters and notes roof-slope limitations and lateral bracing requirements for long ties. The middle diagram shows purlins beneath the rafters supported by inclined struts that transfer load to a central bearing wall, with annotations for strut angle, size, spacing, and bracing. The lower diagram shows dwarf walls or vertical struts supporting a lower-slope roof above the ceiling joists. Its annotations call for deeper ceiling joists to carry the added load and solid blocking between joists where the ceiling below is finished.

Components

roof rafters collar ties lateral bracing ceiling joists purlins supporting struts bearing wall exterior walls knee walls dwarf walls solid blocking

Materials

wood framing lumber