Collar Ties
Collar ties span between and connect opposing rafters, and they are typically placed in the top third of the attic spaces. Collar ties hold the rafters down against the ridge board and, to an extent, help prevent rafter spread. Home inspectors should take note of attics without collar ties, especially where the rafters run perpendicular to the joists. Collar ties must be at least 1x4 (nominal) and spaced not more than 4 feet on center. A lack of collar ties may allow roof rafters to separate and cause sagging of the roof's ridge and structure.www.nachi.org