Contradictions in Use of Collar Beams

Abstract

Recommendations for the use of collar beams—horizontal timbers connecting opposing rafter pairs in gable-type wood-frame roofs—are often confusing and contradictory. This is due in large part to their peculiar history: four distinct types of collar beam roofs have evolved, each corresponding to a different structural purpose. Those purposes are to support roof rafters at the ridge, thereby preventing thrusts from developing at the eave; to resist the horizontal thrust of the rafters where conventional tie-beams are omitted; to act as compressive braces, increasing the potential span of the rafters; and, finally, to prevent uplift in high winds, by acting as ridge anchors. A fifth, nonstructural purpose can be seen in the use of collars to frame attic ceilings. In light of the many inconsistencies found in their specification, it is suggested that guidelines be developed for the proper use of collar beams in wood-frame construction.