Collar beam

A tie in building construction is an element in tension rather than compression and most collar beams are designed to work in compression to keep the rafters from sagging.

[2][3] A collar near the bottom of the rafters may replace a tie beam and be designed to keep the rafters from spreading, thus are in tension: these are correctly called a collar tie.

Collar beam roofs are suitable for spans up to around (4.5 meters).

[5] A crown post is a compression member, a post, in roof framing which carries a longitudinal beam called a crown plate.

The crown plate in turn carries collar beams which help support and carry the rafters, thus collar beams are always found in crown post roof framing.