Collar beam

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A collar beam or collar is a horizontal structural member placed between two rafters to resist compression. Common in domestic roof construction to prevent rafter sag, a collar beam is often colloquially called a collar tie, but is unlikely to be one unless placed high or low on the rafter, as a tie in building construction is an element in tension rather than compression. A collar placed near the top of the rafter is in tension, as is one placed below its midpoint towards the bottom. Each would keep the rafters from spreading, and be correctly called a collar tie. A collar beam may also double as a ceiling joist, providing a member for attaching drywall or other material while also serving to resist rafter sag.