How To Look At A House
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What is a collar tie?
Monday, September 24, 2018
A collar tie is a horizontal piece of lumber connecting two rafters near the ridge of a roof. It is typically a 2x4 or larger, but can be a 1x4, and may be on one or both sides of the rafters. Collar ties are installed to resist the separation of the rafters from the ridge beam under a high wind or other unusual lateral load, and are typically placed at every other rafter pair.
Collar ties are not really adequate to resist downward forces—primarily the weight of the roof itself—that can cause rafters to sink downward at the ridge and splay the exterior walls outward. But we sometimes see them added when a homeowner decides to “open up the ceiling” in a living room of an older home to expose the roof rafters and create a cathedral ceiling. The ceiling joists that are removed do more than just support the ceiling. They also connect the base of a pair of roof rafters to create a triangle shape to resist the spread of the base of the rafters.
While collar ties do create a small triangle that adds some structural enhancement, they are not sufficient to replace ceiling joists, and over time the ridge will begin to sag noticeably. A slight movement of the top of the wall out-of-plumb will also occur, but it is a less obvious defect to a casual observer.
When a horizontal piece of lumber is installed in the bottom third of the vertical distance from the top plate of the wall to the ridge—instead of near the ridge—it is called a rafter tie (see diagram above) and is an acceptable structural alternative when removing a flat ceiling. However, the size and spacing of the rafter ties should be specified by a structural engineer, along the connection bolts.
Also, see our blog posts How much of a roof truss can I cut out to make a storage platform in the attic? and What causes a sagging roof ridge line?
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To learn more about roofs and attics, see these other blog posts:
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