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:
• Why is my roof sheathing sagging between the trusses?
• Why is granule loss a problem for an asphalt shingle roof?
• What are the mistakes to avoid when doing attic improvements?
• What causes roof shingles to curl up at corners?
• What causes shingles to buckle along a line on the roof?
• What causes leaks at a fake roof dormer?
• What causes a sagging roof ridge line?
• What causes bubble-like blisters in a built-up and gravel roof?
• Why does it cost so much more to replace a steep roof than a low slope roof?
• What is "ponding" on a flat roof?
• Is an attic required to have a light by the building code?
• How can I inspect my roof for hurricane damage?
• Why is premature curl of roof shingles a problem?
• How can I tell if a roof has more than one layer of shingles?
• What are the common problems with attic insulation?
• What is the life expectancy of an asbestos cement shingle roof?
• What's the average lifespan of a roof?
• Why is it a mistake to replace a roof and not replace its flashings?
• Why is there no attic access hatch in the house?
• What is the building code requirement for an attic access hatch, scuttle, or door?
• Does a roof with multiple layers of shingles last longer?
• What can I do to prevent roof leaks?
• Are roof trusses better than roof rafters (stick framing)?
• Why is a popped nail in a shingle roof a problem? How do I fix it?
• What are the most common problems with wood roof trusses?
• What causes a lump or dip in the roof?
• If my roof is not leaking, why does it need to be replaced?
• How can I be sure my roofing contractor got a permit?
• How many layers of roofing are allowed on a home?
• What are the dark lines running parallel to shingles on my roof?
• Can metal roofing be used on a low slope/pitch roof?
• How can I make my roof last longer?
• What are the warning signs of a dangerous attic pull-down ladder?
• How can I find out the age of a roof?
• Should I buy a house that needs a new roof?
• Should I buy a house with an old roof?
• What are those metal boxes on the roof?
• What does "lack of tab adhesion" in an asphalt shingle roof mean?
• Why do roof edges start leaking?
• Why do my dormer windows leak?
• Do home inspectors go on the roof? Do they get in the attic?
• Should I put gutters on the house?
• How much of a roof truss can I cut out to make a storage platform in the attic?
• What's the difference between an "architectural" and a regular shingle roof?
• What does a home inspector look for when examining a roof?
• Do stains on the ceiling mean the roof is leaking?
• How can I tell if the house needs a new roof?
• Why does my homeowner's insurance want a roof inspection?
• What are the hazards to avoid when going into an attic?
Visit our ROOF AND ATTIC page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.
Diagram - CodeCheck