Collar ties or rafter ties? What a gable roof actually needs to keep its walls from spreading

A poster on r/Roofing asked Wednesday whether a gable roof needs collar ties. The answer starts at the bottom of the rafters. Gravity loads from roofing and snow push rafter feet outward against the walls. The American Wood Council's Wood Frame Construction Manual defines a rafter tie as a member in the lower third of the attic that ties rafters together against that thrust.

Collar ties sit higher and do something else. Where they are used, IRC section R802.4.6 puts them in the upper third of the attic space, at least 1x4 nominal and no more than 4 feet on center, fastened per Table R602.3(1). Code reference site Jaspector notes that ridge straps meeting the listed gauge and nailing can replace them. Their job is keeping opposing rafters together near the ridge when wind tries to lift them. Houston home inspector Fred Willcox calls them high wind-load members.

The low tie is often the ceiling joists. When joists run parallel to the rafters, sit in the bottom third and are nailed to them, they tie the building across. When they don't, the IRC calls for separate rafter ties, or for the ridge to be carried by a beam designed to accepted engineering practice. A plain ridge board is not structural and depends on the low ties.

Trouble starts when low ties come out. Jaspector lists joists taken out to open a cathedral ceiling without adding a ridge beam, and ties cut for pull-down stairs or HVAC platforms, among common inspection failures, and says collar ties added near the ridge do not fix the problem. Willcox writes that missing rafter ties show up as wall cracks and out-of-square door frames that owners often blame on the foundation.