One for the refurbers, need advice on collar ties

renners, are you working in a code environment? Had to look u collar ties, so take this as uninformed… Collar ties support against uplift and are located at or above the upper third height wall plate to ridge. Metal straps could have been placed on top of rafters warping over the ridge joining all three.
I’d have more concern with the birds-mouthed rafters in the area down slope area of the purlin. Would not the birds-mouth notching effectively reduce the rafter carrying capacity? But hey, it’s still up.
Just an uninformed opinion, perhaps someone really knows the answer.

http://www.nachi.org/collar-rafter-ties.htm

Thanks for that link, there are differences between regs here and where you are, but basically the way things are done is the same. The rafters in this job are at 16” centres, each pair of rafters will have a tie on it for the drywall, so there will be three times the number of collar ties as would be in just a roof.

renners, your “golden rule” is pretty much how we do it.

You’re correct that a slate roof is lighter than a concrete tile roof , but it is still considered a heavy roofing material so adequate bracing is still needed. We brace them the same has we do when bracing for a tile roof, that’s why your rafters are on 16” centers.

If I needed to “cheat” a little and raise the collar ties for head room, I’d use plywood gussets on both sides of the rafters with the 2x collar ties cut to fit beneath the rafters. The height of the gussets would start at the bottom of the collar ties and fit tight to the bottom of the ridge board connecting each pair of rafters at the ridge.

-- Darrell, making more sawdust than I know what to do with

You could probably get by with raising the ties a little and if the first guy did good work and things are flat I would use glue on my new ties. I have seen rafters pull away from the ridge. I am a home inspector and I was in the attic. They had used eye bolts and turn buckles to hold the framing together. Just when you think you have seen it all you find something like that. I think the walls were leaning out. You don’t want a mess like that later.

The term collar tie is really a misnomer as most are actually in compression…. Especially when they are mounted 1/3 down from the ridge. Rafter stiffeners would be a better name.

The load on a pitched roof is translated into vertical and horizontal components. Under heavy load, the rafters want to flex, but high mounted Ted collar “ties” stiffen them at the top and the horizontal reaction is all concentrated at the bottom. This is refered to as thrust.

This is especially a problem on vaulted ceilings, where there is no attic joist to tie the frame together.

To get collar ties to carry tensile loads and oppose thrust, they need to be mounted low.

The cable, eye bolt, turn buckle arrangement referee to by the inspector, is a retrofit to oppose the thrust loads.

If the roof was framed with low mounted collar ties, I personally would take that as a sign that somebody knew what they were doing, and wouldn’t mess with it without having it reviewed by a competent individual.

Good luck!

-- It’s the knowledge in your head, skill in your hands and motivation to create in you heart that makes you a woodworker. - Mainiac Matt