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MtnDon
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# Posted: 25 Feb 2011 08:46pm - Edited by: MtnDon
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This discussion/tip concerns gable roofs and their collar ties and rafter ties. I have found some confusion out there when it comes to these two vital gable roof components. Thay are not the same, they are not interchangeable.
A picture is worth a thousand words...
That should be self explanatory, but take note that the collar tie is in the top third of the space. The collar ties help keep the roof together at the peak. High winds blowing over a roof up to the ridge can act like an airfoil, causing lift forces. Collar ties keep the peak together, metal hurricane ties keep the eves connected to the walls.
Rafter ties are located in the lower third. Rafter ties keep the wall tops from spreading apart. Loads on the roof, snow, wind, rain are transfered by the rafters to the wall top. Some of that force is down the wall and some horizontally outward. The lower the pitch the greater the outward horizontal force. With a pitch of 12:12 the downwards and the outwards forces are equal.
Rafter span tables are calculated assuming the rafter ties are located at the bottom; across the wall tops in other words. When rafter ties are raised to increase headroom corrections must be applied to the rafters. This will sometimes result in requiring a larger size rafter.
Loft floor joists across the walls that are below the side wall top plate do not count as rafter ties. The so called kneewall above the loft floor results in increased side forces on the side walls. Wall studs are meant to support axial loads, top to bottom, as well as wind forces against the wall. If there is a kneewall with rafter loads being applied above the loft floor wind forces may total up to exceed the carrying ability of wall studs. Stud grade lumber has the least resistance to bending of all the commonly used lumber grades.
Just FYI
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KinAlberta
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# Posted: 13 Mar 2016 12:14pm - Edited by: KinAlberta
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This is a great post!
However, in searching google images of open beam and cathedral ceilings, I see lots of photos of ceilings where the ties appear to sit about half way down and there are no ceiling joists/rafter ties.
I also see larger beams going from wall to ridge, spmetimes with a large exposed ridge beam, sometimes not.
In terms of eliminating both collar ties and ceiling joists or mostly eliminating them the collar ties, (which I want to do in removing a cabin's ceiling) rare are the photos of 3 beams - a ridge beam with two outlying beams, one on either side midway up each side.
In terms of my last point, I'm not sure why this seems so rare in my searches. Is there a name for it. Similar to what you can see here:
http://homerenovations.about.com/od/wallsandtrim/a/VaultedCeiling.htm
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toyota_mdt_tech
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# Posted: 13 Mar 2016 01:12pm - Edited by: toyota_mdt_tech
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Also, a added note, in my cabin, a collar tie was on every 3rd rafter (with 16" OC spans) and if you have a 24" centers, every other one. I suppose this would vary depending on snow and wind loads too. And a guy could certainly add one on every rafter section too for that HD upgrade.
My ridge beam was of the non load bearing type, but I did install a post in the middle of the ridge in the attic over an inside load bearing wall (load bearing in the sense it held up the ceiling joist/rafter tie span mid way point) and when the concrete guys did my foundation, I had them pour an extra pier pad to extend that point load in the middle of the ridge all the way to that extra pad in the crawlspace. It was not needed ie the plans was signed off, but I thought it could help if a large tree or branch ever hit the roof, help to keep it from breaking the spine. This was not in the plan, but it was a very low cost HD upgrade, which I like. 
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Don_P
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# Posted: 13 Mar 2016 01:16pm - Edited by: Don_P
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This is one place where mixing up terms creates confusion, review before posting.
The most common way of eliminating rafter ties is to use a ridgebeam rather than a ridgeboard. If the beam is sized to support the rafters hanging from it, there is no spreading force needing to be restrained.
what you are describing I believe is a purlined roof, I've attached a pic of one with several purlins rather than rafters. I suppose the terminology for this one would be principle truss-purlin roof. The purlins are sized for the span between trusses at their spacing up and down the roof. Especially on some of these smaller builds the purlins could run from gable wall to gable wall and eliminate the spreading issues.
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MtnDon
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# Posted: 13 Mar 2016 01:31pm
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Wow, that OP sat there for over 5 years before anyone commented. 
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bldginsp
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# Posted: 13 Mar 2016 01:55pm
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Cabinbuilder recently began including photos and links to past posts at the bottom of the page, as you may have noticed, which probably caused Kinalberta to see it, correct me if I'm wrong. But I think it's a good way to bring up old posts with good info, thanks Cbuilder.
Only things I'd add to the post is that it's easy and more effective to replace collar ties with straps across the rafters on top. Collars just join the two diaphragms, straps do it better.
Don is correct in stating that rafter ties located below the top plate don't work well, but not just because this design strains the studs. It also replaces a triangle with a 5 sided polygon. The triangle of ceiling joist/rafter tie and rafters is a rigid unit that tightens up the whole roof assembly.
Don is also correct that ceiling joists/rafter ties located higher on the rafters, above the top plate, alters the engineering and may affect rafter engineering. But it does retain the integrity of the triangle design so long as the joists are in the lower 1/3rd of the rafters. I've seen engineers do this and call out through bolting of joist to rafter since this connection becomes more stressed the higher it is on the rafter.
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KinAlberta
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# Posted: 13 Mar 2016 08:55pm
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Not the case with me. I want to remove the ceiling in a cabin and have been searching this site and everywhere via Google. I've encountered this posting a few times.
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MtnDon
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# Posted: 13 Mar 2016 10:22pm
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If I interpret what you state correctly, you want to remove a "normal" flat / horizontal ceiling and end up with a vaulted cathedral ceiling. If this was built in a more or less normal way the ceiling joists are doing double duty as rafter ties, assuming the ceiling joists are resting on and secured to the side wall top plates. I'm already making too many assumptions so affirmation of the assumptions so far would help eliminate more guesswork.....
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Quoting: KinAlberta I see lots of photos of ceilings where the ties appear to sit about half way down and there are no ceiling joists/rafter ties.
That is one of the reasons I made the OP. We can find lots of pictures of this and most of them are poor design, poor construction, because most of them do not have a properly engineered ridge beam.
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KinAlberta
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# Posted: 26 Mar 2016 09:42am - Edited by: KinAlberta
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Yeah, I've been surprised at the potential depth of possible ridge beam options over the span, so I figure I'd have to support them midway, up from one of the bedroom walls.
As for tying in the walls: On another thread I also discussed the possibility of a horizontal lay perimeter beam to hold the walls in. Since then I've realized that this could be done not only on the inside but also on the outside tied to the eaves since the rafters form the underside of the eaves in my cases.
A couple less imposing rods might work well and be easier to attach.
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Don_P
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# Posted: 26 Mar 2016 07:02pm
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The horizontal beam would work, but as you said, attachment is the tough part, the end supports need to be capable of resisting the accumulated horizontal force and take it to ground.
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