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Old 04-06-2012, 11:52 AM   #1
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Roof structure question


My attic consists of a 2x6 ceiling joist, 2x6 rafters, and a 2x8 ridge beam, spaced on 16" centers. Every 3-4 frames is a collar tie (?) that consists of a 1x6 nailed to one side of the rafters, and it looks like this board was meant to be flooring as it has a tongue and groove on either long edge. These boards are warped badly, one of them kinda looks like a egg noodle. My question is, are these boards needed? (I'm thinking yes) If they are necessary, can I replace them with 2x4s fastened a little higher on the rafter, giving me more crawling room? I could even put 2x4s on either side of the rafters if the extra strength is needed. The house was build 1977-78 and is located in PA, so I would expect snow loading. The roof consists of two layers of sheathing and asphalt shingles. Also it is a fairly shallow slope of about 20 degrees, leaving me about 4ft clearance at the peak.

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Old 04-06-2012, 12:52 PM   #2
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Roof structure question


Hi.

The tongue and groove boards were likely what happened to be kicking around by the time the roof was framed.

What is the span of your rafters? Depending on the span these ties are basically useless. Without ceiling joists, the ties would serve as anti wall spread, and anti rafter sag. You've got the anti wall spread, and most likely your rafters are robust enough and not spanning such a great distance that they will sag without them.

With that being said, you can certainly replace them with 2Xs. However, once you get too close to the ridge with them, they aren't doing anything.

Godspeed.

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Old 04-06-2012, 01:35 PM   #3
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Roof structure question


The purpose of collar ties is often misunderstood. They are there to equalize uplift on the roof from wind loading. They are not there to resist thrust load on your walls, your floor joists do that. Collar ties are used when you have a ridge board, which is what you have, rather than a ridge beam. A ridge beam is a structural element, typically a glulam or similar beam, that is held up by posts on either end, and sometimes with a post in the middle. A ridge board, which I believe you have based on your description, is a non-structural board used as a nailing surface for the rafters.

Typically collar ties are installed on every other rafter, sometimes every third rafter, but never as you describe on every fourth rafter. If you remove them, your roof is not likely to fall down, but it will not perform well in high wind situations, because the uplift pressure will not be equalized from one side to the other. Collar ties are generally installed approximately 1/3 of the way down from the peak of the rafter to the floor, if they get higher than that they do not work as well. You can certainly use 2x4 lumber rather than 1x4 or 1x6, or whatever you have. I do not recommend removing them due to the uplift issue.
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Old 04-06-2012, 05:55 PM   #4
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Roof structure question


Thanks for teaching me something about roof construction! The roof is 24' across, not counting the eave overhang. They are every third rafter, I just couldn't remember the spacing and wasn't at home to check. They are also about a third of the way down from the peak too. So from your description of how they function, the collar tie is under a tensional force during wind, correct? I couldn't imagine putting a compressive force on them, they would fold up like paper. The reason why I was asking about raising them is that at one point a vent duct runs underneath the collar tie, leaving only about a foot to squeeze my fat butt between. I suppose I could install a 2x4 on the other side of the rafter, the tops of the two being even, which would give me 2 inches of extra clearance. My biggest concern was the sorry state of this lumber; one is cupped and has a nice 's' curve to it from old age. I'm currently fixing my attic insulation, and once I'm done I'll have so much up there that I don't really want to have to climb back up there to replace them in the future.
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Old 04-06-2012, 06:23 PM   #5
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Roof structure question


Quote:
Originally Posted by Daniel Holzman View Post
The purpose of collar ties is often misunderstood. They are there to equalize uplift on the roof from wind loading. They are not there to resist thrust load on your walls, your floor joists do that. Collar ties are used when you have a ridge board, which is what you have, rather than a ridge beam. A ridge beam is a structural element, typically a glulam or similar beam, that is held up by posts on either end, and sometimes with a post in the middle. A ridge board, which I believe you have based on your description, is a non-structural board used as a nailing surface for the rafters.
Brilliantly put. The only thing I would add is that the floor joists also resist out-thrust of rafters when the joists are running parallel, and are fastened, to them. When joists run perpendicular to rafters, an additional member is required: rafter ties. Rafter ties are usually located right above the floor joists and connect opposing rafter ends. Often, people confuse rafter ties and collar ties. They perform very different functions. Think: collar ties=above, rafter ties=below. When joists and rafters run in the same direction, the floor joists are the rafter ties.
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