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bobbotron
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# Posted: 4 Sep 2012 02:29pm
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I know this has been covered before, but bear with me. 
I have a 12/12 roof that's 16' wide, 22' long. Loft with ceiling joists for 8" 6" of it, so that part of the building is "rafter tied" quite well. Trying to decide what do do about the 14" section. I'm thinking doubled up rafter ties every 4', about 6" up from the top plates, and doubled up 2x8 with bolted connection.
As for snow load, we get a lot of snow here, but it's a metal roof; I can't imagine anything sticking to this sucker.
Thoughts?
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MtnDon
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# Posted: 4 Sep 2012 06:13pm - Edited by: MtnDon
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Quoting: bobbotron As for snow load, we get a lot of snow here, but it's a metal roof; I can't imagine anything sticking to this sucker.
It is not likely, but if the weather does just the right, or the wrong things, snow can build up. Case in point; I have seen snow accumulate to a depth of something like 18 inches on a metal roof with a pitch of 8/12. First it rained and sleeted and as the temperature dropped the sleet stuck to the metal roof. Sleet turned to wet snow and added to the layer of pebbly ice. And it snowed for hours and stayed well below freezing for a few days. This was in northern NM not far from our property. So if it was me I'd design with that in mind and not skimp because the snow should slide off.
As for rafter ties in the cathedral ceiling area, if the ties are only six inches above the top wall plates then they will still tie the walls together adequately. Be sure the walls are 100% plumb before nailing the ties in. How many nails to use to connect the rafter ties to the rafters depends on snow load, the spacing of the rafters and the pitch. That will be a minimum of three 16D common (not box) with your pitch; doesn't change even with 70 PSF snow load. But the rfater depth and spacing will be greater for 70 than 20 to 30 PSF.
Quoting: bobbotron ...and doubled up 2x8 with bolted connection.
I'm puzzled by this. I am not able to draw the same metal image as you have on these 2x8's. Are these the rafter ties? Usually bolts are not the best connections for wood joints. Not that the bolt may not be strong enough, but that the bolt concentrates the load on the wood fibers and the fibers may fail.
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bobbotron
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# Posted: 5 Sep 2012 11:48am
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You're right, I guess there is the chance that snow will stick to the roof in certain circumstances! Even so, I'm not too worried.
Bolts vs nails. I read somewhere someone bolting their rafter ties instead of nailing them. I like nails though, so will probably stick with that.
I was thinking of doubling up the tie, one on either side of the rafters that got ties, to look like this. I like the look but I realize the connection to the rafter might get a little overloaded with nails.
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Martian
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# Posted: 5 Sep 2012 12:48pm
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Considering that your countering a sheer load, I wouldn't be worried about using bolts if you like the look. Especially considering that your rafter ties (we call them 'collar ties' around here) are so far down the rafter. You have a great deal of leverage countering the spread load at that point. If your collar ties were 2-3' from the peak, the loads would be much greater. I would consider using a plate on both sides, rather than a washer, to spread the load.
Tom
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MtnDon
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# Posted: 5 Sep 2012 02:43pm
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Not to pick on your nomenclature Martian, but more to try to bring everyone into using the same definitions for components. In recent years with the evolution (good and bad) of the construction codes there are now two generally recognized parts to a rafter assembly as well as the rafters themselves. Collar ties and rafter ties. They are different and a stick constructed gable roof should have both of them.
Collar ties belong in the upper third and are best as close to the peak as possible. They help hold the upper part of the roof together in high winds; keep lift from pulling things apart. Collar ties can be eliminated in a gable roof by using metal ties over the top of the ridge. That works well when applied after the sheathing; nailed to the rafters of course, not just the sheathing.
A rafter tie is located in the lower third, and are best if installed on the top plate of the walls, well nailed to the rafters which are well connected to the wall tops. Rafter ties should never be installed lower down in the wall framing. Their job is to prevent the walls from spreading at the top. They make a dandy ceiling and upper floor as well. If the rafter ties are moved up the rafter the rafter depth may have to be increased to counter the additional forces that can develop. There are footnotes to the code tables that explain by how much. If the rafter ties are higher than in the lower third their effectiveness at restraining the walls is greatly reduced.
Again, not to be critical, but simply an effort to get everyone who may read this at some time on the same page.
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Martian
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# Posted: 5 Sep 2012 03:14pm
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I'll have to share this tidbit of knowledge with my cohorts. I may even win a beer bet!
Do rafter ties become ceiling joist when the sheetrock goes on or are they always rafter ties?
Tom
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toyota_mdt_tech
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# Posted: 6 Sep 2012 12:08am
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Martin, my rafter ties are my ceiling joist. I sheeted both sides with insulation in between (gives me a nice floor in ther attic, but my main reason to sheet the top was ease of walking around building the rafter roof). I have also collar ties, I was only require to have a collar tie on every 3rd rafter (16" OC spacing, ie 4 foot spacing) My rafter ties overlap the sides of my rafters, nailed in, rafters are nailed and strapped to the top wall plate. Rafter ties are resting on the wall top plate.
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MtnDon
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# Posted: 6 Sep 2012 12:25am
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Quoting: Martian Do rafter ties become ceiling joist when the sheetrock goes on or are they always rafter ties?
They are always rafter ties. They become ceiling joists when sized for the anticipated loads.
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