collar ties

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  #1  
Old 02-06-18, 10:26 AM
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collar ties

hi all,

in addition to my ventilation issues in my attic, I have learned that I need collar ties (I have none). This is likely the cause of the 'bellys' I have developed on my roof

I was told that I need lumber of the same size as the rafters (2x6), on every other rafter. My question is exactly where to attach them? Halfway up?

I took this picture prior to adding about 24" of blown in cellulose:

 
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Old 02-06-18, 10:30 AM
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Typically, collar ties should be in the lower 1/3 of the rafter span. The purpose of collar ties is to prevent the ridge from sinking and the rafters from pushing your walls outward.

If your rafters are sagging in the middle of their span, it's because they are over spanned and collar ties do not help with that. To correct over spanned rafters you typically need knee bracing.
 
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Old 02-06-18, 11:14 AM
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One thing to watch out for if you add knee braces is where they go on the ceiling joists as the roof load is then transferred down to the ceiling joists. This can cause the ceiling to bow down under the load. In your case, at least on the left hand side of the photo, this would be particularly bad since the braces could all end up bearing on just one ceiling joist.
 
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Old 02-06-18, 11:39 AM
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I was taught that collar ties are placed in the upper third of the rafter to prevent the rafters from separating from the ridge beam and to help support the roof load.
 
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Old 02-06-18, 12:00 PM
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Yes, a collar tie in the upper third is sometimes done... but for a completely different reason. He needs a rafter tie in the bottom third because there are no ceiling joists tying the walls and rafters together.
 
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Old 02-06-18, 12:19 PM
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I guess I shouldn't have said "collar tie" in post #2... thanks for catching that. I was thinking rafter tie as I wrote because of his photo and the title of his thread. To be clear, IMO he needs rafter ties, not collar ties.

Here is a nice article that explains the difference.

https://www.sbcmag.info/news/2014/au...l-roof-framing
 
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Old 02-06-18, 12:51 PM
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as usual, fantastic info! Thank you all for the replies!

So, I am a bit confused as to what I should do. Should I still do collar ties, as I have none (I now understand that will have nothing to do with the sag (incidentally, mostly on the left side of the roof)?

With regard to knee braces: on the right side, I am ok. And in the upper attic (not pictured, this house is a split level/two attics). On the left side, it would definitely be on one joist. What does one do in that situation (after bumming out)-- is this engineer territory?
 
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Old 02-06-18, 02:03 PM
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You would usually try to find some interior partition walls that are sitting perpendicular to the floor joists... build a header, place it against the mid-span of the rafters, then kick the ends of that header down to your interior partition walls. Any bracing you put on the ceiling joists is then not really putting pressure on the ceiling, its primarily just to stabilize the header. Yes, it does take a little engineering know how.

As for the collar ties, probably not needed but you do get the occasional "Sandy" there. Won't hurt if you do it... might be a little tough getting up there now with all the blow in. Too bad you didn't fix the framing first... that makes it a million times harder now.
 
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