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Old 01-01-2010, 01:07 PM   #16
 
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Thanks everyone, I think I got a little of the education that I was looking for on this subject.

This is what I have learned:

1) Sounds I need to make sure my outer walls don't spread from the roof load.

2) If the ceiling joists provide this "tension" between the outer walls then I'm probably ok. If not, then I need something to provide that continuous link between. Maybe the ties were for that purpose originally .

In my case my ceiling joists probably partially accomplish this. For the most part there is a link but I need to verify this and possibly add some linkage (I'm currently remodeling and could easily do this)

3) In my case the ties defintely don't provide that any more (they are old and sagging), however maybe they should be repaired or replaced with something that does the job

My final question is on ideas to replace the sagging boards?. One suggestion was to put ties higher up, that is an option, but it would still be a very long span (probably 30 feet) which will result in more sagging boards.

How about a steel cable? This would be pretty easy to accomplish.


THanks for everyones input.

Steve..
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Old 01-01-2010, 01:43 PM   #17
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IF the purpose is to resist outward thrust, you are limited to placing them in the bottom 1/3 of the rafter.
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Old 01-01-2010, 02:55 PM   #18
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From your description, you may very well have a ridge beam. A ridge beam is a structural element that supports the rafters. This is completely different from a ridge board, which simply supplies a nailing surfact to nail the rafters to.

This is the structural difference. A ridge beam is supported in the vertical direction either by structural walls on the end, or in some cases by posts. In your case, it sounds like you have posts (2x4's) supporting the ridge beam every 8 feet or so. IF the ridge beam is adequately sized, and adequately supported, the rafters exert NO outward thrust on the walls. This is the advantage of a ridge beam versus a ridge board, a ridge beam completely eliminates the need for outward thrust resistance against the walls.

You still need to design to transfer uplift from one side of the ridge beam to the other, sometimes that resistance is supplied by the rafter connection detail to the ridge beam, other times by internal framing. IF you in fact have a properly designed ridge beam, then those horizontal "collar ties" were probably installed as temporary braces, and are no longer necessary.

The best way to tell exactly how your framing is working would be to hire an engineer or architect to examine your framing and give you a written opinion as to whether you can simply remove the "collar ties" and not replace them. This would entail performing calculations to see if the ridge beam is adequately sized, and whether the connection details are acceptable.
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Old 01-01-2010, 03:41 PM   #19
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"The best way to tell exactly how your framing is working would be to hire an engineer or architect"

Any decent carpenter with skills could make that call as well, and I don't mean a weekend warrior. Even if there is a beam there, which it doesn't sound like there is based on the age of your place and the description, he could take the dimensions to the lumberyard and they could size everything for him through the engineers that work for their suppliers.
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Old 01-01-2010, 04:30 PM   #20
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He said the ridge was 3-1/2" which does not cover the full height of the plumb cut on the rafters. Whether ridge is supported or not I feel additional wood needs to be added to the ridge to support the rafter face
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Old 01-01-2010, 06:36 PM   #21
 
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Quote:
Originally Posted by tpolk View Post
He said the ridge was 3-1/2" which does not cover the full height of the plumb cut on the rafters. Whether ridge is supported or not I feel additional wood needs to be added to the ridge to support the rafter face
Sorry, just realized I mis-stated the ridge height. The rafters meet the ridge plumb, so the ridge height is 5 1/2".

But in reviewing the overall structure, I don't believe that the ridge is really being supported directly by the 2 x4's. The supports aren't all that "substantial" and appear to be more for stablization than as load bearing support. (they are 2x4 stock and sit on an interrior wall, however when you look at them they don't appear to be attached substantially at either the ridge or at the top of the load bearing wall directly below. It could be sloppy construction but it just doesn't look to be intended as a support).

So, it would appear that the entire load is indeed sitting on the exterior walls of the house, which I think indicates the structural need somehow to keep the exterior walls from spreading. (either with a tie or through the ceiling joists).

I understand and will have someone take a look at this as well before I start cutting anything. I'm an engineer (not structural, but I do understand the static load concepts. It is the carpentry and framing techniques I'm trying to understand better).
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Old 01-01-2010, 06:37 PM   #22
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I agree with Daniel, except for the reason why the rafter ties were installed. When framing a house with a stick built roof, you always install the ceiling joists on the walls first, to walk on when installing the rafters. I think the builders installed them thinking they were shortening the rafter span with collar ties- not really familiar with where or what they were for: ceiling joists act as rafter ties when installed parallel to rafters, otherwise rafter ties are required when ceiling joists run perpendicular to rafters. You don't need both in the same roof area.
I think they installed them thinking it would reduce your over-spanned 2x6 rafters.... 24"o.c. = 11'9" (no snow load) OR 16"o.c. = 14'1" n.s. I don't know the on center as you skipped my #4 question. Either way, with the 20-22' span (measured horizontally), I would add collar ties or purlins and struts to strengthen the roof system. Then remove the 1x4's.

Page 39, rafter ties: http://books.google.com/books?id=iwS...joists&f=false

Page 257, collar ties- 1/2 way up, or purlins: http://books.google.com/books?id=1fI...20ties&f=false

Be safe, Gary
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Old 01-01-2010, 07:08 PM   #23
 
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Quote:
Originally Posted by GBR in WA View Post
I agree with Daniel, except for the reason why the rafter ties were installed. When framing a house with a stick built roof, you always install the ceiling joists on the walls first, to walk on when installing the rafters. I think the builders installed them thinking they were shortening the rafter span with collar ties- not really familiar with where or what they were for: ceiling joists act as rafter ties when installed parallel to rafters, otherwise rafter ties are required when ceiling joists run perpendicular to rafters. You don't need both in the same roof area.
I think they installed them thinking it would reduce your over-spanned 2x6 rafters.... 24"o.c. = 11'9" (no snow load) OR 16"o.c. = 14'1" n.s. I don't know the on center as you skipped my #4 question. Either way, with the 20-22' span (measured horizontally), I would add collar ties or purlins and struts to strengthen the roof system. Then remove the 1x4's.


Page 39, rafter ties: http://books.google.com/books?id=iwS...joists&f=false

Page 257, collar ties- 1/2 way up, or purlins: http://books.google.com/books?id=1fI...20ties&f=false

Be safe, Gary
Oops, I did miss one of your questions..... The rafters are 24" OC. And independent of the ties I can see the need to strengthen the roof system as in looking at the rafter spanning tables the first thing I noticed is that they seemed to be undersized for the span.

Strangely (or maybe lucky) enough the 50 year old roof seems not to have any signs of sagging or other bad structural things, however the actual roof loading looks to be pretty low and I'm in southern california so no snow or hurricanes. But I'm not sure what would happen if in the future someone changed the roof from composite to concrete tile.

Going to look at the references and see what makes sense....
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Old 04-13-2010, 04:20 PM   #24
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collar tie


Quote:
Originally Posted by scatkins View Post
Here is a simple cross section of the roof. THe red is the "collar tie" that I would like to remove.

thats not a collar tie...but a rafter tie instead
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