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Ceiling Sistered Joists Preventing Rafter Sistering


whorneff310's Avatar
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NJ

04-16-16, 06:56 PM   #1  
Ceiling Sistered Joists Preventing Rafter Sistering

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I'm installing a 44 inch wide by 27 in skylight so the header span is greater than 4 feet necessitating double header and double trimmer joists. The framing in the ceiling is already done. However due to the location of the sistered joists in the ceiling I can't run a full sister on either rafter. The ceiling sisters are actually a 2x6, and then two 2x8's. The reason it was done like this was due to trying to center the skylight in a particular area where from left to right it went ceiling joist---->Rafter---->extra 2x8 joist so a third 2x8 was added in between the original 2x6 and the new 2x8 and then bolted together to make a beam. In hindsight I wouldn't have done it like this and just not had it perfectly centered over the vanity. In the pic below it's the joist to the left. You can't see the 2x6 in this picture as its to the left of the 2x8's.

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Structurally can I sister the rafter from the top of the roof all the way down and secure it on top of the ceilings sistered joist vs the top plate. Not being a structural engineer I don't know the implications of this. It seems like it would be okay for deflection but I have no idea if that would be putting too much on that sister vs the top plate. Any advice is greatly appreciated.


Last edited by whorneff310; 04-16-16 at 07:11 PM. Reason: add picture
 
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Bud9051's Avatar
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04-16-16, 08:02 PM   #2  
Your sisters have me confused.
First, if this is under a permit and will be inspected, then it is the AHJ (authority having jurisdiction) who needs to answer your question. If no inspection is involved, and I'm not an engineer either, I believe your assumption is correct. Sister it as far as you can and it should be fine. Your snow load in NJ isn't that bad.

Now, about the black coloring we see in the first picture, that looks like a moisture problem so review your ventilation, especially since a skylight may block some of the needed air flow. More advice on that topic if desired.

Bud

 
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04-16-16, 09:11 PM   #3  
Your confusion is very warranted especially since it's very hard to explain. I would really appreciate any info about the ventilation issue. It's all over the attic. Thanks a lot

 
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04-17-16, 05:53 AM   #4  
For the ventilation, you might want to start another thread so this one will remain on topic.
Add some outside pictures and describe what you have for soffit (low) venting and ridge or gable (high) venting.

Bud

 
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04-17-16, 01:33 PM   #5  
Will do bud thanks for the help. I've been wondering what that was in the attic.

 
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04-17-16, 06:18 PM   #6  
I'm no engineer either, but I would cut it to fit exactly on top of the joist so that it would bear on top of it. Then put blocking between it and the adjacent rafter to prevent it from ever coming off the top of the joist. Check to ensure a doubled stud falls underneath the doubled rafters. Hope you plan to add joist hangers to your joists.

 
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