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I have a living room that I'd like to vault the ceiling on. I've had a contractor come out and tell me that my ceiling joists are not supporting the roof in anyway, they are just tying the walls together.

The room has 3 exterior walls and is 14'x20' with the ceiling joists running the 14' direction currently spaced 16" o.c. (built in the early 50s). The 20' walls are both exterior. I'm in an area with little to no snow, wind, and seismic activity (Raleigh, NC). The roof pitch is pretty low.

Ideally I'd like to remove the ceiling joists at the top plate and move them up to the 1/3rd mark so they are a bit higher and also reduce as much as possible the number of ties needed.

I'm thinking two or three 2x6's spaced fairly evenly across the room should do it, but I'm just guessing.

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FYI, Raleigh IS subject to wind (hurricane), snow, and seismic loads. In fact your hurricane loads and seismic activity are moderate for the US. You can look up seismic activity maps and hurricane wind load maps if you're curious. –  Henry Jackson Jan 30 at 21:39
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Is the roof pitch created by one of the walls higher than the other? or is there a ridge board or ridge pole as some call it? In other words are the roof rafters pitched each way from the center of the 14' span? –  Jack Jan 31 at 13:24
    
Stop. No. Don't. Your walls will have a HUGE outward force on them if you eliminate the rafter ties, and moving them up will put a much larger load on the ties as well. I would never attempt to do this type of renovation to my own home, but PLEASE go talk to an engineer if you're still bent on doing this. Explanation of some of the physics involved here: diy.stackexchange.com/a/9497/20 –  Doresoom Jan 31 at 20:38
    
Thanks for the feedback. I get that removing ceiling joists would cause the walls to push out and the roof to collapse. I just thought that ceiling joists every 16" was more for holding up the ceiling dry wall than keeping the walls from folding out. I was wondering if there was maybe an accepted rule of thumb like a joist was only needed every 4 ft to keep the walls from pushing out. I'll definitely consult a structural engineer if this is not a well known. –  michael Feb 1 at 3:55
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"...ceiling joists are not supporting the roof in anyway, they are just tying the walls together"

Unless you have a big ridge beam you failed to mention, this is a brilliant piece of misdirection. While superficially correct, what do you suppose would happen if the walls were not tied together? The roof would collapse under it's own weight! No snowload required.

The lateral force exerted on the walls from a low pitch roof is substantial. While it is possible to stabilize the roof with collar ties alone, the end connections become very critical in low pitch applications. You'd be much better off with a high pitch roof in terms of spreading forces from the roof.

If you're going to do a major structural change anyway, I would install a ridge beam along with proper end supports in order to achieve maximum openness from my efforts. Regardless of which approach you select, for any major structural change you should consult with an engineer to ensure everything is done safely. Small details can make a big difference.

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Consult an engineer! The potential cost of not doing so is your house (or a substantial part of it, anyway) collapsing. Pretty sure insurance won't cover any claim that begins with "Well, I removed some joists from my ceiling..." –  gregmac Jan 30 at 23:22
    
A ridge beam would be nice, but it has to bear wieght straight down to a supporting wall, and down to a footing. Without vert load bearing, the lateral force will stay the same as without a larger ridge beam. –  shirlock homes Jan 31 at 23:35
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