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Rafter/Ridge Beam Sizing


aclayola's Avatar
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MI

02-18-08, 02:43 PM   #1  
Rafter/Ridge Beam Sizing

I am working on a DIY project to build a 30x36 great room inside an existing barn. The room will have cathedral ceilings, with my ridge beam spanning the 30 ft width.

I believe that I want a 4:12 pitch, but will consider something different if it significantly reduces costs/size of my lumber.

I have found several on-line calculators to help in determining the rafter sizes, but none on the ridge beam.

Given the fact that it is inside another structure, there is very little that this needs to support other than the ceiling/roof of course. No worries about wind, snow, etc.

We'll be using 3/4" T&G knotty pine to finish the ceiling and something cheap on top (thin OSB w/ tar paper)... noone will ever see it :-)

Aside from that structure, we'll be insulating, mounting 4 ceiling fans and 4 in ceiling speakers.

Do the rafters hold each other up, applying pressure on either side of the ridge beam?

What is the likely outcome of an undersized ridgebeam for my application... sagging?

So basically my details are a 30' span for my ridge beam and 18' spans for my rafters.

Is an engineered beam my best option for this size?

Thanks,

Andy

 
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aclayola's Avatar
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02-18-08, 03:03 PM   #2  
More info on Rafter/Ridge Beam Sizing

This room is being built with three of it's four walls right next to the barn walls. This includes both walls on which the rafters will sit.

I make this point after reading another posting made by stpami "When is it a ridge board vs beam with load?" where he says...

"The ridge beam becomes load bearing if the rafters are not held both up, and in, at the foot."

If so, and my ridge beam is a ridge board, what are the requirements for a ridge board?

Thanks,

Andy

 
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02-21-08, 03:02 PM   #3  
Andy,
Since you referenced an earlier post of mine it seems only right that I pick up the ball. It sounds like you have a good plan for keeping your bearing walls and roof framing immobolized, assuming the existing barn walls are secure and solid, and, that they are going to stay there. If they are incorporated as an integral part of your design, and then later, removed for any reason, problems will occurr. I would be inclined to install rafter ties, at what would be the ceiling joist ht, every 4' or so, just to be more secure but your span creates some dificulties with that (see better idea below).With a pitch of 4/12, rafters sliding just 1/4" away from center will result in a drop at the ridge of 0.85", or @ 3/4" (according to my calculations). I would also suggest using rafter ties at the wall. Your roof will be getting heavy w/ sheathing on both sides and this energy will be directed disproportionately outward to the walls, rather than as weight down on the wall. Draw a triangle to scale, representing one side of the roof (rise, run, rafter) and think of it as a vector drawing.

As for the ridge beam, I would probably use 2x material--it should be as wide as the rafter ridge cut is long, in order to provide support to the entire rafter. An engineered bm to span 30' would be rather large and heavy.

Another suggestion for tying the walls together, especially since it's hard to find 36' lumber to cover that span, would be to use wire cable with turnbuckles. located maybe every 4' to 6' o.c.. Run the cable thru a small steel plate, then thru the top plates, across the room, thru opposite wall and another steel plate. Adjust tension in the center with the turnbuckle. Make sure all your fittings are well rigged so you don't punish yourself with a flying/snapping cable (very dangerous). Your not dealing with incredible loads here, but don't be fooled--they add up. Hope that helps,

Steve

 
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02-21-08, 06:42 PM   #4  
If I understand what you are doing, with an open cathedral ceiling, with insulation but no roofing and no ceiling joists, are the barn walls structural enough to hold up the walls and roof? Typically, I think barns are only strong enough to hold up themselves as they usually don't have foundations but a post and beam type construction. Why not just buy trusses for a cathedral ceiling and elimnate the problem of keeping the walls from spreading? I would REALLY like to know why you are building a "house in a barn"? Good luck.

 
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02-22-08, 09:31 AM   #5  
Thanks to MikeTN and stpami for your replies. I am really intrigued by the idea of using wire and turnbuckles... What gauge cable would be sufficient?

Currently my design is to use 2x10 rafters into a 2x12 ridge, probably a 5-12 pitch. Then put 2x6 rafter/collar ties at about halfway down. We were going to put on every rafter and just make the ceiling flat in the middle. Now with the cable idea, I think I may go back to the peak..

As for why we are making a "house in a barn". It's my fun room. Bar, game tables, Home theater (barn theater). I'm an avid home brewer and it will make a great place to enjoy my spoils.

The barn is over 100 years old, gambrel roof and in fantastic shape. The barn has a massive foundation of rocks that was reinforced with concrete at some point. It was a working dairy farm until the 50s. There is part of me that would love to be able to look up and see the real barn ceiling, it is quite impressive, but I live in Michigan and heating that volume of space is not practical, plus there are many bats that make their homes up there as we see evidence on the floor below. :-(

I posted some pics on my website.

http://www.clayola.com/barn

 
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02-22-08, 06:44 PM   #6  
That is one gigantic barn! It looks strong but Mike has a point about additional loads. The design looks well balanced and might not take new, unevenly distributed stress very well.
As far as cable size I'm really not sure what you'll need. I can tell you that I've had to pull together walls that were collapsing and I used this method. We pulled a large section (50') of front and back walls back into plumb, and the ridge back up where it was supposed to be with 3 jacks and three cables w/ comealongs. I think we used 1/4' cable but I must tell you that one snapped when the homeowner didn't heed warnings and gave an extra pull. Maybe you could figure a number for your roof wt and assume the cables had to hold all of that, plus some for error and safety. You will need to check the ratings of all the hardware, fittings, etc also.
The collar ties are a good idea, especially on that long span-- I think I would keep them.

Steve

 
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