Cathedral Ceiling

I'm working on some plans to go over with my local building inspector for an addition to my mobile home. This is a free-standing structure that will have minimal attachment (no structural attachments) to the mobile home.
The dimensions are 16x20, and I'm thinking about a cathedral ceiling. The pitch is going to be 3/12. What sort of ridge beam would you need for something like this? At 16 feet, and supporting rafters that would be about 10.3 feet in length, I was thinking that maybe that a laminated beam made up of 3 2x8s would be sufficient, or more than sufficient. I'm pretty sure I've seen 2x8s this long at the lumber yard so I wouldn't even need to splice.
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Aaron Clausen snipped-for-privacy@gmail.com

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I'm no engineer, but can offer some rough calculations for you.
It partly depends on what part of the country you're in (i.e. snow loads), but I'll go with 30psf as a rough guess.
16' x 20' x 30psf = 9600 pounds total
Half is carried by the walls, the other half by the ridge beam (assuming it runs down the center of the building), so the beam would have to support 4800 pounds over the 16 foot span.
According to the charts I have, you would need a 6x12 beam to support that load over 16 feet. You could use a solid beam, or build it up out of three 2x12's (easier lifting). Of course, you would need adequate support posts and footings to support the load at each end.
While a ridge beam is doable, you might want to check with a truss company to see what they can provide. I suspect they can make trusses that will meet your requirements, be easier and faster to install, and probably about the same cost as trying to go with the beam.
Regardless of which option you choose, you will probably need to accomodate insulation (again, varies by region, but R30 is fairly standard for cathedral ceilings), so you would need 2x12 rafters to accomodate the insulation and still leave airspace above for ventilation. With that in mind, the trusses become a lot more affordable. :)
Good luck,
Anthony
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If a 6x12 beam is called for (5.5" wide), you would need to use four 2x12s (6" wide).
Yours, Wayne
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The chart I used has a calculation when substituting built-up beams for a solid beam. I think it was multiplying the listed load times .887, but I don't have the chart in front of me at the moment. In any case, even with the derating for using three 2x12's, it was enough to support the 4800 pounds over the 16 foot span.
But you're right, that's something you would want to consider when building up a beam from individual boards.
Anthony
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Use engineered lumber. It has superior properties over solid wood (straighter, stronger, won't shrink and probably cheaper) and it comes in as lengths longer than you'd ever need. Try the Trusjoist web site - they have online span tables. Ask your building inspector what the design loads are in your area.
R
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