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
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. :)
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.
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.
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