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friesendw@saskt el.net
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# Posted: 18 Feb 2013 11:46am
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I am planning an addition (16 x 14) to our chalet cabin (20 x 24 with 12/12 pitch roof). The addition will have a 6/12 pitch and the roof will run perpendicular to the main roof (addition will extend 14' out from cabin side wall). I want to use an 18' ridge beam for the addition supported by the end wall of the addition and a post at the cabin side wall ( 1' overhang, 14' open span, 3' to tie into cabin roof=18"). Will use 2x8 rafters on 24o/c. Will 2-2x10s work for the ridge beam?
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MtnDon
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# Posted: 18 Feb 2013 12:20pm
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What is the ground snow load in PSF? And what is the foundation plan to accept those column loads?
Is the idea to have an open, cathedral ceiling in the addition, or why the chosen use of a beam rather than a ridge board. Just curious...
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friesendw@saskt el.net
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# Posted: 18 Feb 2013 03:38pm
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Yes, open cathedral ceiling...gsl at 20 psf. Foundation has 3 rows of cement pads 6' apart...rows are 8' apart. Posts to hold up main beam go straight down to a cement pad. An engineer calculated the beam for a 14' span to be 4-2x12s. I think this is overkill.
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MtnDon
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# Posted: 18 Feb 2013 08:31pm
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The engineer is correct. Three 2x12 Southern Pine, #2 non dense, Fail on the fiberstress in bending test for a 20 psf snow load and 14 foot span.
The suggested two 2x10 also fail on deflection as well as fiberstress in bending, by a lot.
Four 2x12 Southern Pine grade 2, non dense, pass all test calculations.
Size the footings for the support columns correctly and of course make all footing as deep as frost depth. A steel reinforced concrete pad 16x16x8 should suffice depending on the bearing capacity of the soil.
I'm taking a holiday but wanted to come back and complete this as I did ask for the snow load.
You might want to rethink the 2x8 rafters as well. Not because they are not sturdy enough for the span; they are fine for strength. I'm thinking more about whether or not 2x8's will provide sufficient depth for insulation IF the insulation is to be placed in the rafter bays. There are other ways to achieve higher levels of insulation such as rigid foam on the exterior sheathing. But that is another subject.
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