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01-25-2012, 10:54 AM   #1
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## Cabin Roof Design and Beam Sizing

I am in the process of designing a cabin 18’x24’ (external) that I am planning to build out of milled 6”x8” D Southern Pine logs and need some help with the roof calculations and sizing … I have a lot of numbers but can’t seem to find the calculation I need to size the beams. First the celling is open “cathedral” So the Center beam is 28’ (2’ overhang on each end) Center supporting wall divides the 24’ into 14’ and 10’ spans - So I assume sizing is based on the 14’ span ?? I assume I will have to buy a 16’ and 12’ beams/timbers then join them on the center wall.

Roofing material 2x6 T&G running vertically (wall to center beam) covered by felt, horizontal nailing strips 2 feet apart, insulation between the strips, and then a metal roof.
Snow load in my area in 15 PSF
2x6 T&G lumber weights 2lb per linear foot – so 4lb per sf
The insulation/ nail strips and metal roof - approx. 3lb per sf
So the roof will weigh 7lb per sf
The 2x6 length (ridge to wall) is approx. 11’ with 8/12 pitch
So the roof weights is 11’ x 28’ x 2 x 7lbs = 4312lb
The roof section over my ‘great room’ is only 14’ Eve to center wall
So that section of roof weights is 11’ x 14’ x 2 x 7lbs = 2156lb

Here are my questions
What did I miss and where am I screwing up…

I was looking for a chart to give me the allowable span for 2x6 T&G (basically laid as flooring on the roof) it seems logical that it should span 5.5 feet with only 3lb dead load and 15lb live (assuming I put an intermediate beam half way down the pitch.)
Then I found the following document online… http://www.awc.org/pdf/wcd2.pdf
And the table on page 8 infers that I could span the entire 11 feet without an intermediate beam??? This fails my common sense test… thoughts?? All I can picture in my mind is putting an 11’ 2x6 on 2 cinder blocks and standing in the middle… not… ??

The center beam in this configuration… it is more of a floor joist as it will have the 2x6 T&G nailed into the top of it I am trying to figure out how to size it (and the intermediate beams(I think they are referred to as perlins)
2156 lbs across 14 feet = 154lb per linear foot
I can’t find a chart with 14’ spans to get the dimensions of what I will need
(Is this one of those things where I have crossed a magical span number?)

--russ--

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