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Ditchmonkey
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# Posted: 9 May 2016 02:48pm
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I'm throwing together a summer only quick and dirty guest cottage that is 8X12. I picked up a nice 8 foot wide window from a window warehouse for 99 bucks. Given that this is a super-simple cabin, single story, no snow load, what would you think would be an adequate window header height? 2 2x8's or 2x10's?
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MtnDon
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# Posted: 9 May 2016 03:06pm
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Is it to go in the 12 foot wall? Is that wall going to be a load bearing wall, as in a building with a gable roof with ridge running the 12 ft direction? Or a shed roof with the 12 foot walls being the high and low to make the slope?
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Ditchmonkey
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# Posted: 9 May 2016 03:17pm
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Yes window goes in the twelve foot wall, with shed roof as you described.
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Don_P
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# Posted: 9 May 2016 10:05pm
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Assuming a 1' overhang plus half of the 8' rafter span... 5' tributary width x 8' header span=40 square feet X 30 lbs per square foot=1200 lbs. ... I'm getting a pass at a double 2x6.
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Ditchmonkey
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# Posted: 10 May 2016 01:36am
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Quoting: Don_P Assuming a 1' overhang plus half of the 8' rafter span... 5' tributary width x 8' header span=40 square feet X 30 lbs per square foot=1200 lbs. ... I'm getting a pass at a double 2x6.
Thanks for the info. What is tributary width?
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Don_P
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# Posted: 10 May 2016 02:04am - Edited by: Don_P
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the tributary width is the portion of, the roof in this case, that is applying load to the header.
Each rafter is supported by a wall near one end and the header near the other. Half of the rafter's load is resting on the wall, half is resting on the header, then add the load on the overhang. So for our example the trib width is half the rafters horizontal span, 4' plus the overhang 1'... 5' total tributary width bearing on the beam.
Tributary AREA, the trib width X the header span is the total square footage loading the beam, 5'width X 8' header span=40 square feet.
Then I applied 10 pounds per square foot for the weight of the materials of the roof, plus, 20 psf Live load which is the minimum used in a non snow area, wind, should be good for 90 mph if memory serves.
So there's where the 1200 lb load came from, notice several assumptions were made, correct them if needed.
From there I checked the beam on a simple beam calc I wrote;
http://www.timbertoolbox.com/Calcs/beamcalc.htm
Inputs for the lumber were roughly #2 SPF
Fb-1000
E 1.4
Fv 135
A couple of ways of looking at the output if you check the calc. Notice a double 2x6 is right there, it is none too large. And then I look at that roof and think 1200 lbs is probably gracious based on the description.
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Ditchmonkey
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# Posted: 10 May 2016 12:38pm
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Thanks Don!
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