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Paul_w
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# Posted: 7 Oct 2018 08:28am
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Hi. I have a mono pitched roof build coming up. Rafter length is around 13.5 feet (a little over 4 meters); will 2x8 at 16in on center be sufficient or should I go narrower and use a 2x10?
I've done some research online and looking at this for example, looks like 2x8 is more than enough (they use c16 there as well, while I have stronger c24 grade wood):
https://www.broxbourne.gov.uk/sites/default/files/Documents/Building_Control/Mono%20p itched%20roof%20withvaulted%20ceiling%20%20Oct2010.pdf
Just hoping to get some extra opinions from people with experience.
Cheers.
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ICC
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# Posted: 7 Oct 2018 09:46am
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Go here----
https://www.awc.org/codes-standards/calculators-software/spancalc
Enter your species, grade, span, snow load, etc.
FYI, for snow load a min of 20 PSF is usually used even where snow is seldom seen
Furthermore, Examples of code-prescribed deflection limits and live load values are:
* Main room floors L/360 & 40 psf
* Bedrooms and habitable attic floors L/360 & 30 psf
* Attic floors with limited storage L/240 & 10 psf.
* Rafters L/240
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ICC
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# Posted: 7 Oct 2018 09:59am - Edited by: ICC
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Tutorial
https://www.awc.org/codes-standards/spantables/tutorial
And remember if the calc says span is good to 13'6" that is max. It never hurts to go one rafter depth larger or one closer spacing, of up the deflection limit one stage
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Paul_w
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# Posted: 7 Oct 2018 01:36pm
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From what I gather playing around with that is that if I go with 2x10 at 24in center, or 2x8 at 16in center, it's more than enough for a 4x5 meter building. Checking locally too though, I'm in the EU and the trees may not be exactly the same.
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ICC
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# Posted: 7 Oct 2018 01:51pm
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On that note, it may be interesting to look at southern pine here in the US. Five or 6 years ago SP was derated. Virtually all the SP has been grown on plantations with trees spaced for optimal speed of growth. It was noticed that modern SP was not performing as good as the old stock. So it was re-rated.
Some mills in the US use a machine stress bending test to arrive at the grading (MSR). The traditional grades are visual. It is a skill that is learned. The grader is the highest paid worker in a mill as a rule.
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rockies
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# Posted: 7 Oct 2018 05:13pm
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How much insulation do you want in the roof? If you're in a cold climate and you build a vented roof using a rafter that is 7 1/2" deep you'll only have 5 1/2" of batt insulation (once the 2" of air space is deducted above the insulation.
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Paul_w
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# Posted: 8 Oct 2018 01:47pm
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Yeah, I was just trying to figure out the minimum. Local guys I've now spoken say 2x8 at 24in center is enough with the C24 classified sorted wood we have here. But yeah, might go thicker for insulation.
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