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backattheshack
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# Posted: 9 Feb 2011 09:20pm
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adding on too the shack using 2 x 8 on 24" centers wondering what kind of distance i can get? we are in the south so no real snow load to deal with will have osb board and then metal roofing.just trying to get about 16 feet total.
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MtnDon
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# Posted: 9 Feb 2011 10:07pm - Edited by: MtnDon
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Maximum span for any given size rafter or floor joist depends on species and lumber grade as well as spacing and what the load is.
Here's a handy online calculator
Note there is a second version called "span options...." click on the bar near the lower area of the screen.
The deflection limit may need some explanation for those not familiar with it.
Examples of code-prescribed deflection limits and live load values are:
*Rafters L/240
* Living room floors L/360 & 40 psf
* Bedrooms and habitable attic floors L/360 & 30 psf
* Attic floors with limited storage L/240 & 10 psf.
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MtnDon
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# Posted: 9 Feb 2011 10:13pm - Edited by: MtnDon
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The calculator gives the maximum spans. If near the maximum and if a solid floor is wanted it is best to go up a size, choose a better grade or reduce the spacing. That is also a good idea if one is not certain about snow loads. The county building dept is often the best source for snow load info. That info can also be found in the IRC code books for many areas.
Also for those who are not sure...
Span for rafters is measured along the horizontal distance, not the slope. Span is also measured from the inside edge of the support, not the outside edges. Rafter span is generally one half the distance between the wall tops minus the thickness of the ridge board.
Floor and ceiling joist spans are also measured inside their supports.
Note the calculator provides the minimum amount that must rest on the supports. More is always better.
If ya'll already knew all that please don't be offended.
Most common grades are #2. It appears that it will be difficult to reach 16 feet with most common species. Southern pine almost makes it in grade#2. Switching to 19.2 inch spacing will work with SP.
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Borrego
Member
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# Posted: 9 Feb 2011 10:36pm
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MtnDon - I must say, you are the man! An answer for everything! We can always count on you :-) Cheers!
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backattheshack
Member
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# Posted: 9 Feb 2011 10:43pm
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thanks guys looks like i will use 2 x 10s
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MtnDon
Member
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# Posted: 10 Feb 2011 12:03am
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Borrego, I don't have all the answers. I do know where to look for a lot of them though. That is one of the handiest online tools.
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