Re: OT Max rafter length

Posted by jdemaris on May 17, 2007 at 06:01:26 from (66.218.11.244):

In Reply to: OT Max rafter length posted by John S-B on May 16, 2007 at 20:15:07:

Besides the dimensions of the wood, the type of wood makes a big difference. Years ago, for certain jobs I used to special-order Douglar Fir for long spans and heavy snow loads - but can't get it anymore (here in New York). Next best option is Southern Yellow Pine - since it's usually commonly available in pressure-treated lumber - but costs a lot. I wish I could get it untreated.

We get heavy snow loads where I live and 60 lbs. per square foot is the minimum code requirement for most buildings. For a 15' span, if you are using common spruce lumber, 2 X 12s will just barely make code if used on 16" centers.

The strongest wood, e.g. Douglas Fir, Southern Yellow Pine, can carry a longer span than weaker wood - e.g. spruce, white pine, red pine, hemlock, etc. In my area - all standard framing lumber comes as spruce - but I am in New York. If I buy rough lumber from a local mill it will be hemlock. If I lived down south - the wood would likely be much stronger as Southern Yellow Pine.

30 lbs. snow load - 16' span - 24" on center - 16 feet max. span - 2 X 10 best wood. If spruce, only 14 feet.

40 lbs. snow load - 14 1/2 foot span max. - 24" on center - 2 x 10 for best wood, 13' for Spruce.

I've built several buildings over the years with 20 foot spans using 2 x 12s on 14" centers. That meets the new code regs. and will hold a lot of snow in case I'm not around to shovel it off right away. It's not an issue with most of my 5/12 roofs with steel - but flatter roofs can be a problem. Technically, a 2 x 12 of the strongest wood is rated for a 17 foot span at 60 lbs. of used on 16" centers, and 14 1/2 foot on 24" centers.

Replies:

• Re: OT Max rafter length Walt Davies 11:22:01 05/17/07 (1)
  • Re: OT Max rafter length jdemaris 11:37:08 05/17/07 (0)

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