Shed roof 2x8's?

Discussion in 'Remodel Forum & Blog' started by rick.a, Jun 1, 2009.

  1. rick.a

    rick.a New Member

    Messages:
    84
    Location:
    Palmer, Massachusetts
    I am building a shed that will be attached to a porch on the rear of my cottage.
    The classic shed roof will be tied into the roof of the porch (side of the porch). My question is: Can 2x8's on 24" centers span 8 ft on a roof with a 3 pitch, and 1/2" sheathing, in central Massachusetts (snow load)?

    The existing porch roof has rafters on 24's and I want to sister into them, over the porch wall, then run out 8 ft, maintaining the existing pitch. The porch roof seems to have held up fine for many years with this same construction ( 2x8 on 24" centers).

    I know that a permit will give me this info but this will not be happening at my summer cottage on a dirt road in a hick town.

    thanks,
    Rick.
  2. Engineer Ben

    Engineer Ben In the Trades

    Messages:
    24
    Location:
    Chicago
    If you're not worried about a permit you will be fine. I would walk on it. You may get a very little bit of flex inside the porch if it will have a ceiling (paneling?). It certainly won't fail, but you may have a very little bit of flex.
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,515
    Location:
    Cave Creek, Arizona
    joist

    It depends on the type and grade of lumber. Select doug fir will give a 60# /sq.ft. loading for a 10' span, while #1 would drop it down to 10#/sq.ft for a 9' span.
  4. sjsmithjr

    sjsmithjr Geologist

    Messages:
    295
    Location:
    Knoxville, Tennessee
    Before reaching for the span tables, you need to calculate the roof load.

    Snow load - A rough rule of thumb is that roof snow load equals 70% of ground snow load which, depending on where you are in Central Mass, ranges from 55 to 65 psf. So your roof snow load will be approx. 40 to 45 psf.

    Dead load for light roofing is 10 psf.

    For the wind load, and you'll need to check this one, I believe you would add in another 10 psf to the dead load as you are located in the northeast.

    That would give you a conservative roof load of 20 psf dead and 45 psf live for a combined load of 65 psf.

    Now you can reach for the span table for the species and grade of lumber you plan on using. In the alternative you can reach for the phone book and your check book and have a structural engineer spec the job for you.
    Last edited: Jun 3, 2009
  5. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    I prefer 16" OC with 1/2"
    Heavy snow load up here every now & then its better for support
  6. rick.a

    rick.a New Member

    Messages:
    84
    Location:
    Palmer, Massachusetts
    Thanks for your replies. I think that I will go for putting them on 16" just to be safe. The additional level of difficulty and expense will not break the bank. I'd rather the peace of mind.
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