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Thread: rafter size

  1. #1
    Robert A. Kuzmick's Avatar
    Robert A. Kuzmick Guest

    Default rafter size

    In a cabin that is 20' front to back by 25'wide, I plan on using 2x8 on 12" ctr for rafters on a vaulted ceiling. Underneath will be an exposed beam. Snow load is pretty heavy as it is in upstate NY. Should I switch to 2x10 rafters on 12' ctr? Currently, there are trusses that have been damaged by previous owner(ie: cut/sistered/etc) Any input is very appreciated.

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  2. #2
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    Default Re: rafter size

    You should pay an engineer (or an architect with access to an engineer) to design this for you.

    "Baseball is like church. Many attend but few understand." Leo Durocher
    Bruce Breedlove
    www.avaloninspection.com

  3. #3
    Bruce King's Avatar
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    Default Re: rafter size

    The charts I have require the following info. in order to figure.

    Horz. span (not rafter length) of rafter
    On center spacing used
    light or medium or heavy roof covering
    live load required by code in lb per SF
    dead load required by code in lb per SF
    drywall ceiling or not
    wood species used
    lumber grade used


  4. #4
    Robert A. Kuzmick's Avatar
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    Default Re: rafter size

    This is what I know Bruce:
    1. 20' span
    2. 12 oc
    3. medium-I'm putting on a metal roof over existing shingles.
    4. live load-minimal,
    5. dead load = 60
    6. no drywall. t&g pine
    7. spf
    8. #2

    thanks,


  5. #5
    Bruce King's Avatar
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    Default Re: rafter size

    Are you sure about the 60 psf dead load? Is that due to the T&G ceiling or code?

    The best match I can find in the tables are:

    50 psf live
    15 psf dead

    SPF is not even in the table, too weak probably even if 2x12.

    Southern Yellow pine on 12" centers:
    #2 2x10 = 17' 5" span
    #1 2x10 = 19' 5" span
    #2 2x12 = 20' 5" span

    Hem-Fir will not work unless #1 grade = 20' 5" span

    D.Fir-L
    #2 2x12 = 19' 8" span
    #1 2x12 = 20' 9" span
    Select Structural grade (special order) 2x10 = 20' 3"


    I would consider eng. I-joists for this application.
    i-level.com or search for engineered I-joists
    These products come with free eng advice.


  6. #6
    Jerry Peck's Avatar
    Jerry Peck is online now Member
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    Default Re: rafter size

    Quote Originally Posted by Robert A. Kuzmick View Post
    This is what I know Bruce:
    1. 20' span
    No, as you described it, it's a 10 foot span for the rafters, unless you are planning on a 'shed' style roof.

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

  7. #7
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    Default Re: rafter size

    Yep. 10' span. You should think about hiring an engineer or an architect with an engineer to design the roof for you. In my neck of the woods Regional Building Dept. would require the roof design to have a PE stamp before they would issue a building permit.

    "Baseball is like church. Many attend but few understand." Leo Durocher
    Bruce Breedlove
    www.avaloninspection.com

  8. #8
    Philip Desmarais's Avatar
    Philip Desmarais Guest

    Default Re: rafter size

    The higher the pitch the more impact on the roof by wind. I would think pitch would need to be considered.


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