Roof structural issue

Discussion in 'Remodel Forum & Blog' started by Bill Arden, Sep 6, 2008.

  1. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    I've ran out ideas on beefing up this roof.

    It's a Hip style roof 20 feet by ~40 feet. The house was a garage once and there are no supports. it's just one large open space.

    There is a 2x6 ridge beam and 2x6 corner beams.

    The worst part seems to be the outer connections. The bottom 2x4's have a 2x4 nailed on top of it flat. and then the upper 2x4's are notched into that flat 2x4.

    In need to increase the strength to handle 2 feet of snow load and am wondering where I would start in terms of having a engineer figure out what to do?

    My structural engineering skills are not good enough to calculate out how much force the corners can handle.
  2. leejosepho

    leejosepho DIY scratch-pad engineer

    In theory, the sheeting on top of the rafters keeps the corner beams in place, and you likely already know that. But if/when the ridge beam begins to sag or lower for whatever reason, the rafters can begin pushing out at the tops of the side walls (while the sheeting slips around a little) all the way to the corners. The 2x6s should be able to handle the snow load, and placing vertical support under the ridge beam should keep that beam it in place so the rafters will not push out on the walls. Placing support under the ridge beam should not be too difficult if the building has a center wall underneath. Other than that, I do not know what might still be possible.
  3. maintenanceguy

    maintenanceguy In the Trades

    Messages:
    107
    Location:
    Pennsylvania
    I don't completely understand the detail about a bottom 2x4 with a flat 2x4 and a notched 2x4. But, in general, trying to beef up a roof structure is not worth the trouble.

    There are simple span tables in the building code that will tell you what size rafters you will need for different types of loads. Anything beyond that needs an engineer.

    If it really is undersized (I'm not sure it is) an engineer can tell you what additional framing needs to be bolted to each pair of rafters to make site-built trusses that will support the load but things like bolt spacing and placement and lumber size will be important so have an engineer do it.

    the easiest and fastest way is probably to remove the existing roof and order trusses through your lumber yard. A 20x40 hip roof can be set and sheathed by a couple of guys in a couple of days.
  4. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,309
    Location:
    Cave Creek, Arizona
    roof

    If it is really weak for some reason, the place to start with an engineer is to contact one. A structural engineer can evaluater your structure and advise as to any upgrades.
  5. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    I gather this garage was converted to a house without benefit of permits, which would have had an engineer do the calcs on it at that time.

    Nonetheless, it didn't just start snowing this year , did it? How has the roof fared in previous years? Is there already a sag you need to take care of?
  6. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    Here is a diagram of typical framing for a hip roof.
    http://www.blocklayer.com/roof/roofeng.aspx

    Are there any ties at the top of the wall that would make it into a truss? There is usually a ceiling and the joists tie the roof framing together.

    What is the roof sheathing? Plywood sheathing can help carry load to the end walls.

    Can you post some photos of the structure, and tell us how the framing compares to that shown at the link above?
  7. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    1. The roof sheeting is just loose tong and grove planks.
    I am also confused as to why someone would cut slots into the 2x6's that form the Hip ridge beams. This makes it only as strong as a 2x4

    2. I am planing on adding more insulation and that will stop the snow from melting.

    3. Permits weren't required when this was built and the existing county building codes grandfather this house. (We haven't even formed a township yet)

    4. I've added some vertical supports from a interior wall to the ridge board, but that seems to have caused some more shifts where the two roofs join together.
    The original hip roof was cut into and the new section was tacked on to.

    5. I'll try and get some photos put up.

    I think the main failure point if I support the ridge beam would be the 2x4 (2 foot centers) Rafters pulling away from the ridge beam or breaking in half.
  8. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,309
    Location:
    Cave Creek, Arizona
    roof

    We are having a problem trying to figure out if your roof is properly triangulated or not. That would determine its structural integrity.
  9. Rich H

    Rich H Civil Engineer

    Messages:
    20
    Location:
    Wisconsin
    Post some pics or a diagram of what you are trying to bring across. Then, ignore the useless advice you'll get here on a plumbing forum and track down a structural engineer.
  10. Gary Swart

    Gary Swart In the Trades

    Messages:
    7,309
    Location:
    Yakima WA
    Surely there is at least one company in your area that makes trusses for the construction trade. They build and deliver trusses that will meet or exceed structural requirements. Even professional builders don't even try to build their own trusses.
  11. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    Replacing the roof would be very expensive!

    If propping up the middle and adding a few extra boards won't be enough, I would replace the entire roof with a gable roof using trusses.

    But this would involve.
    1. Cutting the existing roof off.
    2. removing the soffits
    3. Adding a 2x4's on top of the existing celing 2x4's around the perimeter to support the new trusses.
    Note: The existing ceiling, insulation, wiring, ect.. would have to be left in place.
    4. Adding trusses, sheathing, shingles
    5. Adding new soffits
    6. Siding patches?

    The result would be a new roof raised about 10 inches higher than the existing roof.

    It reminds me of when a friend put up a pole building roof over the top of a motor home.

    All that makes me think I would be better off starting with a bulldozer :p
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