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Ridge beam collapsed

Michael Kalman | Posted in General Questions on

Hey guys!
 
I am building a tiny house on a trailer. Today I was installing my rafters (while not installing the H1 hurricane ties for each rafter to save time, I thought I would do it all at once at the end) – and the roof collapsed. I had spliced 2×6 boards to make a 38′ long ridge beam/board. We are grateful that we’re not injured. 

I would like some advice on what went wrong, and how to avoid the ridge beam coming down, it snapped where we did diagonal 18″ splices screwed together with 5″ #9 screws and wood glue. I really thought it would hold. Now I am thinking of rebuilding with 2x6s laminated together to be 4×6, and adding horizontal beams to tie the walls 10′ in distance apart together, and to put a post to support the ridge board/beam, sort of like a King Truss.

What is your opinion? Should I build the ridge board with laminated 2x6s to make 4×6 or should I do 2×8? Would I benefit from the horizontal support beam? I also am wondering if I should instead use plywood gussets, and if so what size would work?

I want to avoid having the thing come down on me again.

Thanks to all!

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Replies

  1. Expert Member
    Akos | | #1

    You generally never splice a beam like that, it needs to be a single piece of lumber. If you can't get an LVL the right length than install posts.

    38' is a very long length, even with the small supported span of a tiny house roof, about the only way to span that is with a small steal beam. Post in the middle with LVLs on either side is probably cheaper and lighter. Breaking it into three section (two posts) would let you use dimensional lumber.

  2. Expert Member
    Malcolm Taylor | | #2

    Micheal,

    There are essentially two types of gable roofs: Ones that support the rafters with a load-bearing ridge, and ones that don't.

    The ones with a load-bearing ridge need that ridge to be an appropriately sized beam. As Akos said, that's something like a LVL, glu-lam or steel, not something site-laminated from 2"x6"s.

    The ones that aren't supported at the ridge need something to counteract the resulting horizontal forces. That's how gable-trusses get their triangular shape. - to deal with both the vertical and horizontal forces. There are a few ways to deal with the tendency of the unsupported rafters to push outwards. You could tie the exterior walls together at regular intervals with metal rods, or attach wood rafter ties to every second one. With a good connection of the walls to the rafters, coupled with either strategy, the size, or even presence, of a ridge board becomes unimportant.

  3. Expert Member
    Zephyr7 | | #3

    You can use king trusses to support shorter sections of ridge beam. King trusses are essentially extra beefy trusses intended to support extra loading like this.

    You can’t splice a beam the way you describe. 18” of overlap on even a 12’ beam is concentrating force as a lever. You end up with HUUUGE shear forces acting on the screws and they’re sure to fail. Normally you overlap by staggering the boards 50%, but in a critical application like this steel, LVL, or a glulam (a type of factory-built laminated wooden beam) is really the right way to go.

    Don’t guess at your structure here. Whatever you build right now is going to be subjected to dynamic loading as you drive down the road. Bouncing down the road WILL FAIL any marginal structural elements.

    Bill

  4. Michael Kalman | | #4

    Thank you all for the replies. What is the next best thing if LVL or Glulam is not available? The laminated 2x6s to get 4x6 and king trusses, with additional gussets? And what size should I seek for a LVL? 2x8 x 38'? or is 2x6 fine?

    For the future, how do I know if my ridge is load bearing to avoid this?

    1. Expert Member
      Michael Maines | | #8

      Michael, you can't span 38' with a structural ridge made of wood. Or, more accurately, you could, but the beam would be extremely large and heavy. You need to either support it at mid-span or use rafter ties (not collar ties) to keep the walls from spreading.

      Please don't take offense, but with your lack of understanding of building physics you are putting the inhabitants and others at risk--please considering paying a structural engineer to design a safe assembly for you.

    2. Expert Member
      Malcolm Taylor | | #9

      Michael,

      Your ridge is load-bearing on any building where there is no provision to counteract the outward thrust of the roof with horizontal structural members like rafter ties, trusses, or a second floor with no knee walls.

      Fold a piece of cardboard and place it on a desk so that it forms a tent shape. Push down with your finger and watch the edges move outwards. The two ways you can stop that are to either tape another piece of cardboard to the bottom forming a triangle, or place a support under the peak of the tent.

  5. Jason S. | | #5

    Michael,

    As Malcolm suggested, you have the choice of making the ridge non-loadbearing. To do this, consider making the shorter cross-spanning rafters into something more like a truss, triangulated or parallel chord or otherwise. The connections of the truss members must resist moment forces to both hold the truss shape under full load and transfer vertical load only to the sidewalls. They shouldn't push the walls outward.

    If you go that route, the ridge no longer requires an egregiously long structural beam, but don't forget truss racking resistance! Dimensional lumber 45 degree 'kickers' can work to brace the top of one or more trusses to the top plate of the end walls.

  6. James Howison | | #6

    I think you’ll find that a structural engineer costs in the order of $350 to figure this out for you. Not nothing but think of the time you’re going to invest building on top of this structure, all the finishes etc. Only one shot to get this right :) ok, two shots!

    Also: 38’ is a tiny house?

  7. Expert Member
    Akos | | #7

    If you don't want a lot of collar ties or beams in the middle of the house, one option is to support the ridge support columns on a set of beams that run across the top of the wall.

    Because the wall to wall distance is so small, the beams would be pretty small. With 3 cross beams and 3 columns, the span of your ridge beam now goes down to 9.5'. Your local building code should have tables to spec both the beams accross the top of the walls and the size of the ridge beam needed. My guess it is around a double or triple ply of 2x10 for both.

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