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Structural Ridge beam

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  • Structural Ridge beam

    Can anyone explain why you do not need collar ties when using a micro-lam (parallam) beam as your ridge on a simple gable roof? Is it because it simply will not sag or is it because the structural ridge has greater "nail holding strength" then conventional lumber. Maybe i just don't understand the purpose of collar ties to begin with.

  • #2
    Re: Structural Ridge beam

    Collar ties prevent the roof From spreading under its own weight. -
    Imagine 2 sheets of plywood leaning on each other in A- Frame style. Put weight on them and they will collapse. Tie them together & they will not.
    In a roof you must prevent your outer walls from spreading. Collar ties do that -- If you remove the ties -You need another way to counter the spreading effect. A structural weight bearing ridge beam will do that. The beam supports the roof to prevent the spread.



    • #3
      Re: Structural Ridge beam

      A structural ridge in affect, hangs the roof from the ridge. It however, requires adequate support at the ends. The structural ridge is designed for minimum sag. The walls of a house cannot be pushed out if the ridge doesn't sag. In fact the roof has a tendency to lean inward at the bottom when a structural ridge is used.

      Collar beams in conventional construction do not prevent spreading, they only help - sometimes. With todays taller roofs, a collar beam is not so effective any more. If it is installed low enough to resist the load, it will overload the rafters if minimum rafter sizes are used. This is a common mistake. A collar beam actually puts a bending load on the rafters which can overdeflect them.

      The collar beams do not receive any load however until the ridge beam sags and the walls are pushed out. If this happens, and it does, the collar beam joints will fail, or the rafters will often overdeflect. When a collar beam is placed very high such as just under the ridge, the load is leveraged and the reaction at the ends of the collars is very high. If you have only 3-16d nails in each end, it can hold only about 300 lbs before the joint fails. That is easily exceeded.

      For these reasons, structural ridges are a great alternative, as are stuctural hips. If you decide to use them, get an engineer to help with the sizing. The mfgs will oversize them (probably unintentionally) because they don't take into account the resistance provided by other framin members. For example, a room over a garage can be balloon framed by extending the wall studs above the floor/ceiling. These cantilevered studs provide a lot of resistance especially if they are less than 3 feet. If you have a knee wall, it will also provide a significant resistance. I have also noticed that some engineered wood mfg will add unfactored wind load into the software as well which is not necessary. In one case I am familiar with, the mfg spec'd a 24" LVL for a structural ridge when a 11 1/4 was more than adequate.



      • #4
        Re: Structural Ridge beam


        After reading your engaging discussion on ridges and the relative merits (or lack thereof) of collar beams, I concluded that the 52 year-old roof under which I presently sit should have collapsed years ago. The ridge piece is slim and narrow, and collar beams are placed high. Interestingly, vertical 2X4s spaced roughly 4' apart run from select rafters to the top of a load-bearing wall. I am not sure how consequential those 2x4s are (I suspect not very). In fact, I hypothesize that the roof has remained intact because the rafter bottoms are securely attached to the sides of the ceiling joists; this prevents the walls from being pushed out.

        All of this leads to a question. Reversing a premise found in your first paragraph, I ask: if the walls of a house cannot be pushed out, can the ridge sag (assuming the rafters are adequately sized)?


        • #5
          Re: Structural Ridge beam

          To answer your question, If the walls of a house cannot be pushed out, the ridge cannot sag. At least in the context you ask it, that would be a true statement. I actually can't think of a possible exception at the moment, unless as you suggested the rafters buckled which is not likely to happen.

          Many older homes, at least in the South have ceiling joists that actually cross the exterior walls and tie to the rafter tails. In this case there is no horizontal load to the top of the outside wall. It is ofcourse important that the ceiling joists be continuous to the other side in this case.

          Problems generally occur when there is a vaulted ceiling (no ceiling joists to hold the walls together) or when a builder turns the joists in the opposite direction which seems to happen rather often these days.


          Notwithstanding collar beams are inherantly inadequate to hold a roof together. Other members have been doing the job and the collar beams have usually gotten the credit.


          • #6
            Re: Structural Ridge beam

            the way i c it is this:

            the rafters form the compression chords and the ties form the tension tie,now the joints must be designed for the loads just as a truss is which uses 2x4 members for outragious spans.glen can u do a few calcs showing the force polygon that woould result from a 30' spann with no ceiling joist but with collar ties at 1/3 up per code.


            • #7
              Re: Structural Ridge beam


              Yes I can (do the calcs), but I'll need a pitch.
              At a low pitch, say 4/12, the loads are incredible if you assume that the walls are incapable of holding any load, and the load is carried entirely by the collar beams. In fact, in this ideal condition, i think it would fail.

              This is easy to calculate. Simply sum the moments around one of the wall connections and set it equal to zero. Solve for the unknown tension in the collar beam. The last time I did this, I concluded that it was not possible to put enough nails in the collar beam at the proper spacing to carry the load. A gusset plate or shear connector is a possibility but if you have those kind of loads, you have to look at what that is doing to the rafters.

              What is interesting is that this same discusssion is going on in the letters to the editor in Fine Homebuilding Magagzine in the current issue. Two opposing views (one right and one dead wrong)-- very interesting.

              I'll posts these loads sometime over the weekend when I have a little more time.