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Thread: Ridge beam and rafter sizing?

  1. #1
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    Ridge beam and rafter sizing?

    Considering adding an addition with open "cathedral" ceiling (to avoid trusses). Span will be 20' with 2' roof overhang on each end. I'm considering 2 or 3 2X10's laced together as ridge beam (since it will be supporting the load) supported by several vertical 2X4's burried in wall to support the beam. Rafters probably 2X8's or 2X10's on a 18' span resting on beam. Is this enough or do I need a engineered wood lam for the ridge beam? I would probably do mockup beams accross acting as cross beams or ties on 4' centers. I usually overkill. Am I on track?
    Jerry

  2. #2
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    Any particular reason to avoid trusses? A scissors style truss will give you the cathedral ceiling and more than likely allow more room for insulation plus eliminate the ridge beam. Should go up much faster, too. Downside is that a it's not really a true cathedral ceiling; the inside pitch won't mimic the exterior pitch because of the need for the interior truss components- the inside pitch is shallower.
    Last edited by John Callahan; 06-25-2009 at 5:57 PM.

  3. #3
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    Smith Mountain Lake, Virginia
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    How about going with a true timber frame? It's a fine look.
    Regards, Frederick C. Wilt

  4. #4
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    Jerry, is this what you mean (with 2' overhangs instead of the 1' I drew)? In this case, 2x8's will work, but you might want to use 2x10's for added insulation. I'd need to know how far the ridge will be spanning to say whether 2x10's would work, or you'd need an L.V.L. As for the wall ties at 4' o.c., they would probably be overkill, since the ridge is resisting the outward pressure of the roof on the walls.
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  5. #5
    How much snow do you get? How much wind? Does the ground move in Illinois (New Madrid fault)?

    We've just had a house built at Lake Tahoe. Design snow load is 200 psf. In an earthquake. Each new home built up there empties a lumberyard (figuratively speaking).

    Consider hiring an engineer to do the calcs?

  6. #6
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    Hello Jerry. I would call or go by your building inspectors office. They can advise you of local minimum requirements and may have a hand out with info.

  7. #7
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    Nov 2008
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    Smithfield, UT
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    Jerry,

    First a disclaimer: you should always check with the building department. That said, they will probably just refer you to the codes that won't tell you the particulars of what you are asking. So I will tell you what I would suggest as I have just a few extra moments here.

    Consider just using scissor trusses manufactured locally. They will build them to code and they will be a much easier and faster install. But the interior pitch won't match the exterior pitch. Personally I hate it when I ask for advice on a certain method and all I get is alternate method advice, but just in case you were open to the easier alternatives.

    My #2 suggestion would be to use engineered lumber. Since you are suggesting using regular dimensional lumber I am going to assume that the length of the room is less than 16' and that the span width is your stated 20'. I'll assume a modest 6:12 pitch and being in northern Illinois I'm assuming a live load of 40#. In this case you'll be ok with two 11.875" LVL beams ganged together for the ridge beam. If the roof length increases to greater than 16' but still less than 20', then you'll need two 14" LVL beams instead.

    Create a beam pocket in a gable end, with a continuous run to a footer. If you don't know what that is, picture four 2x6 studs nailed together with the center two being 12" shorter than the outer two. You'll also need a layer of 1/2" ply between the center two in order to fit the 3.5" thick beam you've just created.

    Oops.. out of time. Gotta go., I'll try to finish up later.

  8. #8
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    Westminster, co
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    Hire an Engineer

    Jerry:

    Hire a licensed professional engineer in your state to determine your roof loads and size the structures. As an engineer, some of the information in the postings is correct, but not all. I do not think you will get a building permit for this roof without stamped structural plans - my two cents as a professional engineer
    Last edited by John P Clark; 06-26-2009 at 7:18 PM.

  9. #9
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    ++ for John Clark's response

  10. #10
    Jerry, I have been a carpenter for over 20 years now, and will tell you from experience NOT to use microlams glued and nailed together. I have seen them pull apart and fail. Use a glue lam that's rated for your application. A quick call to a quality local lumberyard should be all you need to figure out what you need. Personally though, I'd go with at least a 5 1/4x16" glue lam.

  11. #11
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    Well I don't mean any disrespect for the rest of the pros around here at all. What I am suggesting is that the OP is asking for advice on something that is very simple and typical of residential roof construction.

    Indeed, most places will require a stamped set of engineered documents for the roof anyway. My suggestion to use the LVLs was based on the manufacturers specs, which is exactly what the engineer is going to use. This is not a complicated engineering feat, but rather framing 101.

    5.25x16" beam is very much overkill for the application. A little 20' x 16' addition doesn't really need a beam that large. But if it makes you sleep better at night. Also, I usually lag screw the beams together. In any case, if I were building this addition, I would do the following, and it wouldn't require engineered docs here because it is in accordance with the lumber manufacturers specs.

    Two 12" LVLs glued and lagged together. 9 1/2" TJI for rafters spaced 24"o.c. Attach to beam with simpson variable slope joist hangers and simson strap tied over the peak. Strap the TJIs together according to manufacturers specs and apply 5/8" sheathing.

    That really would more than get the job done. In fact even the lowest rated TJIs could be used for this job as there is only a 10' horizontal span.

  12. #12
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    The other benefit to using the LVLs instead of the glulams is that they are a not nearly as awkward to install. A 5.25x16" glulam is really quite heavy, at something like 320#. He'd either need a crane or about four other guys on ladders to get it up there. OTOH, each of the LVLs I suggested would come in at about 100# and can be managed by just 2 people really easily.

  13. #13
    You gave the span as 20' but didn't give the length of the addition. 20' is not much, 2x6's would be strong enough, but no room for insulation. Personally I prefer an attic, and therefore like to build a beam at the ceiling height preferred, and then brace the roof up off the ceiling beam. Also, not a fan of plywood beams, usually they sag of their own weight, and you need to brace them up.

  14. #14
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    There is not enough information to answer your question. If you are running your rafters to walls as Brian suggested in his sketch then you are relying on that ridge beam far more than if you are adding a bonus room for example and the rafters meet the floor.

    You should have it engineered: snow and wind loads, the pitch of the roof, the length of the rafters resting on the ridge beam, and the free span of the ridge beam all need to be considered. If it is a 20' span in the ridge beam you could easily need 3-18" LVL's if not more and 18' rafters will probably need to be 11 7/8's I joists. Trusses are usually cheaper than stick framed roofs.

    Do it right and safe. Get a building permit and have your plans engineered.
    Last edited by Glen Butler; 06-29-2009 at 5:33 AM.

  15. #15
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    Quote Originally Posted by Jim Andrew View Post
    Also, not a fan of plywood beams, usually they sag of their own weight, and you need to brace them up.
    Not necessarily true. I restored a Carl Strauss designed house built for the Dracket family that had great high ceilings[27'], carried on plywood box beams spanning 26' themselves. It was standing proud from 1962 until about 5 years ago when the current owner let the roof leak into the box beams. I built new box beams in the affected sections and all is well.

    Interesting house. It had 87 windows and twenty one exterior doors. Each bedroom had two exterior doors, and some rooms were not accessable from inside the house, you had to go out on the deck to get to the bedrooms. We replaced all of the windows and doors with units built in our shop.

    Jerry: I could tell you what you need to do here, but that would not mean anything to you. I would go to your local building department and ask politely. Some departments have a god complex, so keep that in mind and be humble. They will tell you what is the minimum, go one size over. I would not even consider trusses if you have the ability to do it with rafters. Scissor trusses are usually so inaccurate that I find them very frusterating at the finish stage, I prefer rafters in any case. Old school and not afraid of a framing square I guess.
    Last edited by Larry Edgerton; 06-29-2009 at 7:19 AM.

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