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Thread: bci for roof joist

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    Member akriverunner's Avatar
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    Default bci for roof joist

    People that have used BCI's for roof joist how did u go about insulating and venting?

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    Member AKDoug's Avatar
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    You basically treat them the same as a standard 2x12 if you are using a 11 7/8 BCI. If you want ventilation you will have to buy cathedral ceiling R-38 that is only 10" thick. that will leave 2" of airspace above. You vent the eave end just like you would a standard rafter. In my opinion, and I sell both, there is no real advantage to using BCI's in 9 1/2 or 11 7/8 size vs. 2x10's or 2x12's in a roof application other than weight. BCI's are more expensive in initial cost and cost much more when you consider the Simpson straps required to tie everything together plus the pain and expense of cutting all the filler blocks needed to do it right. For snow load they are not stronger and the straightness advantage generally isn't a big deal on a roof, especially for a cabin.
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    Member Dirtofak's Avatar
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    If you have exposed eaves, they just don't look right either.
    I don't mean to sound bitter, cold, or cruel, but I am, so that's how it comes out.
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    Member akriverunner's Avatar
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    Maybe I will just use 2x12s, I want a 8/12 pitch the cabin is 24' wide 36' deep with a half loft. I assume I will need a ridge beam and go on 16" centers? Do I have to plate each side of the 2x12s at the peak angle? Thanks

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    Member akriverunner's Avatar
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    Where the 2x12 sits on the wall do I have to notch it or use a Simpson tie or what are the methods for this? Thanks for all the help.

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    Member Dirtofak's Avatar
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    I used site built trusses. My cabin is 20 wide. You would have to buy 20 foot 2X6s to do it. It your place remote?



    I don't mean to sound bitter, cold, or cruel, but I am, so that's how it comes out.
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    Quote Originally Posted by akriverunner View Post
    Maybe I will just use 2x12s, I want a 8/12 pitch the cabin is 24' wide 36' deep with a half loft. I assume I will need a ridge beam and go on 16" centers? Do I have to plate each side of the 2x12s at the peak angle? Thanks
    Quote Originally Posted by akriverunner View Post
    Where the 2x12 sits on the wall do I have to notch it or use a Simpson tie or what are the methods for this? Thanks for all the help.
    16" OC is overkill for rafters IMO. 24" centers should be sufficient for your roof. 16" is standard for your walls, but usually 24" is sufficent for roofs and floors as long as the span isn't too long. Put a birds mouth (a notch) where your rafter tail overlaps your top plate. Cut an 8/12 pitch on the top and nail it to your ridge beam. For added measure you could use some type of joist hanger at the ridge beam to give it added strength. But this might be overkill as well. As for your insulation, I am assuming since you will be building a half loft, that the ceiling will be vaulted. The main concern here is a hot roof. Whenever you have a hot roof the main concern is moisture buildup in your rafters. Without proper ventilation your rafters could go rotten and mold over time. If I were to do it, I think I would use batt insulation and lay it in loosely on top of the vapor barrier with the 2-3 inches of air space on top of the insulation to give a little ventilation. Leave a little bit of air space on top of your bird blocks and install bird screen first. Also leave a 2 inch gap on your roof sheeting before installing your ridge cap. Good luck.

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    Member akfirefighter's Avatar
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    Quote Originally Posted by Bushwhack Jack View Post
    16" OC is overkill for rafters IMO. 24" centers should be sufficient for your roof. 16" is standard for your walls, but usually 24" is sufficent for roofs and floors as long as the span isn't too long. Put a birds mouth (a notch) where your rafter tail overlaps your top plate. Cut an 8/12 pitch on the top and nail it to your ridge beam. For added measure you could use some type of joist hanger at the ridge beam to give it added strength. But this might be overkill as well. As for your insulation, I am assuming since you will be building a half loft, that the ceiling will be vaulted. The main concern here is a hot roof. Whenever you have a hot roof the main concern is moisture buildup in your rafters. Without proper ventilation your rafters could go rotten and mold over time. If I were to do it, I think I would use batt insulation and lay it in loosely on top of the vapor barrier with the 2-3 inches of air space on top of the insulation to give a little ventilation. Leave a little bit of air space on top of your bird blocks and install bird screen first. Also leave a 2 inch gap on your roof sheeting before installing your ridge cap. Good luck.
    If you leave a 2 inch gap in your sheeting on the ridge will this be enough for air movement? If you use metal roofing should you drop that a couple inches too and cover with ridge cap, or will it be ok to put the metal right up to the peak? It seems that if you drop you metal down a couple inches snow and rain could get under your cap and create a problem, but sealing it up tight reduces your air flow too? What is the best way to vent a cathedral roof?

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    Member AKDoug's Avatar
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    Half of my house is 14" BCI's with 12" of insulation. The 2" of air space has been plenty. My sheething is held back 2" from the ridge on each side of the ridge. I ran bird screen down the ridge under the ridge cap to seal it up from bugs. I ordered ridge cap with 12" of coverage on each side and I've had zero issues with snow or rain getting into my roof. However, I live in a relatively wind free area.

    To use 2x12's for rafters on a 24' wide roof you are going to need to go 16" o.c. to hold any real snow load. The problem is, you will also need a huge ridge beam to make it work. I know exactly why dirtofak stopped at 20' wide. Beyond that width requires some decent engineering to get the roof to work with any sort of real snow load. It can be done, but it takes using mid span beams between the ridge and eave walls to make it work if you actually want to lift them up with man power. You may also require posts at the center of each beam to make them work. I literally have dozens of folks in my store every year wanting 24' wide and wider rafter roofs and the solutions are often expensive. Remember that a roof with no collar ties or ceiling joists gets all it's strength from it's ridge beam and the outer walls. The ridge beam actually supports 1/2 of the snowload of the entire roof. If you have a 24x24 structure the ridge is holding up a 12x24 area of snowload. I use 100# per square foot as a baseline for snowload around here. 28,800#'s of snow is like parking a Kenworth on your ridge A whole bunch of structures in snow country only survive because they shed the snow. An 8/12 will not reliably shed snow if you are not in it all the time. One of my employees has an 8/12 steel roof on his house and we've seen it hold an entire winter's worth of snow. We are talking 5' compressed. Even the 12/12 portion of my house will hold up to 3' at times.

    As for wall to rafter connections, a Simpson VPA hanger is an excellent solution and allows you not to have to make any birdsmouth cuts.

    For making BCI's look "right" at the eave overhangs you can sister on cantilevered 2x6's for tails and cut the BCI's plumb with the wall. Use 2x6 lookouts and barge rafters to clean up the gable ends. You can't even tell my roof on my house uses anything other than 2x6's for the rafters, but they are all 2x12's and 14" BCI's.
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  10. #10
    Member Dirtofak's Avatar
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    <---Thinking Doug nailed it. Wide costs $$$$$$
    I don't mean to sound bitter, cold, or cruel, but I am, so that's how it comes out.
    Bill Hicks

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    Member junkak's Avatar
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    Nice construction DirtofAK!

    How quickly did you build your last joist? Start getting a groove going with the jig and bam you are done!

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    Member akriverunner's Avatar
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    Dirto, what did you use 2-2x6 together so you have 11"? What insulation did you use also?

  13. #13
    Member Dirtofak's Avatar
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    The brackets were all cut at home on my table saw. I had 2 guys assembling them while I cut 2X material. They are put together with construction adhesive and a Paslode with ring shanks. I think it took us an hour to get the jig set up for the first one. Then about 20 minutes each after that. Room to store them was tough. It is 17' to the top of my walls. They are on 24". 3 years, no problems. We got the ceiling up and only had to shim twice for less than 1/4". 12/12 pitch, metal standing seam roof and it doesn't always shed snow. I can make it shed by pounding on the bottom of the rafters though. It is quite a sight to see all that snow shedding.

    A person would have to use 2X8s to get more insulation. I figure R34 is enough for a weekend cabin. Once the walls are warm it is very easy to heat.

    If anyone is doing this, I have a stack of the plywood gussets a couple feet tall that I can contribute to their cause.
    I don't mean to sound bitter, cold, or cruel, but I am, so that's how it comes out.
    Bill Hicks

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