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Thread: roof ideas??

  1. #1
    Member akfirefighter's Avatar
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    Default roof ideas??

    So I am trying to figure out what to do for the roof of my cabin. The cabin will be 20x24 with 12/12 pitch roof. I am going to use rafters made out of 2x10's. The questions I have are how should I vent and insulate the roof? I plan on using metal. Do I sheet the roof with OSB or put the metal directley over the rafters? With the 2x10's have enough room for insulation or should I go with 2x12's. I was thinking about putting 2x4's horizontally across the rafters a couple feet down to make the inside of the cabin have a flat cieling, would this be a good idea for ventilation and insulation?

    Thanks

  2. #2
    Member dkwarthog's Avatar
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    I am not a building expert, but I built my own home and this is how I made my cold roof...


    I have 2x10 rafters and I nailed 1x2's flush with the top of each rafter on each side. Then I put the 1/2" plywood sheathing on the roof, which works good because you now have 3" of rafter edge to hit with your plywood seams.

    Then from inside the house, I put 2" blueboard in between each rafter up against the 1x2's. That gives you 1.5" of airspace between the blueboard and sheathing. That gives you a cavity to fill with insulation that is about 5.5". I used spray foam in that space for a total R value of about 38, but in my addition I used R25 fiberglass for a total R value of about 35.

    I installed 12 foot long 2x4 ceiling joists between the rafters to creat an 8' ceiling in my second story (only the center 12' of a 32' wide home). I insulated above the ceiling with fiberglass to R50 and put vents in my gable ends above the insulation. I seems to work slick, the only problem I have seen is that a couple of my blueboard pieces stopped at the wall top sill plate blocking and melting frost/condensation would drip down on top of the wall, I solved that by making sure the blocking is short enough to allow the blueboard to extend all the way past the outside of the wall..

    The other advantage of having that blueboard in there is that if I ever have a roof leak, the water will drip onto the blueboard and run to the outside thru the eave vents instead of running into my insulation....

    I know this is probably hard to follow, but it's the best I could do

  3. #3
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    My cabin is not a full time residence so the insulation requirements are less important to me that for my house. I use it on weekends all winter and heat only when there. I used 2x10 conventional lumber for my roof, which is similar size and pitch to what you're planning. I insulated from the bottom with 8" insulation leaving a couple of inches of air gap under the sheathing and then crafted some ridge venting and eave vents. I've never had any problem heating the place and there's no indication of condensation anywhere I've opened the ceiling up over the years.

    My primary cabin is sheeted with 7/16 OSB and the addition is an extension of the 2x10 rafters with 1x4 horizontal battens instead of sheathing. The battens are less expensive and easier to install but the roof screws back out every season. Every spring and fall I have to get on the roof and reset the screws on the batten portion of the roof. I've never had a screw back out of the OSB. For that reason I'll use OSB sheathing for any future roof projects.

  4. #4
    Member dkwarthog's Avatar
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    I think Mr. Pid's method using insulation and leaving an air gap is the most common method. I wanted spray foam and needed something to spray against so I installed the blueboard. In hindsight I would have used 2x12's and fiberglass probably...

  5. #5
    Member AKDoug's Avatar
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    The Spenard Builders/Galco stores sell a high performance R-38 insulation for 10" spaces. Couple that with 2x12 rafters and you will have adequate ventilation for your roof. I prefer to sheet with OSB and use #14x1 roof screws. If you use the 1x4 purlins I would also use the #14 x1 screws. They have much better holding power and I haven't seen one back out yet.

    The nice thing about 1x4 purlins running 90deg to your rafters is that you have a nice ladder to climb up and down when installing the steel.
    Bunny Boots and Bearcats: Utility Sled Mayhem

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    Member cdubbin's Avatar
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    The company I work for, when we're redoing roofs, we usually use 5/8" cdx with Grace barrier on top. That's a bombproof roof, baby! My house is just 2x4 purlins with ring-shank rubber washer roofing nails.
    " Gas boats are bad enough, autos are an invention of the devil, and airplanes are worse." ~Allen Hasselborg

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    I also used fiberglass matched to my rafter space. If you are using fiberglass make sure you have a good airspace when installing the batts and good way to keep the squirrels out of the roof space that leave good airflow up the channel you create. Cheat the plywood down at the peak to give a good gap for airflow.

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