Anyone found a quality underlament for garage roof? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 11-02-2010, 08:00 PM
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Anyone found a quality underlament for garage roof?

I am building a 24 X 38 garage with a 3/12 roof pitch. I am using 2X6 rafters on 24 inch centers. I was planning on using 4X8X7/16 plywood under a metal roof. When I went to the big box stores to get the initial supply of plywood to re-enforce the corners of the building, I found the quality of the plywood to be extreamly bad for the CDX normally used on a roof. I had to sift thru about 60 sheets of plywood to get 10 sheets of fair quality, and this is at $14.00 a sheet. They had some better quality plywood there but it was extremely expensive.

I had considered using OSB but the odor was so bad I could not stand it. The underside of the roof will be exposed to the inside of the garage so this was enough of a problem to keep me from using it.

The metal roof supplier told me that I could install 2X4's across the 2X6 rafters and install the metal roofing over that, but he said if I heated the inside of the garage in the winter time, the underside of the roof would sweat, so I really don't want that either.

Any ideas on how to solve this problem?

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Old 11-02-2010, 08:24 PM
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seems like ive seen a styrofoam insulation between the metal roof and the 2x4 runners.
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Old 11-02-2010, 11:11 PM
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The insulation may be your only way out given what you have found..personally I have not had or been told of anyone having an odor issue with OSB..CDX ply is a structural grade and is not as pretty as an appearance grade.


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Old 11-03-2010, 06:40 AM
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osb or any other similar composite material will stop "offgassing" [smelling
] after a short period of time .but.... osb does not hold fasteners as well as plywood. I'd put the osb dowm, then use 1x or 2x purlins 3' o.c. toattach your metal.
. [ 35 year builder ]

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Old 11-03-2010, 06:48 AM
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They just did a small church accross the street from my house with steel on the roof. I watched close as I am thinking of going that way myself. They used osb first and then nailed 1x4s at the peak running the length of the roof then used a 2ft x 8ft 3/4 in. thick white foam running the length of the building between the 1x4s and nailed with a plastic cap nail. Then 1x4 again then the foam and so on. Then after that they covered it with 30# roofing felt, they said the felt would work as a barrier to help keep any moisture from getting to they roofing structure. This is how the roofing crew did it. Hope this helps you out.
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Old 11-03-2010, 07:36 AM
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Look for the aluminized composite decking material. It looks like pressed board but is much more durable than pressed board or even plywood. It has an aluminized coating on one side that goes toward the inside of the structure. It will dramatically cur down on energy use especially in the simmer. It eliminates the need for a heat barrier in your attic.

Vince
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Old 11-03-2010, 10:28 AM
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I see you are down south, so I guess things are much different there......2x6s and 3/12 pitch wouldnt come close to code here. Also, we dont insulate up in the peak. We insulate the ceiling above the room of the garage. That keeps the sweating factor down.
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Old 11-03-2010, 10:44 PM
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if it were me, i wouldnt leave the rafters open.. makes it harder to light, harder to heat without a ceiling, plus painting would be almost out of the question.. i put in those drop stairs to the attic in my garage, and wired a 4 foot light off of one of the circuits for the 8 foot ceiling lights.

if i had it to do over again, i would hang white metal siding for the ceiling and walls in my garage instead of drywall or particle board.. friend of mine has a shop with metal siding walls and ceiling its very air tight and heats and cools good, his paint bay is the same way.. shop sprays down easy and the walls are easy to clean..

now that i think of it thats how we did my dads shop..we used the 1x4 runners on the roof with the white stryofoam sheets of insulation between them..
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Old 11-04-2010, 06:00 AM
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7/16 is on the thin side for 2' spacing, I believe 5/8" is minimum code around my area. When I built my garage, my trusses are on 24" centers and used 3/4" sheathing.
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Old 11-05-2010, 07:34 PM
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Depends where you live. Here in PA 3/12 pitch is pushing it. I would go with at least a 4/12.
If you go with 2x4s on the 2x6s and then metal roof, you can then insulate the ceiling. This will give you less space to heat or cool.
That is what I did and have no sweating.
Good luck!
You might consider going with a 9' or 10' ceiling in case you decide to install a lift.
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Old 11-06-2010, 03:07 PM
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I thought of something today.
If you go with truss system they will be engineered for your area and they will have the ceiling joust already installed.
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Old 11-06-2010, 06:34 PM
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Since he is in Alabama he is probably not worried about heating it and certainly is not concerned with snow load.
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Old 11-09-2010, 11:03 PM
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Garage roof underlayment

Firestone makes an interesting product for such a use. It is in common use for flat roof commercial construction. (Can't remember what it is called - came across it a couple of years ago) Try googling "Firestone roof insulation"
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