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2×6 T&G car decking install advice

Wunderbar | Posted in General Questions on

Im redoing a large section of post and beam roof and looking for any advice on installing the 2×6 decking.

Zone 4, Roof assembly is

4×10 beams 4′ o/c
2×6 t&g
1/4″ plywood sheathing for shear
7″ of polyiso (fiber paper backed)
1/2″  Zip sheathing taped
metal roof

Questions are

1. how to airseal at the overhang? I’m thinking ill install the v groove side up and tape/caulk where the wall meets the t&g.

2. is 1/4″ sheathing for shear good enough or do I need to do 1/2″?

3. I’ve got a source to buy a fair amount of the 2×6 T&G at a really great price but they are all only 8′ long. Doesn’t seem like a big issue as my beams are 4′ o/c but i couldn’t find if there was a code requirement on the length?

4. Is it better to install 1×4 furring strips on top of the polyiso and then the metal roof instead of the zip sheathing? I’m technically in a wildfire area (not high risk though) so was wondering if having no air gap under the metal roof would be safer.

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  1. Expert Member
    Akos | | #1

    "how to airseal at the overhang? I’m thinking ill install the v groove side up and tape/caulk where the wall meets the t&g"

    The best way is to stop the T&G half way across your outside wall plate and tape the air barrier (in your case the first layer of CDX/OSB) down to your wall sheathing. From there continue the T&G to your overhangs. This also lets you change the direction of the T&G which can simplify overhang details sometimes. Make sure to tape the seams of the CDX/OSB as well.

    " is 1/4″ sheathing for shear good enough or do I need to do 1/2″

    The thinnest our code allows is 3/8" for a roof. Our code also allows T&G itself as structural sheathing so anything else is optional. You might want this layer thicker so you have a bit more depth for screws this way you don't have to worry about trying to hit the beams bellow for all your fasteners.

    "if there was a code requirement on the length"

    There isn't in our code but you should check your local code.

    "Is it better to install 1×4 furring strips on top"

    At the current OSB/CDX prices, I would not bother with 1x4s. With the current trends and dryer weather, mild fire area will be high risk eventually.

    7" Polyiso seems overkill in Zone 4. With expensive insulation there is no ROI for going above code min here. Since continuous external insulation works better, most codes allow for U factor based compliance. I would check your local code as most allow for an R34 assembly, so in your case this might mean only needing 5" of polyiso.

    1. Wunderbar | | #3

      Great thanks for the tips!

      good advice on the air barrier for t&g. that seems great.

      Code is 3/8th here too and allows for t&g as sheathing but seems good practice to nail an additional plywood sheet for better shear strength. I'll likely just do 3/8th.

      I only went with the 7" as I got a good deal on a ton of 3.5" iso from a commercial site so doubling up on that for the roof.

  2. Expert Member


    Akos is right - the best way to seal the T&G is to stop it at the exterior wall. If it supports the overhangs and that isn't possible, bed the T&G in a flexible caulk (like Contega HF) as you install it, and run a bead on the groove before putting the next piece in place.

    I wouldn't lay it with the v-groove up, that just moves the problem to the top, and looks like hell.

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