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I have a 2-car garage that is not cooled or heated. Above the garage is an attic/loft space. The attic has beams (they are large beams, not 2-by-4s) that are spaced 16 inches apart. The beams have pink fiber insulation in-between them. The insulation has a vapor barrier on the underside. I did a lot of research online and think my questions will apply to a number of people. I couldn't find a single source that answered all of my questions.

1) Before I add a plywood subfloor do I need to add an additional vapor barrier (the insulation already has one)?

2) I watched a Georgia Pacific instructional video on YouTube and they said to leave 1/8 of an inch in-between the 4x8 plywood boards, I thought you were supposed to butt them up against one another?

3) Will 1/2 inch thick plywood work in my situation?

4) What kind of screws should I use? Will 1 inch decking screws work?

5) Should I lay the plywood perpendicular to the beams?

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  • Not sure what you plan to do up there, but I wouldn't secure it, run it perpendicular to the beams and only where needed. Also be mindful that even if your garage had ridge and soffit vents which it probably does not, your belongings are going to see 140 degree temperatures.......
    – noybman
    Commented Aug 21, 2017 at 3:54
  • It does have ridge and soffit vents. I have a bunch of stuff stored up there but want to tidy it up and make it easier to get to. Why wouldn't you secure the subfloor?
    – John
    Commented Aug 21, 2017 at 3:59
  • If you need it to be permanent, then its ok. I just know I have often had reason to move the boards so never bothered in my own home. As for spacing, it is good to have the gap for expansion, 1/8 wont hurt. Prevents binding, you would have plenty of room to secure it. You shouldn't need additional vapor barrier
    – noybman
    Commented Aug 21, 2017 at 4:05
  • Thank you. My biggest concern is that a subfloor would somehow increase the chance of mold or fire. I highly doubt either but wanted to confirm, especially the mold part? I say fire because it does get very hot up there (about 140 on the very hottest days) and with a subfloor, the insulation will be sandwiched between the drywall and the new subfloor. Thanks again for your advice.
    – John
    Commented Aug 21, 2017 at 4:10
  • Plywood is good up to 200degF so I wouldn't worry about that front. As to mold? Well, keep the water out and you're good -- no water, no mold!
    – ThreePhaseEel
    Commented Aug 21, 2017 at 10:58

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If the cords on the trusses are larger than 2x4 than it sounds like they used storage trusses designed to carry a little more load than normal.

1) No don't worry about another vapor barrier especially since it is a garage. The first one helps keep moisture out of the insulation which degrades its ability.

2) Good idea. A small gap allows the plywood to expand and contract with humidity and temperature without bowing.

3) If you are walking on the plywood then I like 5/8" since it doesn't spring as much under my weight (225 lbs), and even more so if you are storing items with significant weight to them.

4) Decking screws should be fine. I would go to 1 1/2" but you are just trying to keep the wood from shifting. 8d nails would work fine too.

5) Plywood and OSB have a higher strength to them lengthwise to span the rafters and carry the weight of the roof. So, yes lay the plywood or OSB perpendicular to the trusses. There is a good discussion of strength orientation here and they say plywood is generally stronger in its long dimension unless otherwise marked.

Good luck!

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    Roof decking sheets (4'x8') are laid with the 8' across the rafters.
    – Jim Stewart
    Commented Aug 21, 2017 at 20:09

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