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Old 09-14-2011, 04:35 PM   #1
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Laying second layer of subfloor


My question is about laying a second layer of subfloor over the existing 1x6 (or 1x8) diagonal subfloor. Existing floor is ceramic tile over cement over the 1x subfloor. Joists are 16" centers. Second layer of subfloor needs to be 3/4" to match floor height in adjacent family room which is a 3/4" plywood with some sort of finish on it. Both rooms will be finished in 3/4" hardwood.

I have found plenty of articles with different opinions for laying a second subfloor over plywood but none concerning a first layer that is the old diagonally laid 1x dimensional lumber.

I am planning to use 3/4" or 23/32" T&G plywood. I have been told I can use OSB but have concerns if we should have some sort of flood, this being a kitchen. I am planning to fasten with #8 deck screws. The questions, based on other reading:

1) Do I really need T&G wood? A family relative had a second floor laid recently in which 5/8" ply was first laid over vinyl tile. The installers did not use T&G ply.

2) Should I put a layer of felt paper between the subfloors? Or should I glue the second subfloor to the first as well as use screws? If gluing is the answer, should it be wood glue or construction adhesive?

3) Should I fasten the second subfloor into the joists? Or try not to attach to the joists? I have seen both opinions expressed in other articles.

Thanks,
George V.

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Old 09-14-2011, 05:00 PM   #2
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Laying second layer of subfloor


2nd layer of 3/4" over 1x6 slats should have buiding paper to protect from squeeks. Should line up butt joints on center of joists and try to get all screws into joists. Plywood is a better answer than osb but t&g is not necessary.

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Old 09-14-2011, 06:34 PM   #3
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Laying second layer of subfloor


No T&G.
No glue.
Today's OSB's will take moisture the same as plywood will.
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Old 09-14-2011, 06:39 PM   #4
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Laying second layer of subfloor


Hi George & Harry, & Bud,

The opinions I give come from a tile setter's view, but should be valid when installing hardwood too.

Quote:
1) Do I really need T&G wood? A family relative had a second floor laid recently in which 5/8" ply was first laid over vinyl tile. The installers did not use T&G ply.
Not necessary, right.

Quote:
2) Should I put a layer of felt paper between the subfloors? Or should I glue the second subfloor to the first as well as use screws? If gluing is the answer, should it be wood glue or construction adhesive?
I don't see any benefit in placing tarpaper between the subfloor and the underlayment. If anything the paper should go between the underlayment and the hardwood. Check with the manufacturer's instructions.

The underlayment should not be glued to the subfloor, just fastened.

Quote:
3) Should I fasten the second subfloor into the joists? Or try not to attach to the joists? I have seen both opinions expressed in other articles.
It's best to avoid the joists and fasten into the subfloor. No problem if you use the correct length screws. Choose flooring screws that are a little longer than both layers. I don't believe it's a big deal though with hardwood.

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Old 09-15-2011, 08:42 AM   #5
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Laying second layer of subfloor


Hi Jaz,

Ive had issues with screwing to 1x6 slats in the past thats why i recommend into joists. If there was subfloor underneath then i would agree with you
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Old 09-15-2011, 09:14 AM   #6
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Laying second layer of subfloor


I know what you mean about fastening to old planks. So, I agree, if the planks are in poor condition or have wide spaces that some screws will hit air, I would then switch to longer screws and hit the joists too.

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Old 09-15-2011, 09:48 AM   #7
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Laying second layer of subfloor


Quote:
Ive had issues with screwing to 1x6 slats in the past thats why i recommend into joists. If there was subfloor underneath then i would agree with you
Where Jaz is coming from is like he said; his advice is "from a tile-setters point of view."

And that is...the tile installation industry recommendation is not to screw underlayments or tile backers into floor joists in an effort to help separate the tile installation from the structure. They are to be fastened into the subfloor only. That's not to say we all agree with or understand that theory, but that's the information from the tile Gods.

In this case where hardwood is being installed I suppose it really doesn't matter.

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