New addition: double subfloor to match existing?


Old 09-01-14, 09:19 PM
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New addition: double subfloor to match existing?

Hello, first post here. I've been trying to figure out my best option on my subfloor sheathing. I have the new floor joists on a kitchen addition (hardwood floors to come later) but because I'm adding onto an old house, the dimensional differences in lumber mean that the tops of my floor joists are now a good 1" to 1-1/8" from the top of the existing subfloor.

I think I want to double layer my subfloor to come up and match the existing (I can't find just 6 panels of 1" or 1-1/8" subfloor sheathing). This is not going to be a subfloor with an underlayment; both layers will be permanent subflooring and I plan on setting the walls on top of the second layer so everything is even with the existing subfloor.

My questions are:

1) Should I go with double 5/8" subflooring or 3/8" plywood topped by 3/4" subflooring?
2) Should both layers be oriented perpendicular to the floor joists or should/could one layer be perpendicular to the other layer?
3) Should I put felt down between the layers?
4) Should I use adhesive glue between the layers?
5) Should I nail both layers into the floor joists or just the bottom layer into the joists?

I'm looking for the strongest available option (within reason). Thanks!
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Old 09-01-14, 10:28 PM
Handyone's Avatar
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a good 1" to 1-1/8" from the top of the existing subfloor.
I would say measure accurately and set the floor height exactly where you want it.
If it's 1-1/8", use 3/4 + 3/8. I would lay 3/4 down first.
Place both perpendicular (you could overlap joints), use adhesive to stop squeaking. You don't need to get carried away with adhesive. Nail both into joists, not necessarily at the same time.

Not sure about felt, others may comment. For me, it would make sense not to. The walls will be bearing on subfloor and I wouldn't want anything to interfere with compressive strength.

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