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Old 03-03-2016, 01:11 PM   #1
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Attic subfloor help


In the planning/beginning stages of finishing the attic in my 1.5 story cape. Joists are 2x8 and 16" OC. The existing plywood floor is 1/2" and in pretty good condition except for a handful of spongy spots.

It was suggested that I put another 1/2" or 3/4" layer of plywood on top of the 1/2" - is 3/4" overkill?

And should I be gluing and nailing the additional plywood down, or is the glue not necessary? And just to confirm - I'm simply attaching the additional sheets of plywood to the existing, correct? Not attaching the 2nd sheet to the joists?

Thanks!

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Old 03-03-2016, 02:00 PM   #2
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What's the spans on those joist?
2 X 8's is the bare minimum for joist and 1/2 sub floors has not been used as a rule for about 20 years.
If it was mine I'd be using 3/4" T & G Advantec not plywood. It's stronger, smoother, no voids, 50 year warranty.
Glue should never be used under an underlayment, only on the joist for a subfloor.
Also make sure the seams do not line up with the seams below and never attach to the joist.
What's the finished floor going to be?

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Old 03-03-2016, 02:23 PM   #3
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I'll double check the span on them tonight. I ripped up one board to get a look at the rough plumbing that was upstairs to see what the options were for adding a bathroom. The room that we're going to be finishing on the left half of the attic will be a bedroom with carpet.

20 years makes sense...the house was built in 1986.
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Old 03-03-2016, 02:50 PM   #4
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May be opening up a whole can of worms making this into a finished space.
Once again we have no idea where on the planet you are so this is just general rules in most of the US.
To make it into a bedroom, it's going to need several permits, egress windows, code approved stair way, health dept., building dept, zoning approval.
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Old 03-03-2016, 03:04 PM   #5
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I'm in Connecticut. Windows are plenty big and meet egress requirements. The attic was rough plumbed for an additional living area upstairs. Stairway shouldn't be an issue as it's already existing...
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Old 03-03-2016, 05:06 PM   #6
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I am also a fan of the advantek product....but it is HEAVY. That said, I use it anyway as it is a great product. Ron
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Old 03-03-2016, 06:18 PM   #7
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Joe, there are a couple of ways an attic floor can be constructed. Look at where your rafters sit on the outside walls. Do they rest on top of the wall or on top of a perimeter rim joist. If they rest on top of the double top plate, the wall, then the plywood probably doesn't cover all of the insulation.

When you add kneewalls and create side attic spaces there are some steps needed that are much easier now than after all is enclosed. I'll attach a link on side attics for capes that will show some of the insulation options.
http://www.finehomebuilding.com/how-...kneewalls.aspx

Be sure to pull all permits required. Just because it was prepared for an expansion doesn't mean the work up there is already approved. AND future buyers are now checking to make sure all work was permitted and inspected. If not you could take a big hit making it right.

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Old 03-05-2016, 06:45 PM   #8
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Quote:
Originally Posted by joecaption View Post
What's the spans on those joist?
2 X 8's is the bare minimum for joist and 1/2 sub floors has not been used as a rule for about 20 years.
If it was mine I'd be using 3/4" T & G Advantec not plywood. It's stronger, smoother, no voids, 50 year warranty.
Glue should never be used under an underlayment, only on the joist for a subfloor.
Also make sure the seams do not line up with the seams below and never attach to the joist.
What's the finished floor going to be?
I think if it was my house i would just remove the 1/2" and put down the 3/4 T&G with glue and ring shank. This way i know its not going to be squeaking on me.

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