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Old 07-10-2008, 06:20 PM   #1
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Glue underlayment to subflooring?


I am in the process of replacing old, improperly installed 3/4" plywood subflooring (SF) with new 3/4" T&G Exposure 1. Since the old floor was terribly squeaky & had mucho "soft spots" (there was no underlayment), I will also be adding a 3/4" plywood underlayment (UL) prior to carpet. I have added blocking between the joists around the perimeter of the rooms to support the new SF & am gluing / screwing the new subfloor down now...

Floor joists are 2X8 with a 10' span, 16" O.C...solid & in good shape.

My understanding is that I should not use any adhesive to bond the UL to the SF...simply avoid the joists & screw the UL to the SF. Is this correct & why would you not want to bond the UL to the SF? Is it a "movement" thing?

Also, I believe I am to leave a 1/16" gap between all the underlayment panels as I install them...for expansion. Sound right?

Thanks in advance...


Last edited by scheenstra; 07-10-2008 at 06:28 PM.
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Old 07-10-2008, 09:20 PM   #2
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Glue underlayment to subflooring?


Quote:
My understanding is that I should not use any adhesive to bond the UL to the SF...simply avoid the joists & screw the UL to the SF. Is this correct & why would you not want to bond the UL to the SF? Is it a "movement" thing?
It's because with a lot of floorings, like sheet vinyl, you glue the flooring right down to the underlayment.

In the future, if you have trouble getting that sheet flooring off the underlayment, you want the option of removing that old flooring by removing and replacing the underlayment.

If you glue the underlayment down to the subfloor, you're stuck cleaning that flooring off the underlayment no matter how tough it is, or easier it would be to just replace the underlayment.

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Old 07-10-2008, 11:37 PM   #3
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Glue underlayment to subflooring?


Actually.....the reason not to glue the underlayment to the subfloor is because most people will use construction adhesive from a tube which because of its thickness, creates many voids just next to the beads of glue. If you could use a thin wood glue and spread it 100%, and work fast enough, gluing would work well.

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Old 07-11-2008, 01:06 AM   #4
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Glue underlayment to subflooring?


Actually, in this case it's not necessary to glue your subfloor down to your floor joists. The reason for gluing and screwing the subfloor down in new construction is because the floor joists are often not as dry as they should be. Wood shrinks as it dries, and if the floor joist was wet when it was installed, a gap will develop between the joist and the subfloor as the joists dry. It's the nail and subfloor vibrating as they rub against one another that causes floor squeeks.

By gluing the subfloor down to the joists in new construction, you prevent any gap from opening up between the two, thereby preventing floor squeeks.

If you're replacing some subfloor in an older house, then the floor joists have already dried, and it's really not necessary to glue the subfloor down to the joists to prevent floor squeeks. Your floor joists aren't going to shrink any more.

Exactly the same thing happening to wooden wall studs results in drywall nail pops when someone presses against a wall.

You don't need to glue underlayment down to a plywood subfloor because the plies of plywood have to be dry before they can be successfully glued together. As a result, plywood typically leaves the factory with a moisture content of only about 4 percent, compared to 19 percent for "kiln dried" lumber. Thus, plywood is more likely to expand after it's installed as it's moisture content goes up, rather than than shrink. It's that potential for expansion that's the reason why manufacturer will recommend leaving a gap between sheets of plywood subfloor.

If you do glue your subfloor down to your joists, and glue your underlayment down to your subfloor, then nail down something on top of the underlayment that you can glue your flooring down to. That way, you always have the option of removing that flooring cleanly by removing what it's stuck to.
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Old 07-11-2008, 09:07 AM   #5
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Glue underlayment to subflooring?


Took the words right out of your mouth Jaz.
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Old 07-11-2008, 12:45 PM   #6
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Glue underlayment to subflooring?


Thanks, All...I'm on it...

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