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Old 08-19-2012, 11:31 AM   #1
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Plywood underlayment for 2nd flr of home?


Hello everyone

I'm the proud new owner of a cape, 2br /1.5 bath / 1600 sq ft. The home is a modular home built in 2001, and what is unique about this house is that all 2br /1.5 bath / 1600 sq ft is on the first floor of the home. There is an entire ~1600 sq ft. 2nd level of this house that is unfinished..... It has a full dormer on the rear of the house, will get approx 7'5" ceilings, already has a permanant stairway built up to the 2nd flr, and already has plumbing and heating piping roughed in from the basement up into the floor cavities of the 2nd flr. The former owner used this space as an attic, however it was of course designed to be finished up here at some point in the future. I intend to build additional living space up there.

THUS.. my joining this awesome forum and posting this first of many questions here!

so, first step in this big project:

There is lots of discussions about specific types of underlayments for a particular application, tile, vinyl, hardwood, etc. But I can't find any posts for a "general purpose" plywood underlayment... My 2nd flr will have variety of finished floor materials by the time i'm done (pre-finished 3/4" hardwood in hallway, carpets in bedrooms, tile in bathrooms)

This unfinshed 2nd flr currently has a single layer of 1/2" OSB as a subfloor laid over 2x10 ceiling joists spaced 16"oc. This 1/2" OSB does not appear to be glued to the joists. It was attached using staples, I am unsure of the length of the staples. It was laid down staggering the joints, edges appear to hit the joists. It feels like blocking was NOT installed at the seams running perpendicular to the joists. And this 1/2" OSB is not T&G.

I intend to lay a 2nd layer of plywood "underlayment" across the entire 2nd flr to kick off this project. I plan to finish 1/2 of this space within the next year, and leave the other 1/2 as attic space.

Questions I have / looking to confirm are:

1. How can I prevent against squeaking without removing, gluing, and reinstalling this sub-floor? (that would be a huge pain)
2. Is 1/2" thick plywood acceptable for my new underlayment, giving me a total of 1" ?
3. Should I go with 3/4" thick plywood just for the heck of it since it might not cost too much extra? Or is this crazy overkill?
4. What is the proper grade to use in this type of application? BC? CD? CDX? Looking to of course choose the cheapest, since i'll be covering it up anyway. (I will not be using any vinyl finished floor.)
5. I plan to use nails to fasten the underlayment (i have a pneumatic framing nailer) . Do I need to hit the joists with long nails, like 2" or so? Or can I just hit all over all parts of the new sheet using 1" nails?
6. I will stagger the joints so that they don't overlay the existing joints.
7. Based on what I've read, I do not need to glue the underlayment to the subfloor.

thanks in advance for any advice that anybody has!

Mike

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Old 08-19-2012, 11:59 AM   #2
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Plywood underlayment for 2nd flr of home?


First off, check the height of the top step to the osb and compare that to the rest of the steps. This needs to be within 1/4" to pass code. Possibly they figured in a sheathing thickness when the steps were built. Was the osb laid in after the walls were built? If it was, it can be easily removed and proper blocking and 3/4" plywood sheathing can be applied. If not, you can go over it and use screws to secure your top layer. If you end up having to rebuild the stairs, that is probably left to a professional.

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Old 08-19-2012, 12:01 PM   #3
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Plywood underlayment for 2nd flr of home?


If it was mine to do I would take the time to get rid of all that subflooring that's in there now.
It's about useless and will just act as a shim on top of the floor joist making the subflooring higher then it would be if it was done right.
It may also be a source for future squecks.
I use nothng but Advantec T&G 3/4" for my subfloors, not plywood.
Reson being I find less flaws, it holds nails and screws great, it's perfectly flat, hold up far better if water ever gets on it.
If that 1/2" was gone you could use the constrution adhesive on the joists making the floor about 25% stronger and far less likly to squeck.
Then I go over that with 1/2 subfloor rated reguler plywood with no glue, making sure the seams do not line up with the seams below, and do not attach to the floor joist.
I like to use a narrow crown pneumatic staple gun with 1-1/4 staples.
Staples will sit just below the surface and will not leave dents like a nail will.
It needs to be fastened every 4" on the edges and every 6 to 8" in the field.
With the gun set in touch and go I can do a while sheet in less then 2 min.
Done this way any flooring you choose can go right over it.

Have they already built the knee walls?
Has the HVAC been run yet? See any return air vents up there?
It's always best to post a picture so we all can see what your seeing.
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Old 08-21-2012, 12:30 PM   #4
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Plywood underlayment for 2nd flr of home?


Quote:
Originally Posted by sixeightten View Post
First off, check the height of the top step to the osb and compare that to the rest of the steps. This needs to be within 1/4" to pass code. Possibly they figured in a sheathing thickness when the steps were built. Was the osb laid in after the walls were built? If it was, it can be easily removed and proper blocking and 3/4" plywood sheathing can be applied. If not, you can go over it and use screws to secure your top layer. If you end up having to rebuild the stairs, that is probably left to a professional.
So, there are no walls up there now. It would seem that they accounted for a 2nd layer of 1/2" plywood, because the top step height matches the rest of the steps.

Quote:
Originally Posted by joecaption View Post
Have they already built the knee walls?
Has the HVAC been run yet? See any return air vents up there?
It's always best to post a picture so we all can see what your seeing.
knee walls were already framed in the front of the house. The rear of the house is dormered so no knee walls on that side.

No HVAC ducts - I have forced hot water. hot water supply and return lines were run up a wall and capped beneath the sub floor of the 2nd level.

I am really hoping to not have to rip up the existing 1/2... If you were to leave it as is, would it be any benefit to glue the 2nd layer of 1/2" to the existing layer of 1/2"?

I will post up some pictures tonight.
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Old 08-21-2012, 12:43 PM   #5
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Plywood underlayment for 2nd flr of home?


Glues not going to do any good.
I've had to finish out several modulers on the second floor, the customer that cheaped out and want to just go over the OSB that was alread down tryed to sue me because of tile cracks, grout failing, and squecks. Good thing I had it all in writing and took pictures.
Do a couple of things before doing anything. Pull a string on the top and bottom plates to see how straight that knee wall is. Most of the time they just through it up there and do not check anything. If the wall is wavy it will stick out at the corner like crazy.
Second pull a tape measure on the cross ties from the outside walls.
Make sure there set so you can you can run full sheets of drywall. I've seen them run them on the wrong side or switch from side to side as they were installing them leaving no way to run full sheets.
Most likly they also never install any nailers for the sheetrock in the corners.
With some preplanning you can use some of the area behind those knee walls for storage.
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