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Attaching the second sheet of plywood to the subfloor...

Attaching the second sheet of plywood to the subfloor...

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  #1  
Old 03-15-11, 03:25 PM
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Attaching the second sheet of plywood to the subfloor...

I read somewhere that when adding a second sheet of plywood to the subfloor, you shouldnt screw/nail to the joists, can someone tell me why?

Also, I am installing two 3/4 sheets, what size screw should I be using for the top sheet of plywood? Thank you all for commenting.
 
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  #2  
Old 03-15-11, 04:52 PM
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That is correct. Theory is that the first sheet is adhered to the joisting. The second sheet will allow for ever so little movement between the sheets and not transfer total movement from the joisting through to your final flooring. 1 1/2" - 2" screws will work fine. Don't use sheetrock screws, use decking screws.
 
  #3  
Old 03-15-11, 05:26 PM
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Should I be gluing and screwing the second sheet down to the first one?
 
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Old 03-15-11, 06:43 PM
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Screwing only to allow for that movement.
 
  #5  
Old 03-16-11, 05:43 AM
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This is interesting. How long has this method been in practice? Does it actually work?
I would have thought that adding 1-1/2" of subfloor to existing floor joists by screwing the top sheet to the joists would have made a stronger floor.
 
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Old 03-16-11, 04:10 PM
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Don't know. You have wood attached to each other at different planes. There will be an inevitable movement as wood just does it. This movement doesn't compete with the San Andreas fault , but there is movement. To reduce this transfer of movement you separate the layers and allow that slight movement to happen.
 
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Old 03-16-11, 04:22 PM
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OK, I respect the theory but wouldn't you think that it would loosen up up time?
 
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Old 03-16-11, 04:41 PM
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Why would they loosen? They are screwed together every 12" or so. If you mean 100 years..well maybe....
 
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Old 03-16-11, 04:59 PM
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We are fastening a piece of plywood to another piece of plywood so that it can absorb vibration correct, I just think that if its constructed to absorb vibration, at some point the areas that vibrate the most will probably loosen up. One year, five years or twenty years down the road it will need maintenance. Ever own a solid mount Harley?
 
  #10  
Old 03-16-11, 05:03 PM
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Jeez...it doesn't vibrate..it moves...very slowly, due to temp and humidity changes.

Do what you wish..but correct is to solidly fasten the first layer to the framing and fasten the second to the first.
 
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Old 03-16-11, 06:05 PM
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Just a side point, but another reason you don't glue and screw additional layers is because someday someone is going to need to tear that floor off, and they will surely want to reuse the original layer of subfloor.

If it's glued on, and someone wants to remove that additional layer of plywood for whatever reason (let's say they want wood floors and don't want them to be another 3/4" higher) they now need to demolish the entire subfloor down to the joists and start over, turning a $1000 demo job into a $10000 one.

The original layer of subfloor is permanent. Anything else on top of that is just temporary. You want to make it permanent by gluing it, go ahead. But everything you read and those who have commented here seem to be in agreement.
 
  #12  
Old 03-16-11, 08:25 PM
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OK, I'm sold, I will be screwing the second sheet without the glue.
How many screws per sheet should I be using for the top sheet, I plan on using 1-1/2" #8 screws?

Thank you all very much for your comments
 
  #13  
Old 03-17-11, 05:26 AM
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I'm with XSleeper on why not to glue. I recommend ring nails instead of screws. They are faster and cheaper and have more than enough holding power.

I am going to ask the same question I always ask whenever thermal/humidity expansion of flooring is discussed.

How much relative movement is possible if the materials are properly secured? Screw down a sheet of plywood 12" in the field and 9" around the perimeter I just don't see where the movement comes from. It seems to me that with that number of fasteners the two pieces are pretty much integral.

I copied this from a plywood site -

Thermal expansion may be of importance only in assemblies with other materials where the moisture content is maintained at a relatively constant level. Plywood and wood expand upon heating as do practically all known solids. The thermal expansion of wood, however, is quite small and requires exacting techniques for its measurement.

The effect of temperature on plywood dimensions is related to the percentage of panel thickness in plies having grain perpendicular to the direction of expansion or contraction. The average coefficient of linear thermal expansion is about 3.4 x 10-6 inch/inch per degree F for a plywood panel with 60 percent of the plies or less running perpendicular to the face. The coefficient of thermal expansion for panel thickness is approximately 16 x 10 minus 6 inch/inch per degree F.

If my math is correct that's about 0.005" of expansion for a 48" width of plywood seeing a 30* delta T.
 
  #14  
Old 03-17-11, 05:35 AM
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I'd never use ring shank nails. There going to rust. Have no pulling power to pull the two sheet together. Will come loose over the years.
Use cermaic coated decking screws and use an impact screw driver to drive them in.
Ryobi makes a cheap impact that cost 1/2 as much as the other brands and works just fine.
A cordless drill is not going to cut it.
 
  #15  
Old 03-17-11, 06:59 AM
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Guys, what are we saying here? Are we saying there is a problem attaching the plywood with #8 zinc plated screws? Whats going to happen to the screws? Whats going to happen to the plywood?
 
  #16  
Old 03-17-11, 08:54 AM
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This is an interior application. Ring shank nails are not going to rust any more than any other framing nail. Neither are they going to loosen. IIRC Code requirements for securing 3/4" plywood subfloor to joists are 6d - 8d deformed shank. According to my local inspector ring nails are considered deformed shank.

It's been a few years and the code may have changed to allow screws in this application but IMO they are really unecessary.
Plus driving nails is a lot more fun and if you're in a hurry you can break out the framing nailer.
 
  #17  
Old 03-17-11, 04:19 PM
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Originally Posted by Wayne Mitchell View Post

How much relative movement is possible if the materials are properly secured? Screw down a sheet of plywood 12" in the field and 9" around the perimeter I just don't see where the movement comes from. It seems to me that with that number of fasteners the two pieces are pretty much integral.
OK, I now understand and agree that the second sheet shouldnt be glued because its not the actual subfloor but

please explain
Screw down a sheet of plywood 12" in the field and 9" around the perimeter
. Are you saying that it doesn't make a difference if you nail/screw to the joists below?
 
  #18  
Old 03-17-11, 07:38 PM
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Actually I think the nailing schedule is 6" around the perimeter and
12" in the field.

I don't know of any screwing schedule and I'll refrain from any further comment on that.
 
  #19  
Old 03-18-11, 07:56 PM
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4d (1-1/2") x 12.5gauge shank diameter ring or screw, 6/8" for 3/4" thick, offset 2-6" from joists, etc.....http://www.apa-europe.org/Languages/.../PDF/R340G.pdf

Gary
 
  #20  
Old 03-19-11, 05:19 AM
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A fine guide for fastening plywood Gary. Unfortunately, it's for Europe. I don't think the same fastener schedule applies here.
 
  #21  
Old 03-19-11, 06:45 AM
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Wayne, did you look at the same .pdf that I did? APA prints the labels on the back of all our plywood, and are based in Tacoma, Washington!

Even if Europe is in the URL, I compared mine with this one and they match exactly. (I have this same exact .pdf in my APA construction folder.)
 
  #22  
Old 03-19-11, 06:47 AM
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That's from the APA. They are headquartered in Tacoma, Washington.
 
  #23  
Old 03-19-11, 10:12 AM
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OOPS - I didn't even go to the site I just saw Europe in the URL and jumped.
 
  #24  
Old 03-19-11, 11:26 AM
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ziziano

What kind of finished flooring will you be installing over the plywood, or did I miss that?
 
  #25  
Old 03-20-11, 06:04 PM
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For those of you not familiar with APA (American Plywood Association):
“APA is a nonprofit trade association that has grown and evolved with the engineered wood industry. APA was founded in 1933 as the Douglas Fir Plywood Association, and was later recognized as the American Plywood Association. In 1994, APA changed its name to APA – The Engineered Wood Association to better reflect the range of products manufactured by APA members and the international scope of the Association.”

International offices in Boumemouth, United Kingdom; Mexico City, Mexico; Tokyo, Japan.

Gary
 
  #26  
Old 03-22-11, 02:31 AM
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Originally Posted by HeresJohnny View Post
ziziano

What kind of finished flooring will you be installing over the plywood, or did I miss that?
Several years ago, I went through the same sub-floor and hardwood finish flooring dance except I didn't have the luxury of this great site to connect with everyone for ideas and advise. I had someone fix the sub-floor and install unfinished flooring, then sand, moisture cure, sand again and moisture cure again. I realized that there were too many variables (things to go wrong) with unfinished flooring and the finishing process so this time around I plan on installing 3 1/4" pre-finished oak that I found reasonably priced at HD for better consistency.
 
  #27  
Old 03-22-11, 06:35 AM
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ziziano

I'm a tile guy. The installation of the second layer of plywood that you describe is the way that you would install plywood underlayment for ceramic tile. That is screwing the second layer of plywood to the first layer of plywood only and avoiding the floor joists. This idea is an uncoulpling effect between the second layer of plywood and the movement of the floor joists.

I don't know that this would be the proper way to install a second layer of plywood for a wood floor. I dont know this for sure cause I dont install wood floors. I think the uncoupling effect would be a lot less important with a wood floor. You may want to ask this question in the wood floor forum.
 
  #28  
Old 03-22-11, 04:01 PM
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I just prepped a 5/8" subfloor for the guys who were going to be installing a 3/4" hickory floor, and they wanted an additional 3/8 ply stapled down. Using 3/4" ply as underlayment for 3/4" solid wood flooring when the floor is ALREADY 3/4" is overkill with a capital O.
 
  #29  
Old 03-24-11, 03:15 AM
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Wood is not absolute and not as straight as metal right, right.

How much is it acceptable to be off? Is there a perfectly level floor?
 
  #30  
Old 05-02-14, 02:15 PM
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Gluing second sheet is fine

If you aren't going to glue down the 2nd sheet because you might have to remove it later on, think real hard about that. If you need to remove that second sheet, you're probably going to remove the first sheet as well. You aren't making anything easier.

Say you have a hardwood floor over the subfloor of 2 sheet of plywood. After you've hacked way at the hardwood we're going to worry about the 2nd player of plywood? Not me. If there is a crawlspace, get under it, drill holes and cut that subfloor off. Believe me, you aren't going to want to keep it. Think about the reasons you are taking off the hardwood floor, bet you have some real issues if you also need to remove the 2nd layer of plywood. You are going to end up going down to the joists anyway, betcha.

On to movement. If you use T$G and screw it down, sorry, it isn't moving every so slightly that gluing the 2nd sheet down is going to matter. It doesn't work that way. If the 2nd sheet is moving through contraction/expansion so much you can't glue it down, something is wrong and you'll end up with a loose 2nd sheet. Think about it. If movement was needed, why glue down the first sheet? You are putting that T&G with no seam gap between them. just what is moving independently in all this? Nothing. The hardwood floors are nailed to that 2nd sheet right? The hardwood floor, the 2nd sheet and 1st sheet are not moving all over the place independently. If they are, you have other problems.

Still dubious about gluing it? How do you think the plywood is made? It is glued. All of a sudden plywood that is glued together to form the plywood is screwed down on other plywood because the glue is going to allow for movement? Not a chance.

Of the components of the floor, what costs the most? The finish floor. If you have to take up that finish floor does much else really matter? You cut off the subfloor if it is damaged and put a new one on.

More? Okay, the 2nd layer comes off. Now what? Use the same screw holes for put a new 2nd layer on? Try to offset them so you don't? Not a chance. Get that whole subfloor off if you are going through that much work or the job is a half done job.

Glue that 2nd sheet down. If you don't and something goes awry, you'll wish you had. Ask yourself if the plys on a single sheet are moving around.
 
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