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Installing plywood OVER old plank subfloor that has left over cutback glue


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08-20-12, 06:55 AM   #1  
Installing plywood OVER old plank subfloor that has left over cutback glue

I am prepping an old plank subfloor from the 60's for a new layer of 3/8" plywood. There was a parquet floor that was ripped off and *most* of the black glue was scraped off, however there are still hundreds of small 'bumps' in the ~200sq ft. area.

What else do I need to do to the original plank subfloor so it's ready for the plywood? Can I just use 2.5" screws to fasten the plywood through the planks into the joists? or just screw into the planks? overall the room is level, it's just the small 'peaks and valleys' from the residual glue I'm worried about.

Ultimately I need to install the plywood floor for a 3mm cork underlayment for a 1/2" t&g engineered walnut floor. third floor level of a townhouse.

thanks everyone

 
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08-20-12, 09:39 AM   #2  
Welcome to the forums!

Are the small bumps big enough to keep the plywood from being snugged up to the planks?
It's always best to screw thru the planks and attach the plywood to the joists. There may be areas where you'll be forced to screw just to the planks. 15# roofing felt between the planks and the plywood should help to eliminate any squeaks if you think that might be an issue.


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08-20-12, 11:48 AM   #3  
Thanks marksr.

When I lay a straight edge over the floor, I can see small dips everywhere, most of them around 1/8", some maybe 3/8". What happens when I put the plywood on top and screw it down to where a dip is? Will the new plywood subfloor take shape to the bumpy plank subfloor?


I could try to place strips and pieces of roof felt ONLY where the dips are, but how practical is that?

thanks again.

 
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08-20-12, 12:02 PM   #4  
additionally, here is what I found online as far as screwing placement for the sheets of plywood:

Screws should be driven at every joist on 12 centers, and on 6 centers along the edges parallel to the joists. If you are counting, that results in the use of 43 screws per 4 x 8 ft sheet when installed with the long edge perpendicular to the joists, and 52 screws per 4 x 8 ft sheet when installed with the long edge parallel to the joists.

 
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08-20-12, 02:54 PM   #5  
3/8" plywood is not going to be enough subflooring over the planking. It will bend and shape itself to the aberrations. Why are you choosing such a thin overlayment? In addition you don't screw through the planks into the joists. You intentionally miss the joists as the two substrates must move independently of each other. It's a daunting task, but I would ensure the planking is secure to the joisting first and add no less than 1/2" or thicker plywood, 5/8" better. The screw pattern you are referring to is for basic subflooring on joists. I would not install subflooring parallel to the joisting.

 
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08-22-12, 06:00 AM   #6  
thanks chandler, a few followup questions:

1. 1/2" is fine - did you have a recommendation on what material to put on top of the black glue before i lay the plywood on top of that? I was told 15# roofing felt and Red rosin paper.

2. If I don't screw the plywood through the planks and into the joists, then what do I fasten the plywood to? Just attach the plywood to the planks *around* the joists? with 1" screws?

It seems to me that no matter what the joists, plank subfloor, and plywood subfloor are going to be connected and NOT be independent. You can't fasten the planks to the joists and then fasten the plywood to the planks and expect any of it to move freely, can you?

Thanks again I really appreciate the help.

 
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08-22-12, 06:07 PM   #7  
I would still try to scrape off the worst of the glue. Or sand it down.
You will fasten the planks with screws to the joists, then fasten the plywood to the planks not into the joists. I know it sounds goofy, but in reality the plywood will "slip", and you are allowing it to do so by not putting it firmly into the joists. Your planks become part of the joists, and the plywood becomes part of the planks. Now you are sure to be confused

 
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08-24-12, 05:43 AM   #8  
Thanks again chandler. If I need to cut the 4x8 sheet of plywood in half for transportation, how should I approach the screw pattern for each 4x4 piece?

 
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08-24-12, 05:13 PM   #9  
Offset the plywood by the width of a joist on each run and pretend it was a full sheet, but adding extra to the edges as you would a full sheet. It would look like a checkerboard.....well almost.

 
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08-25-12, 05:28 AM   #10  
If possible, I'd try to figure out how to transport full sheets. It will be less work and the give you a better floor. Friends, neighbors, surely you know someone that has a pickup truck or utility trailer


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08-25-12, 06:34 AM   #11  
Totally agree with Marksr on the full sheets if possible.

 
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08-27-12, 02:59 AM   #12  
Thanks for the great info. I have a similar wood-plank subfloor from the 60s as well. So, if I install 1/2" plywood (on top of the existing 5/8" plank boards) and then 1/2 cement board (with thinset) on top of the plywood, would that be enough for tiling?

 
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08-27-12, 05:21 AM   #13  
Welcome to the forums Mike!

The sub floor is only part of the equation. It also depends on your floor joist size and how far they span between supports.


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08-28-12, 06:45 AM   #14  
update!

first, I'm picking up plywood this evening, full sheets, using HD's truck rental for $20.

As for the floors, we have been continuing to prep them. scraping/removing glue, removing baseboards, etc. we also prepped the hallway since we will start there (see pic below).

I picked up an orbital sander last night and will be using it tonight. that coupled with the oscillating tool that i've been using should do the trick. I also have 30# roofing felt to use, luckily there aren't too many low spots greater than 1/8" in the hallway.

I am still unsure if I *need* 1/2" plywood as opposed to 3/8".

Here are a few progress pics:

http://jeffsimpsonphoto.com/public/p...s/P8260033.jpg

http://jeffsimpsonphoto.com/public/p...s/P8260034.jpg

http://jeffsimpsonphoto.com/public/p...s/P8260035.jpg

 
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08-28-12, 01:45 PM   #15  
Yaaay, pictures!! Lot of work, huh? If you use a random orbit sander, rule #2, don't bear down on it. Let it do the work. If it isn't working well, increase your paper grit (lower number). Go with 1/2" ply and live well.
Have you decided where your newel post will go on the staircase?? It will need to be fastened through your flooring, so make plans.

 
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01-03-13, 03:07 PM   #16  
1/8" space between 2nd Layer of Plywood over existing Plywood Sub Floor

Hello all,

I'm doing something similar in that I'm installing 1/2" plywood to an existing plywood subfloor in prep for bamboo flooring.

I'm using Subfloor liquid nails, screwing into both joists and existing plywood and am questioning the need for the 1/8" gap between the new plywood boards.

Right now I've put several sheets down butted tightly with screws spaced 6" oc and a slather of liquid nails.

Not sure if i should go back with a circular saw or not to get the 1/8" gap between the boards on the 2nd layer.

Thanks in advance

 
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01-03-13, 04:54 PM   #17  
Welcome to the forums! If you had read the thread before you started, you would have known not to place screws through both plywood layers into the joists. The bottom layer gets screwed to the joists, and the next layer is screwed to the plywood. Could have saved you a little there. You should have 1/8" gap between your plywood sheets, but if it was seasoned with the room (acclimated) for a period, you may dodge the bullet.

 
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01-03-13, 05:02 PM   #18  
Thanks Chandler.

Yes, I had the plywood in the space for over a week. Would that be a fairly decent period for the plywood to become acclimated?

What is the impact of screwing both sheets to the joists, in addition to the base plywood?

Should i go ahead and saw cut the butted seams to get that 1/8?

 
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01-03-13, 06:20 PM   #19  
hmmm?

In looking at the site and this link here:
How to Lay a Plywood Subfloor Part 2 | DoItYourself.com

It's fairly general and doesn't mention holding any sort of space between ply... actual says "cut to fit"


"A plywood subfloor is required when you feel the existing floor needs to be stiffened and when you want to add another layer of flooring over the existing flooring. The subfloor protects the existing floor while adding strength to the new flooring." .... "Cut the plywood to fit." ..... "Start fixing the panels to the joists using nails and screws"


Read more: How to Lay a Plywood Subfloor Part 2 | DoItYourself.com

 
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01-03-13, 06:30 PM   #20  
With two layers of subflooring, they have to be given the opportunity to "slip" against each other, although, granted, this slipping is minimal. Fixing both layers to the joists prevents any possible movement in the top layer. True, not everyone adheres to this, but best practices shows it to be a better way of doing it.

 
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