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randallS1 09-03-2008 11:51 AM

Subfloor Layers
Hi All,

I am a newbie and have already made a mistake by posting a question to a reply, which I apologize for.

I have had some good advice on how to add a layer of 3/4" BC plywood to a 3/4" OSB subfloor.

I was going to glue and screw it per the suggestions I received. But, when I went to order the glue for 1000 sq. ft. the Titebond tech people told me that if I glued the two sheets together it would be a problem. They said that the Titebond II and III adhesives "are brittle adhesives" and when I walk on the floor after the glue has cured, I will "hear crackling sounds" from the glue film fracturing between the two subfloor sheets or panels, because that glu eis not flexible. They want me to use Titebond 811, which will cost $900 more than using white glue or Titebond II for a 1000 sq. ft. space. So, now I am a little concerned about gluing the two layers together and not sure how to proceed. They also said that a water base adhesive will cause the OSB already in place, to swell and create other problems. The Titebond 811 is petroleum based and will not be a problem except for fracturing my finances.

So now I am thinking about just screwing the 3/4" underlayment to the OSB with #12 or #14 1-1/4 or 1-1/2" screws and hoping it won't end up squeaking all the time. Over the top of the 3/4 ply will be 3/4" x 4" solid White Oak plank flooring, nailed down. Floor joists are solid 2 x 10's, 16" OC.

Any suggestions will again be greatly appreciated.

DangerMouse 09-03-2008 01:14 PM

i used only screws to do the same thing and never heard a squeak. just be sure to use enough screws and sit on top of what you're screwing to hold it all the way down. total cleaning first is a must too. vacuum any corners, etc.

*good luck!*


randallS1 09-03-2008 01:50 PM

Subfloor Layers
Hi DM,

Thanks for the reply. :)Much appreciated and that is very good news. What size screws did you use and what spacing or schedule in the field?

DangerMouse 09-03-2008 02:20 PM

one and a half inch screws spaced at about 12" squares over the whole board. use clutch on cordless drill to set them to right level. (if you have one, if not, go get one!) i have 2 cordless and one good old Hilti corded and i use them pretty much daily here.

hope this helps!


randallS1 09-03-2008 02:26 PM

Subfloor Layers
Hi DM,

Thank you again. I have two cordless drills and a Senco with a clutch that I just bought. So, here we go. :)

DangerMouse 09-03-2008 02:32 PM

"and they're off and running folks!" should be nice with the white oak floor..... post some pics when you're done? remember, i'm not advocating doing it this way. others here may pop in and tell you to use glue, but i didn't even consider it when i did mine, i just assumed the screws would be sufficient, and they were.


Termite 09-03-2008 04:23 PM

I don't think that glue is necessary. If the plywood is properly screwed down, you shouldn't ever hear a squeak. Nails would squeak.

JazMan 09-03-2008 04:43 PM

Not only is the glue not necessary, but it makes things worse. Aside from the crackling sound, the new floor would actually have more up and down movement than before. Huh you ask? Yes, it will if you use a glue that comes in a tube at least. The beads of adhesive creates voids next to the beads, hollow spots.

They only good way to glue the two sheets together is to laminate them. That requires the right adhesive, 100% coverage, and extra help to fasten the sheets before the wood glue dries too much.

I am a tile guy and we never recommend gluing the underlayment to the subfloor. Just use a good quality 1 1/2" screw, fastening every 8" in the field and 6" on edges.

On another note, why do you need the second sheet of 3/4" ply in the first place? Wouldn't the proper fasteners work well over OSB?


mike costello 09-03-2008 08:44 PM

Ill add that you can put a layer of rosin paper between the two sheets and screw as already advised.

Not many OSB sheets will accept any type of adhesive anyway

randallS1 09-03-2008 10:41 PM

Subfloor Layers
There are several reasons for the 3/4" plywood underlayment. This building is on a pier and post foundation so the 3' crawl space is fully open and the Kraft faced inulation is up against the bottom of the subfloor, or very close to it. There is also a vapor barrier on the ground but we do not want to penetrate the Kraft facing on the insulation in the floor joists. The 2" nails for the 3/4" hardwood flooring will penetrate the bottom of the OSB about 3/8" if we install the floorin gover the OSB and the plywood will prevent that. Also, part of the floor will be tiled and we wanted to have at least a 1-1/4" floor for the tile.

randallS1 09-03-2008 11:09 PM

It's tough to be a newbie. Somehow I lost about half of my last post. Not sure what I did wrong.

Anyway I could sure use some help on the screw diameter. I have been looking at #12 and #14 in the recommended 1-1/2" size. #14's are pretty big but they have nice deep thread. Also discovered that if the screw is threaded all the way up it holds the sheets slightly apart unless I drill a pilot hole in the plywood larger than the one in the OSB. So, I am also looking for a screw that has a smooth shoulder where the shoulder has a smaller diameter than the threaded part. That way it will pull the sheets together and I will not have to drill two different sized pilot holes. Has anyone used screws like this? What size diameter have you guys found to be the best diameter?

If I had it to do over, I would use ACX Exterior plywood for the subfloor instead of the PS2 OSB. Seems like it would be less of a problem.

This weekend I will attack the underlayment installation with my screw gun and a 12 pack for nerves. :wink:

DangerMouse 09-04-2008 06:24 AM

"Also discovered that if the screw is threaded all the way up it holds the sheets slightly apart unless I drill a pilot hole in the plywood larger than the one in the OSB. "---> wow, how much do you weigh? if you sit on the thing your weight should hold it tight. do you have a "heavy" friend to assist you? -=chuckle=- but go ahead and go find the 'shouldered' screws, you shouldn't have to predrill a thousand holes, for sure.


Termite 09-04-2008 07:40 AM

Use screws like these.
They're specifically for subfloor. You might not find them loose at home centers, but fastener suppliers like Fastenal will certainly have them. They're also available online.|1426474261

#14 is waaaaaay too big. Go with a #8.

Termite 09-04-2008 07:42 AM

Don't doubt your selection of OSB too hard. It is a good product, and won't perform any different than plywood in this application.

randallS1 09-04-2008 11:43 AM


My most sincere appreciation to everyone for sharing your knowledge and experience. You are all gentlemen, scholars, and judges of fine women. Your advice has made all the difference in our confidence in being able to install this 3/4" BC exterior plywood underlayment, and in using the correct technique and hardware.

From what we are seeing, the screws that Thekctermite suggested work the best in our tests, using scrap cutoffs. I thought a #8 1-1/2" screw might be too small but it looks like the way to go. We tried four ways to screw the sheets together.

We tried standing on the plywood and using screws threaded all the way up and the weight did help and did make a difference, but this method did not pull the sheets together as tightly as the following, although it was clearly the fastest method and there was very good contact between the two sheets.

We also tired standing on the plywood and pilot drilling the two sheets with a 1/16" drill bit, countersinking the top plywood sheet in a separate operation, and used the partially threaded shoulder screws in the photo from the post by "Thekctermite" . That worked even better.

For the third method, we stood on the plywood, pilot drilled both sheets 1/16", pilot drilled just the top layer of plywood 3/32", which was the smooth shank diameter of the screw from Thekctermite's post, and screwed the panels together. This gave us the tightest joint but was very time consuming. Maybe this is overkill.

The fourth method may not be very realistic. We used the same method as number three but first brushed on a very thin, barely visible, film of Elmer's white glue in a very small area on the rough side of the OSB panel, maybe 10% of the total area, same as if it were the subfloor. I thought it would be a token effort since the stuff was so thinly spread. After 45 minutes, into the lunch break, we removed the screws and we could not pry the boards apart. Unbelievable. No swelling was seen in the OSB, maybe because we used so little and the OSB is PS2 Exposure 1 rated, and we stayed away from the edges.

This last methoid is really overkill and very time consuming and not very practical but it really gave us the tightest joint of all. Maybe if this were your own home and you have the time...........

So, it looks like what we are going to do is use the screws recommended by Thekctermite, stand or sit on the sheet for help from the weight, pilot drill and countersink both sheets in one operation, then put the screw in for the second operation. The pilot drilling really seems to help the screw bite into the OSB better than if we do not pilot drill. We found that using a pilot drill close to 1/2 the diameter of the smooth shank on the screw gave us the best compromise for time and a good tight joint. The pilot drilling must help the screw threads bite into the OSB a little better and also helps the screw head pull the sheets together. this job will be done this weekend. H:)appy day.

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