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3/4" ply over concrete slab-on-grade


knothandy's Avatar
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04-06-09, 11:42 AM   #1  
3/4" ply over concrete slab-on-grade

If I am using Bostik's Best to secure 3/4" PT plywood to my clean, dry slab-on-grade concrete floor do I really need to also nail it down at the edges? It's going under stapled 5-ply engr hardwood. I'd prefer not but if I do need to nail it can I use "normal" concrete nails like that used for carpet tack strips only longer?

 
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04-15-09, 06:56 AM   #2  
3/4" ply over concrete slab-on-grade

OK - since it seemed like I needed to nail AND glue I decided to only nail the subfloor and will use an RS22 powder actuated gun to set nail the 3/4" thick plywood. I will cut the 4'x8' plywood panels into 2'x8' "planks" first to A) make them lighter (easier to handle) and B) reduce chance of the finish wood falling on a seem.

QUESTION: Is it still wise to kerf the backside of the plywood sheets? When I was planning to glue the subfloor Bostik recomended kerfing them 3/8" deep every 12" or so to avoid binding.

 
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04-15-09, 08:56 AM   #3  
I have never heard of placing plywood under the floor.

 
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04-15-09, 09:21 AM   #4  
If you are finishing a concrete floor in a basement, this link is worth the read.
BSI-003: Concrete Floor Problems —

Bud

 
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04-15-09, 04:10 PM   #5  
Why not use Bostiks Best to glue directly to the slab?

I would never ever ever try to nail or glue PT plywood directly to slab. PT plywood will shrink considerably when it dries out.

I think you need to find new flooring since you didn't research this current floor beforehand.

In order to use what you got for flooring, you would have to frame out the floor using 2x4's on there sides, then lay the plywood on top of those.

 
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04-15-09, 04:24 PM   #6  
In the house I am currently working in, they are laying 3/4 CDX t&g over the cement kitchen floor. It is getting glued down with PL400 construction adhesive and they are using tapcons to screw it to the cement on what looked like 12" centers. I think they will be installing wood parquet over it. Not sure if the cement was sealed with anything prior to the above.

This is not in a basement... the houses in that area have 1st story cement floors on top of cement joists and basements below.

 
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04-16-09, 08:54 AM   #7  
3/4" ply over concrete slab-on-grade

Thanks for the feedback!

It's a 35 year old 1st floor slab. It’s had Parquet hardwood for 10 years with no problems. The slab is very dry - the Polyfilm Test shows no moisture.

SUBFLOOR METHODS “RSEARCHED”
Floating – Mainly DRIcore. Eliminated because Armstrong said it would void the warranty.

Glue down – Plywood system over Bostik's Best and sleeper system in mastic. Sleepers eliminated due to added height. Glue down was "eliminated" in general because nailing it down seems sufficient (see original post).

Nail down – Plywood over vapor barriers vs. plywood over Delta FL (and Delta look-a-likes).

* Armstrong (I’m going with Bruce stuff) is not prescriptive on the issue of subfloor - their main guidance is the usual "Dry, Flat, Structurally Sound" etc. phrasing which covers a multitude of sins.

* NOFMA recommends plywood-on-slab or sleeper sub-flooring systems for concrete. Either is for 3/4" hardwood flooring up to 4" wide. They state to "Fasten plywood to the slab with power-actuated fasteners [no glue!], securing the center of the panel first, then the edges, using nine or more fasteners."

PLYWOOD VARIATIONS RESEARCHED
Different sized sheets (16" x 8' out to 4' x 8'), thicknesses, and PT- vs. non-PT.

VAPOR BARRIERS RESEARCHED
Bostik MVP, 6 mil, felt, the "built in" features of systems like DRIcore and Delta, and all combinations of the above.

* Armstrong doesn’t recommend any VB / VP unless "excessive moisture is present or anticipated" in which case they recommend their “VAP arrest” (0.27 perm), or vinyl sheeting (read 6 mil) is probably a max of about 0.4 perm.

* NOFMA again recommends 2 choices: two layers of #15 over cold, cut-back mastic or 4 - 6 mil poly film laid over the slab. “When moisture conditions are more severe” they recommend to apply the poly over cut-back mastic.

“END SOLUTION” (open to minor changes at this point!) – Ultimately I've decided on the following layers, looking at it from top to bottom layer:
- 3/8", 5-ply engineered hardwood stapled down using R-wire (3/16" crown), 1" Leg, 19 gauge galvanized Sencote coated fasteners. Armstrong recommends the precise flooring gun and fasteners to use.
- Top layer of #15 as a slip sheet.
- 3/4", 4’ X 8’ non-PT plywood cut down to 2’ X 8’ planks power fastened (RS22 gun) into the concrete, using 14 nails / plank, spaced 3/8" between each. I decided I decided to cut the bigger sheets down to 2 X 8 to make them easier to handle. This is NOFMA’s recommendation and they are pretty clear.
- Another layer of #15 felt between the plywood and the slab. Armstrong doesn’t recommend anything and NOFMA’s is much more bullet-proof. I think that the felt’s 5 perm is sufficient.

Anyway, I hope that was all helpful to somebody! Everything is being delivered in a week. I'll try to post pictures etc. if folks are interested.

XSleeper - Thanks. Very useful post, as always. One comment: I would have assumed they MUST be putting some kind of vapor barrier / retarder down first but it sounds similar.

Bud - Very useful as well. Love the building science stuff. It is very no-nonsense. Thanks for the link. Their main concern that was relevant in my case seemed to be debonding of vinyl adhesives.

Airman - I don’t understand your comment. I'm not looking to be flamed but I thought there was always ply under finish wood unless it's floated / glued to concrete or nailed to something like old planks or old solid flooring, right?!

KH

 
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05-26-09, 03:44 PM   #8  
UPDATE - job done

Layed 750 sq. ft. of eng. hardwood after installing a 3/4" ply subfloor over the slab-on-grade. Looks pretty cool! Wife and kids return from vacation in 10 days whihc gives me time to finish the trim and maybe do a closet.

Used 1-1/2" ramset nails with .22 gauge yellow charges but every nail had to be set with a sledge hammer. One blow did it without cracking the cement.

Used 1" staples to attach the 3/8" hardwood.

Happy that it is over )

 
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05-26-09, 03:56 PM   #9  
Well, you gonna post pictures, or what?

 
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07-29-09, 10:00 PM   #10  
Results!

Took me forever to figure out how to get some pix loaded.

Here is one of the subfloor finished : 3/4" plywood over a layer of 15# felt attached to the slab using a .22 Hilti shooting yellow charges and 1-1/2 nails. Spacing was about 16" from all nails. Had to re-set all of the nails with a 30# sledge but one or two well placed taps put them in flush. Cut 4'X8' sheets into 2'X8" before starting which made them easier to handle and allowed me to stagger the ends simulating a diaganal pattern. The last few I just man-handled the full 4'X8' pieces into place because I was getting tired of the cutting and managing. I curfed the backs of these though to protect against bowing...
http://img11.imageshack.us/img11/418...donetvroom.jpg


Below is a picture of the finish flooring. 3/8" engineered oak (Bruce) with staples about every 3" to 5" and another layer of felt under it. New baseboards all around but no 1/4 round...
http://img38.imageshack.us/img38/5262/playroomm.jpg


Comments / questions welcome. Finished 2k sq. ft.

 
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07-30-09, 02:29 PM   #11  
That looks good, I like the look of the wood floors. I was going to do that to my mom's house but she decided she wanted ceramic tile

 
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08-06-09, 09:00 AM   #12  
Nice

Very nice job. I'm doing something very similar:

1. I put down a 6 mil moisture barrier over the slab.
2. Fasten down 3/4" plywood (I'm using full 4'x8' kerfed).
3. #15 roofing felt over plywood. I was told by a guy in Lowes that this is good for keeping the floors from squeaking should the floors become lose over time.
4. Nail down 3/4" solid oak flooring over the felt.

Seeing what you have done makes me feel better about doing my own. When I'm done I'll post pictures if I don't mess it up too bad.

 
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08-22-09, 06:18 AM   #13  
3/4" ply over concrete slab-on-grade

Definately the way to go. I didn't level it like a madman (didnt grind or fill the slab *anywhere*) although *all* instructions made a huge deal of this. To be honest, I think that bolting the sub-floor to the slab first takes care of most of this but I am just not sure that many guys have real experience with heavy ply over slab - they are usually gluing it. Before I laid the finish floor I *did* grind the plywood in a couple of places but there were perceivable zero bows or dips when done.

I used felt for the base layer becuase it was so easy to work with. Same with the 2 x 8 vs. 4 x 8 : the 2 x 8's were just easier for me to haul around. By the end though I have to admit I just kerfed the last few at 12" to 16", 5/8" depth and just put them in - saved a HUGE amount of time. No difference in that room so far either.

I did use a second layer of felt as your Lowe's contact suggested. I guess that worked - I wouldnt know the different. The only thing I would do different on MY installation would be to boost the charge from .22 yellow up at least one notch BUT that meant going to a different gun for me so setting them with a mallet just was the way to go.

Good luck! Let me know if I can help (morally that is!). I'm still waiting for Chandler to weigh in on all of this malarky ... ???

 
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08-22-09, 07:29 AM   #14  
Oh, I'm taking it all in. Understand I am not a fan of installing plywood directly over a basement concrete floor. Reason.....moisture. But others don't have a problem with it, so who am I to question it.
Hopefully you won't have any plywood heaving problems with all the seams. Of course it would have been better to apply full sheets, but if it is working, that's great. Floor looks great, as well. How did the transition to the other rooms work out? Did, or are you going to have to use or make them?
Definitely not "marlarky"!! You were able to do it yourself, and that's the most important part. Thanks for keeping us posted on the progress.
Next project???!!

 
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08-24-09, 06:46 AM   #15  
3/4" ply over concrete slab-on-grade

Happy to hear your still monitoring this case! By the way, it's slab-on-grade concrete fllor and not in a basement - not sure that changes your view but it's definately dry. Ya, not sure how well the 2x8' sheets will work either - we are cycling through the most humid season in Virgina and so far so good, nothing popping out yet!

Not 100% happy with the transitions but there are only 4 of them on the first floor. I bought the basic strips and fabricated different piece to make it match up... 1) is where I changed direction so it is flat and I used a "T", 2) and 3) where into the kitchen - probably a 3/8" step down because the kitchen has a bunch of old lino which brings it up in height a bit. I kept the strips themselves flat and "bowed" the lino up under the lip of the transitions. Looks like a bit of a take-off ramp in there but not a single person has noted it... I'm not sure it's even percievable. 4) Is into the laundry room which is was a big customer job - out of the way so not worried toooo much there.

Next project? Either a deck or a shed. We shall c! Thanks again for all of the support.

 
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