# Plywood over concrete



## jlweber (Dec 5, 2008)

Hi Everyone - I've read posts and articles about my issue until I'm cross eyed. I hope you can help. 

I have a 320 sq.ft. workshop off the back of my separate garage. It was finished back in July and was built on a concrete slab with a moisture barrier according to codes. The concrete is cured and a moisture test was negative. I want to do wood floors on the cheap because its a workshop. So, I have 5/8" plywood that I was planning to cut into 2'x2' squares and gluing onto the concrete floor...almost like parquet on the cheap. Before I did that, I was planning to put a coat of moisture sealant onto the concrete. 

My questions are..

Am I ok gluing directly to the concrete? If so, what kind of glue should I use? Should I instead put a layer of roofing felt as a moisture barrier and glue the plywood to that essentially floating the floor on the concrete? Could I go as large as 4'x4' squares? 

Your help would be greatly appreciated. Thank you!


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## RippySkippy (Feb 9, 2007)

what about putting some PT sleepers where you're flooring joints meet and fill the spaces with rigid foam? Do you have to keep it thin for door swing?

I would not glue to roofing felt...most likely edges will not lay flat


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## jlweber (Dec 5, 2008)

You've got it. No room for sleepers because of door swing. You're probably right....felt not a good idea.


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## RippySkippy (Feb 9, 2007)

Probably not enough space to give the door room to swing then ramp up to the "new" floor level?

The only reason I suggest this is in one of my old shops that had a concrete floor, my feet froze all winter when I'd be out making sawdust. It would take me hours to warm up. The only other thing I'd suggest is to use counter sunk tapcons...but that would be a lot of drilling. Maybe quicker than tap cons are the split anchors (flat head)...drill a hole and drive'em home.


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## jlweber (Dec 5, 2008)

Wanted to avoid drilling, but still a possibility. Are you saying you would not glue directly to concrete?


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## JazMan (Feb 17, 2007)

Gluing will not work. No way to find and keep sheets perfectly flat, plus you'd have to place weights until glue dried. Even if you could do that, wood will absorb moisture from the concrete and from the air and buckle. 

You have to fasten using at least one every 12", more on edges, plus corners. Even then I'll bet some areas will buckle if on grade level and if not conditioned the year round. I wonder how much warmer even this method would be? Sleepers with insulation would be better, and doors can be installed to sling the other way or raised.

Jaz


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## Termite (Apr 13, 2008)

JazMan said:


> Gluing will not work. No way to find and keep sheets perfectly flat, plus you'd have to place weights until glue dried. Even if you could do that, wood will absorb moisture from the concrete and from the air and buckle.
> 
> You have to fasten using at least one every 12", more on edges, plus corners. Even then I'll bet some areas will buckle if on grade level and if not conditioned the year round. I wonder how much warmer even this method would be? Sleepers with insulation would be better, and doors can be installed to sling the other way or raised.
> 
> Jaz


Agreed. 

If it is a slab on grade why bother insulating the slab??? Spend the money insulating the walls and the ceiling instead!


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## Dragon (Aug 1, 2006)

I would go ahead and seal the concrete. Tons of different choices as to the product. In your case something cheap from a big box store should do. Especially as you are planning to cover it right up.

I would not glue down the squares due to issues previously mentioned. Just too many issue to contend with. And if for some reason it needs to come up it will be holy hell getting the glue residue off the floor. Not really a nice thing to do to some future owner of your home.

After thinking about it I was going to recomend a toung & groove system but that would eb a paint to do all the edges on all those squares. What would work just as well would be biscuits. At least two or maybe three per side on the plywood squares. No need for a fancy biscuit cutter if you have a router. Just get a slot cutting bit and set it to the right depth.

This way the floor still "floats" if it expands and contracts from moisture. (and it will) And by biscuiting everything together you will not get raised edges on the 2ft squares. You could then glue them but I would try it with just the biscuits at first, should be just fine. If necessary then you would only need to Tapcon at the very edge in front of the door to hold that edge down. The rest you could leave.

If you want more expensive but easier install option you will find pre-made 2x2 panels with a plastic bottom and tounge & groove edges for sale at the box stores too. (but maybe that is where you got your inspriation in the firts place.)

Let us know how it goes and put up some pictures.


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## jlweber (Dec 5, 2008)

The biscuit idea had been suggested to me earlier. I'm not hung up on the 2x2 squares. In fact, I'd rather put down full sheets or 4x4 squares. If I used biscuits, do you think I could go with full sheets using fasteners where I needed them? If so, would you put maybe a plastic or roofing felt moisture barrier underneath? 

Thanks everyone for your input. Its helping.


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## Dragon (Aug 1, 2006)

If you already have a moisture barrier under the slab I would not think that you would need one again on the top. Especially considering that you are not having any moisture issues at this time. Sealing the concrete along with the previous barrier should be everything you need.

4 foot squares or complete sheets should work fine. I would regulary space the biscuits around the edge to prevent any lifting at the edges. Trip hazard. If doing it myself I would place the biscuits no further that 2 ft apart. The biscuits are cheep and will give a nice tight fitted edge. It would tick me off to space them too far only to have to pull up later to add more to keep an edge from coming up.


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## yesitsconcrete (May 11, 2008)

not sure why you want another flooring surface but, if it were mine, i'd just seal the conc,,, if you want prettier, acid-stain it 1st then seal.

if your tootsies get cold in the winter, buy some rubber mats or wear thick-soled shoes like the rest of us do.


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## jlweber (Dec 5, 2008)

Project complete! I'm very pleased how it came out, although it was alot of work. Just to recap...I laid down liquid sealant directly onto the concrete. I then put down 15# roofing felt over that. I took 8x4 sheets of 5/8 plywood and cut it into 2x2 squares. I used a biscuit joiner to put 2 biscuits on each side of the square so that the squares were interlocking and started placing those on top of the felt. I rented a floor sander to smooth everything up. I didn't want a shiny finish, so I used deck stain to darken the wood. Now I'm just waiting for the smell to subside and for the stain to dry really well before I move the band and my tools in. It ended up taking 2 1/2 days of work. I estimate cost at around .70-.75 cents a sqft. 

Thanks so much for all your advice. It was very helpful.


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## javan (Jun 9, 2008)

Nice!

And here I was going to also suggest the space heater and rubber mats, or a carpet. I went the heater and carpet route in my old garage years ago. It was a real short nap carpet with a thick pad. Surprisingly, it cleaned up easily, and it was free. If I had the money, I would have gone the rubber mat route.

Nice Job!


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## Dragon (Aug 1, 2006)

Looks Great. Nice job!


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## tmskim (Aug 30, 2011)

jlweber,

nice project! I'm looking to do a similar project, and my eyes are also going cross-eyed looking for information on the web. I just wanted to clarify a couple of things:
- what was the liquid concrete sealant product? 
- so the finished floor is not fastened to anything? no nails, screws, or glues? How is it holding up??

thanks in advance!


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## jlweber (Dec 5, 2008)

tmskim, 

Thanks! It's been about 3 years now, I think, and I'm very pleased with how it's holding up. Remember that this is in a workshop, so I'm willing to put up with imperfections. I've got some places that have buckled alittle. It would not be acceptable for me if that was happening in my house, but for my purposes it's no problem at all. I thought for sure I was going to have to pull up a large portion of it when in May of '10 we were hit with the floods you may have read about. Water came over the pad and under the floor. But, all I did was run fans and it was fine. 

I believe all I did was put down some Thomsons water seal on the concrete. Then I put roofing paper on top of that...with the flooring on top of that. Everything is being held together by bisquits and is floating. Nothing is nailed down at all. The reason for buckling in a couple of places is probably because I may not have left enough space around the outside edges. But, I have had many band rehearsals and have built many a project in that room over the last 3 or so years. Would not have done it any differently. Hope that helps. jw


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## molly1590 (Jul 22, 2011)

Hi. I noticed you did not get a reply on your clarifying questions... Were you able to get the answers elsewhere? And if so would you kindly pass them on to me too 

Thanks!!!


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