Hardwood Over Concrete Slab
 

What is the best method to install 3/4 pre-finished hardwood over a concrete slab that was once a garage floor? The slab is not in a basement.

I would suggest floating a layer of 3/4" plywood on the concrete. Then add a 2nd layer of 3/4" ply and be sure to stagger the joints and screw this layer into the one below it. Then lay down some roofing felt paper and staple the floor to the plywood using 1.5" crown staples.

No disrespect meant but, But not a great plan.
Wood should never be in direct contact with concrete unless it's pressure treated. And no pressure treated plywood in this case is not the ansewer!
A whole lot easyer to lay a layer of vaper barrier and use engineered flooring and install it as a floating floor by gluing all the seams.
You would have the same look and feel, could still be refinished at some point, and still have real wood as a top layer.
Engineered flooring is below grade rated (yes I read where your not below grade but it's nice to use a product you know will not be effected)

Plastic film doesn't glue very well. What would be alternative vapor barriers for this application?

Does the engineered product "snap" or lock together?

If you lay the floor the way I suggested the 6 mil. os just layed on the slab, seams get taped, and flooring is just laid down over it. The only glue used is on the seams, not under it gluing it to the plastic.
It's done this way all the time in basements.
In one of my rentals I layed the 6 mil. then 3/4 blue foam panels on the slab before the engineered flooring to act as a thermal break.
Now the floor feels warmer, a the floor does not flex or move at all.

What is the best tape to use on the seams?

Some blue foam panels have foil on one side. Any opinion on this?

The same tape that's used on Tyvek house wrap will work.
I've never seen dence blue foam with foil on it but if there is I'd have it facing up.
Before doing anything take a look at some of the manufactures web site that make engineered flooring and take a look at the hundred or so differant type of wood avalible and also look at there install directions so you have a complete understanding.

You do not say where you are and I think that makes a diff. I am in fla. so everything is built on a concrete slab. So if the garage was poured at the same time it's probably the same thickness. If you are up north and have a basement a lot of times the garage floor is thinner than a slab. My suggestion would be to take couple places, tape down a piece of pretty heavy plastic (make sure it's sealed around the edge) let it set a couple days. Then remove and see if it is wet under the plastic. Depending on if it is wet or how wet it is you may have to explore other options.

Thanks. I live in Southern Pines NC. Will do the moisture test next week and report back.

The main house was built on a crawl space in 1974. No moisture problems under the house.

Iplay,

There's all type of non-approved methods that individuals claim work well, some don't. Although some of these methods may work sometimes, they are not approved for a reason, they sometimes fail. Plastic on a slab is one that can bite you. What does the wood manufacturer say? What brand is it?

Jaz

I sent an email to the mfg. today on this matter. Thanks.

How would you suggest to check for moisture in a slab? This is what I was taught and what I read but am interested in a better way.

I uses a moisture meter on wood, but not sure it would indicate a valid result with impenetrable concrete.

Possibly a block of wood could be measured, then place on the concrete and measured again in a few days. An increase would suggest moisture, but making a valid quantitative interpretation may be difficult.

I like the poly film idea myself for at least a rough indicator.

Is there a sealer that can be applied to concrete to eliminate the issue?

I was hoping Jaz would come back to tell us another way and i'm pretty sure he will. And yes the poly was only ever a rough indicator. And yes there are concrete sealers but i'm not sure they are for this application. Maybe someone else knows for sure.