A:  It is likely that you can install plywood subfloor or underlayment directly on concrete, but it depends on your slab.

If the concrete slab remains high and dry all the time, and is perfectly smooth and stable, you can install the plywood subfloor or underlayment right on the concrete.

You’ll want to lay down a thin layer of mastic to bond a 6-mil (minimum) plastic sheeting to the surface.  First test the surface of the concrete to make sure that it really is dry.  If you’re feeling “old school,” you can use roofer’s felt.  Same thing either way.

Why Is Level and Smooth Such a Big Deal?

You may think:  Well, I’m laying down 3/4″ plywood (or whatever thickness) on this concrete slab…why does it really have to be perfectly level and smooth?  Because even small bumps and imperfections can throw off your plywood.  You’ll end up with raised corners that you valiantly try to smack down with your power nailer.  Or you may even have an entire side that won’t go down.  Small imperfections get magnified when you’re laying down the underlayment/subfloor sheets.

There are 2 solutions here:

1. Fill in the gaps and smooth down the rises with a power grinder.
2. Lay down a system of risers to elevate the plywood.

Laying the Plywood on the Slab

Plywood Subfloor on Concrete

With the surface prepared and the vapor barrier in place, it’s time to lay out and fasten down the plywood.

• You can either lay out the panels in “logical” fashion (that’s what I call it anyway); that is, sheets parallel to the wall.  Or, as some floor installers do it, you can lay out the plywood on the slab in diagonal fashion.  This diagonal layout has one benefit:  you ensure that plywood joints and floorboard joints never line up.  Personally, I think the diagonal layout is a pain in the ass.  Think of all that cutting!  But you’re welcome to try it…
• At the very least, you do need to stagger the joints.
• Space out each sheet with about 1/4″ gap between them and about 1/2″ to 3/4″ away from the walls.
• Use your power nailer to drive three or four nails in the center of each plywood sheet.  Then 2 or 3 nails for each edge.

And that’s that.

It may seem that sealed concrete is a perfect surface to lay down finish flooring.  What could be cleaner and less porous than concrete with sealant on it?

Well, as it turns out, sealed concrete is not the best surface…

Find Out – Is the Concrete Sealed?

Dip your fingers in a cup of water and sprinkle some drops onto the concrete slab.  Does the water bead up?  If so, it could be that the concrete is impregnated with oil or some other substance that repels water–in which case you still need to thoroughly clean the thing.

But if water beads up all around the flooring, you’ll know it’s been sealed.

How to Remove Concrete Sealant

Your finish flooring adhesives actually need a porous surface in order to bond well; slick does not do the job.  There is no substance that I know of that will allow you to blister and peel/scrape away concrete sealant, in the manner that you might strip paint from woodwork.

Your only option is to rent a concrete floor grinder at the rate of around $80-100 per day and roughen up the surface.

While some concrete experts may disagree with me–and hey, they’re the experts–my take on the matter is that not every single square inch needs to be thoroughly blasted away.  As long as the majority of the concrete surface is exposed, that’s enough for your flooring adhesive or mortar to “take.”

Huge concrete projects retain moisture seemingly forever.  Hoover Dam supposedly is still drying out and cooling down from its pour over 75 years ago.

If the tape doesn’t hold down the plastic, then the concrete might be too dirty…or too moist even to hold down tape.

Back to the residential world, how do you know if the concrete slab is ready for flooring?  Moisture is your biggest culprit, so do these things:

• Well, first of all you’ve got to wait a good three or four days.  Concrete will not be dry before then.
• But after that 3-4 day waiting period, you can tape a square of clear plastic to the surface of the concrete.  The plastic doesn’t have to be very big–maybe two feet square.
• Now, tape down the entire perimeter of the plastic with duct tape.  Make sure it’s down good and tight.
• Wait 24 hours.
• If you come back and find fogging, beads of water,or any evidence of moisture on the inside part of the plastic, it’s not time to install flooring yet.  Wait a day, then tape down the plastic again.

While the “traditional” method of building with a raised foundation–joists over a crawlspace or basement–definitely has its merits, there are many parts of the world where concrete slab is the most logical way to go.  But how does this fit in with laying down your finish flooring?  Can you lay the flooring directly onto the concrete slab?  What you’ll find is that some flooring can go right on concrete, while others cannot.  Let’s take a closer look…

Concrete Slab Gets Moisture from Below

The main thing to remember is that concrete slab floors is that they receive moisture from the bottom-up.  Even though the concrete slab floor is laid onto a sand or gravel bed, moisture does come upward.  You cannot escape this moisture.  Whether you live in Buffalo or Tucson, Austin or Hawaii, you’ll be getting ground moisture.  You many not think so, but it happens.

How Does the Vapor Barrier Come Into Play?

Many kinds of flooring–wood, laminate, engineered–are greatly affected by moisture.  Resilient flooring, such as vinyl, linoleum, and other man-made materials, can withstand moisture better.  Your concrete slab already has a moisture barrier in place (let’s hope).  When the builder laid the concrete, he laid down the sand/gravel bed…and then a plastic vapor barrier…and then he poured the concrete.

Types of Flooring Bonded Directly to Concrete

Think:  rot.  What kind of flooring material is most subject to rot and deterioration?  Which kind is least affected?

Well, you’ve already answered your question:

• Ceramic tile
• Natural stone
• Brick
• Resilient flooring

…can all be installed directly on concrete slab.

Ceramic tile, stone, and brick are such close cousins to the materials used in concrete work that they are largely unaffected by any moisture.  Resilient flooring (i.e., vinyl) doesn’t mind if there is moisture, either.