And keep in mind that we’re not even talking about the surface itself, which needs to be absolutely clean, oil-free, stain-free, dirt-free, and sealant-free.  That’s a whole different matter…

Cracks or Depressions in Concrete Slab

One good thing about repairing cracks in the concrete is that none of this is visible.  It’s really difficult to repair concrete cracks and not see the repair areas.  Here, all you want to do is get the thing level and smooth.

1. Use your ever-ready masonry chisel to chip away at the loose edges of the crack.  Anything loose must come out.
2. Then, chip away at the bottom of the crack to form a (rough) inverted “V” shape.  This will help form a space so that the epoxy goes in…forms a plug…and then won’t come out.
3. Clean it all out with a pressure washer or hose.  Shop-Vac the crack thoroughly.
4. Fill the crack with epoxy patch designed for concrete.
5. Trowel smooth.

High Spots in Concrete Slab

High areas in the concrete also need to be brought down.  Otherwise, you’ll be bowing out your flooring.  Theoretically, this can be done.  And that’s one of the benefits of laying ceramic tile:  it conforms to the shape of the flooring below.  After all, you’ve seen ceramic tile laid on all kinds of curved surfaces such as swimming pools; it can certainly lay down on gently undulating concrete slab.

But do you really want this?  Given an option between smooth and flat concrete, you’ll take the “flat” option.

About the only solution is to use a concrete floor grinder.  These are serious machines that rent out for $80 to $100 per day.  High cost, but you probably will not need the floor grinder for more than a day or two.

The fact is, you do need to remove that threshold; there is no way to get around it. Even if you are not installing flooring, there may be times when you just want to remove the threshold so that you can put in a new one.  Thresholds get a lot of traffic and thus they get damaged.  Thresholds that no longer do their job are not effective.

The reason thresholds are terrible to remove is because the ends are trapped under the door casing.  One way to remove the threshold is (obviously) to knock out the door casing.  Good luck there.  That opens up a host of other problems.

Cut Threshold to Remove It

So, what you need to do is cut the threshold into two or more pieces with a miter saw.  This allows you to use a prybar to pry up an edge of the now-cut threshold.  Keep in mind that you’ll still have the end trapped under the casing, so you need to wiggle it to remove it.

What if there is a nail directly under the door casing?  I would recommend cutting the threshold once again, as near to the bottom of the door casing as you can get.

I’ve never one for doing unnecessary work.  For one, I hate the idea of applying subfloor adhesive (nice thing, but not absolutely necessary), but staggering your subfloor end joints definitely should be done.  And it’s not all that hard to do.

What’s Your Subfloor Material?

You’ll most likely be using either plywood or OSB for your subfloor material.  If plywood, remember that the grain goes perpendicular to the joists.  For OSB, it doesn’t matter:  OSB has no grain.

If you want to go fancy-schmantzy, you can use tongue-and-groove plywood subfloor.  Just like it says, it’s plywood, but with a tongue that fits into a groove of the neighboring panel.  This isn’t really necessary, but if you have some money burning a hole in your pocket, why not?

Avoid End Joints that Form a Cross

The worst thing you can do is end up with joints forming a cross like so:

Plywood Subfloor Layout Incorrect

The reason is because those corner areas will deflect and become unstable.  In fact, you don’t want this kind of arrangement with any kind of building material.  Drywall is another application where you never want to have these corners meeting up like a cross.

Stagger Subfloor End Joints

Instead, you need to stagger the end joints like this:

Plywood Subfloor Layout Correct

It’s just like brickwork.  You never see bricks laid out in a grid-like pattern.  Bricks are laid stagger-formation for a good reason:  it’s structurally stable.

Same with subflooring.  Staggering the corners makes for a highly strong subfloor, and a better and smoother surface for your finish flooring to rest on.

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.