Removed Carpet. Replacing with tile. Cracks in Concrete [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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08-31-2010, 01:19 PM
I just joined this forum and have enjoyed reading a lot of informative posts. I can't find one the matches my specifice question.
We had water damage and had to remove all our carpeting. Taking this opportunity, we decided to replace our flooring with ceramic tile.
We have two issues:

On part of the concrete floor, the builder used it as a place to paint the trim molding. There are wonderful painted stipes on about 15% of the floor.

In our living room and dining room, there is one each, a long hairline crack.

From what I read, I need to sand off the paint. Correct?

I also read that I need some sort of underlayment for crack isolation. Is that also true?

If I need the crack isolation, what is the least expensive method. In today's economy and a fixed income(I'm retired), I need all the help I can get.

Thank you for any and all advice.


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08-31-2010, 01:33 PM
Yes, the paint needs to come off the floor. Sanding will be slow. Grinding will be quick. Both will be dusty, so take precautions.

If the crack is even on both sides (i.e. no vertical displacement), then you can use an antifracture membrane over the slab in the area of the crack, or over the entire floor. Redgard is an example of a roll-on liquid membrane, and is about as cheap as they come. It's not as good as sheet membranes, such as EasyMat (both are from Custom Building Products and are available at HD).

08-31-2010, 01:53 PM
So I don't have to do the entire floor with the membrane?

I've got 1500 sq ft to do and only the two cracks.

What would be the prefered tool to remove the paint?


08-31-2010, 01:58 PM
If you are confident your slab has done all the cracking it's going to, then your membrane need cover the area of the cracks to a width equal to 3 tiles.

What would be the prefered tool to remove the paint?I like the telephone myself, but you are on a budget. :D An angle grinder with a diamond cup wheel will remove the paint and expose fresh concrete. It may come in handy if you find a high spot or ridge elsewhere on the floor.

08-31-2010, 02:04 PM
Crawling around on your hands and knees with an angle grinder will work, and it will be fairly fast, but very dusty and messy. Standing up with a surface grinder (scarcifier) will be faster, and some of them have dust pickup. Either way, wear protection and mask off the doorways. A fan to exhaust the dust is a good idea. It will be easier to keep things flatter with the larger scarcifier, but an accidental gouge with the angle grinder won't be as big. If you have a high spot, the scarcifier will take it down easier.

08-31-2010, 02:18 PM
Last week I went through the task of prepping my concrete floor for a tile install. I used a 4.5" angle grinder with a carbide cup wheel attachement to remove adhesive residue and take down some high spots. I highly recommend purchasing a dust containment device to go on your angle grinder if it doesn't have one. This keeps the level of dust WAY down.

This is what I ended up buying:

08-31-2010, 02:20 PM
I have a 4 1/2 grinder with ceramic flap discs. Would this work?

Here are a couple pictures:
By doctordun ( at 2010-08-31
By doctordun ( at 2010-08-31

Houston Remodeler
08-31-2010, 03:01 PM
It will, but not nearly as fast and as well as a diamond cup wheel

09-01-2010, 12:49 PM
From advise I've received for our concrete slab that has a couple long hairline cracks, I should cover the cracks with a Crack Isolation Membrane and lay the tile with Flexible adhesive.

What are the best and most economical solutions? I live just south of Dallas and I'm not sure where to go the get the best products.


09-01-2010, 01:17 PM
Davy is in your neck of the woods. If he doesn't stop by, send him a PM.

EDIT: I sent the PM. Hang tight.

09-01-2010, 05:09 PM
Hi Doc, welcome. Like Bob said, Redgard is about the cheapest membrane out there and I have used it under thousands of sq ft of tile. The key is to get it thick enough. I usually apply it with a flat trowel and leave enough so it's solid pink. It will dry red. If you can see thru it and see the slab, it's too thin. Many times I will go over the area with a paint roller just behind the trowel. It will knock down any trowel lines and add just a little extra. I would do the whole floor myself but you can just go over the cracks. Make sure you mop the floor well and let it dry before putting it down. With a clean mopped floor, you might see more cracks that you didn't know were there. Keep an eye out for those. I would use Versabond thinset from Home Depot.

09-01-2010, 06:08 PM
What kind of coverage do you get with Redgard?

Just trying to find out what I need.


09-02-2010, 04:08 AM
From the Redgard Datasheet (

RedGard as Crack Prevention Membrane:
• 1 gal (3.78 L) pail — 110 ft2 (10.2 m2) at 30 mil
thickness when wet
• 3.5 gal (13.2 L) pail— 385 ft2 (35.8 m2) at 30 mil
thickness when wet

Your coverage will probably be less than this. Those Custom's guys really know how to spread it. ;)