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Old 07-29-2007, 05:30 PM   #1
Lee280zx
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Question Concrete Slab Prep questions (Pics)...

I'm tiling around 1100 square feet of my 1979 single-story slab home in San Diego.

I'm wondering what I need to do to prepare the slab. As you can see in these pictures, there's quite a bit of paint that must've been spilled when the house was built. Some areas also have carpet glue and adhesives that were used for linoleum or vinyl tiles.

I'd appreciate any advice as to what I need to do to prepare this slab for tiling. I have some experience tiling countertops and floors (both using Hardiebacker), but this is the first slab I've tiled.







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Old 07-29-2007, 05:37 PM   #2
chuck stevenson
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I would scarcify the floor. Then re-evaluate.
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Old 07-29-2007, 06:25 PM   #3
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Chuck...

Hi Chuck...what is that and how do you do it?
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Old 07-29-2007, 06:42 PM   #4
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Scarifier/grinders are walk-behind machines which can remove the upper layer of concrete from a slab using various types of carbide cutters. Available at rental places that cater to contractors. Rental prices vary of course but think $70 to $100 a day.
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Old 07-29-2007, 06:42 PM   #5
Brian in San Diego
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Lee,

Who's overseeing the operation in the top picture? Scarifying is a grinding process. Not sure if a rental yard would rent the equipment to do it or not. I believe I've read on here that the equipment can be rented. If you do a search here, you may find out more about it. I have a little difference of opinion. If the slab is in good condition (no cracks), I would be tempted to lay Ditra on top of the slab and tile away. I may be in a minority here, but I recently ripped up some parquet flooring in my entry that had been glued down. It was a mess of glue residue that I aggressively cleaned with a 4" razor scraper. Once cleaned, water absorption wasn't the best. (The glue residue is called cutback.) I laid down a sheet of Ditra with Versabond (which is a no-no) and it stuck perfectly. The Versabond is rated for use over cutback, but it's slightly modified and shouldn't be used under Ditra on concrete. I may be wrong in my assessment, but I would probably lay a 4'x4' of ditra with versabond and see how it sticks. I'd probably choose the worst area. If there is any glue residue, I would aggressively clean it with a razor scrpaer first.

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Old 07-29-2007, 06:45 PM   #6
chuck stevenson
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Lee,

I used to own one. Now I rent them. It is similar to a large buffer, with grinders. You purchase the discs. Depending on your floor, pick up a couple extra discs. If you don't use them, they should take them back.
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Old 07-29-2007, 06:45 PM   #7
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Lee-

You can rent machines to do it from Home Depot. I know that the one in Vista has both the sanding style and the bead blasting style floor prep. machines. The bead blasting one works much better and is easier to use. Next step is to locate the high and low spots. Use a long straight edge and run in along the slab and the highs and lows will be very visible. Grind down the highs with a diamond cup wheel on a grinder (you can rent these from H.D. as well) then fill in the lows with a self leveling compound. Don't forget to use the appropriate primer for the SLC. A little prep. work makes the job go faster and look a whole lot better. Good luck!
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Old 07-29-2007, 07:30 PM   #8
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Thanks

Thanks for all the opinions so far.

I used a grinder with a diamond wheel to prep the garage floor for an epoxy coating that I installed. I'd definitely rent something bigger to do this job, if that is what it takes.

McTile--I'm not familiar with the bead blasting machines, but if they're easier to use, I'm all in favor of that. By sanding type do you mean the kind that grind with diamond wheels?

Brian--That would be me. I'm overseeing the operation in the top picture. Why do you ask?


When you're using one of these bead blasting machines (or one of the walk-behind grinders), where does all the concrete dust that's created go?
...

Last edited by Lee280zx; 07-29-2007 at 07:32 PM. Reason: added information/question
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Old 07-29-2007, 08:08 PM   #9
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Lee-

The grinding type uses carbide grinding pads. Its a little hard to get used to. First time I used one it was like taking a bull by the horns, but after a bit you get used to it and its not that difficult. It just doesn't seem to work that good and yes, dust goes everywhere. The bead blasting type Takes about a 8" strip up at a time and you only need to go over it once. I'm not sure if the ones at HD have vacuum hook-ups but some do. The diamond cup wheels (and grinder) are used for grinding down the high spots in the concrete and the ones that you can rent at HD do have a vacuum hook-up. Wear a mask no-matter how "dust proof" anyone tells you it will be and mask off anywhere you don't want dust. Make sure that slab is clean. I can't stress this more.
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Old 07-29-2007, 09:34 PM   #10
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McTile

I'm always a big believer in safety gear, dust masks, eye protection, gloves, etc.

When you say make sure the slab is clean, are you emphasizing making sure the slab is clean before installing the tile, or are you saying that I need to clean it as much as possible before I either grind or bead blast it?

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Old 07-29-2007, 10:25 PM   #11
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I mean make sure it is clean of any foreign material (paint, adhesives, drywall, etc..) before installing your tile. Whether its your tile or an anti-fracture membrane, anything contaminants on the slab will cause a weak bond now that it is between the concrete and what you are bonding on top of it. This brings up a good point-- go the extra mile ands install an anti-fracture membrane before installing your tile. Mercrete makes a pretty user friendly one (mercote 5000) that comes as a liquid that you roll on. I use an anti-fracture membrane on all slabs no matter how big or small. Its an easy step and a good piece of mind.
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Old 07-29-2007, 10:31 PM   #12
Brian in San Diego
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Lee,

It looks like a dog in the top picture has an interest in what's going on. I agree that you have to find the high and low spots in a floor. Don't necessarily agree that you have to scarify the floor. Over fifteen years ago my then wife had a tile guy come in and tile our kitchen on slab floor and that tile is still there and there are no cracked tiles nor cracked grout joints. I know he didn't install an isolation membrane. If your floor is reasonably clean and relatively flat, I think you will have a surface that you can tile. I guess I look at things differently and I tend to weigh the risk/reward factor. Will my floor fail if I don't scarify it first? If I demonstrated to myself that the floor was flat and I could knock down the high spots with a grinder, then I doubt I would go to the time/money of scarifying the entire 1100 sq.ft. I would be looking at the added benefit of doing so. If I couldn't justify it, then I wouldn't do it. This is coming from a person who has been accused of being a perfectionist. I definitely go the extra mile, but only if the end result justifies the journey.

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Old 07-29-2007, 10:39 PM   #13
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Before I did a thing to that slab, I'd find out how flat it was. Rule of thumb, not off by more than 1/4" in any 10' direction.

Next, I'd find out if it will absorb water with a sprinkle test. It should not bead
up and the water should soak in in maybe 10 minutes.
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Old 07-29-2007, 10:44 PM   #14
Lee280zx
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Brian...

Brian, that's Linda supervising the job. She makes certain no food contaminates any surface to which she has access.

Having worked with concrete coatings, I can definitely see why scarifying the floor would be superior.

But what you say interests me. Having seen the extent of the stuff present on the slab, what, short of scarifying, would you do to prepare the floor for the tile?
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Old 07-29-2007, 11:03 PM   #15
Brian in San Diego
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Lee,

Linda looks like a Jack Russell and knowing a little about their personalities, I'm confident she is ultimately qualified to the task.

I'm not trying to talk you out of scarifying the floor. I am only try to add another perspective. Like GeorgeK said, you want to check to see the floor is flat. If it is less than the 1/4" in 10' he mentioned then the floor is relatively flat. If the entire floor was painted then I might see the need to scarify the entire floor. In my case the cutback was excessive and it didn't really pass the water absorption test, but I proved to myself by setting some Ditra with Versabond that the floor will accept the Versabond/Ditra combination and ultimately the tile installation. I have seen worse cases presented on this site where no scarification was employed (and seemingly got the blessing of more than one professional). If the floor is flat and the remains of any glue were scraped up with a 4' razor scraper, I would go forth and install ny isolation membrane and my tile.

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