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Unread 06-13-2020, 09:24 AM   #1
Join Date: May 2020
Location: Michigan
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Tile over Tile Bathroom Floor-1st Project

This is my first tile project (except for helping my dad when I was a kid) so am looking for some advice. If this goes well, I have a bunch more in mind, so this is sort of my trial run. I am generally a pretty handy guy and like to research thoroughly before getting started on a project.

I have a basement bathroom that has 12x12 white ceramic tiles over a concrete floor. The previous owners then put carpeting over this. The tack strips for the carpet were nailed right through the tile, cracking out the outside 1” or so of the tile in many places. I have not fully ripped up the carpet yet, but given the rest of the tiling in the house is very well done, I would assume this was as well.

I want to remove the carpet and put down new tile. I have LifeProof Linen Wood 6 in. x 24 in. Glazed Porcelain Floor and Wall Tile that I got for about 30 cents per sq. ft. and have plenty of it.

1. Can I tile over the existing tile (assuming they are all well adhered, uncracked, etc.?
2. How should I address the cracking of the existing tiles around the room edges due to the nails from the carpet tack strip?
a. Can I pull out any loose cracked pieces on the edges and level back to the existing tile with a mortar?
b. If yes to a. what product should I use and any tips for application?
3. If tiling over tile should I use a primer product (thinking MAPEI ECO Prim Grip) or just sand with 80 grit, or do whichever is easier?
4. I am thinking to use a large format tile mortar (thinking MAPEI Large Format Floor Tile Mortar mixed with Polymer Additive) applied with a ½” xx ½” square notched trowel. Is this the best combo? FYI I did check the flatness of the tile face to face and it is very flat, no rocking corner to corner or gaps in the middle.
5. I am thinking 1/8” gap with a leveling system and unsanded grout. Is this a good choice for the project? I would love to go to a 1/16” grout line, but would this be looking for trouble for these tiles and my first tile job?
6. Any comments on the materials below? I am not married to MAPEI, but it seems to get good reviews and my local big box store carries a good supply of it.

Thank you in advance for your help!
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Unread 06-13-2020, 11:57 AM   #2
Tool Guy - Kg
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Welcome to the forum, Loren!

1. In a relatively small number of cases where you've got a very solid installation over a great substrate, you can tile over tile. This might be the case for this project.

2, 2a, & 2b. For slivers of tile missing on the perimeter, many pros will use a 'Large Format Tile Mortar' (doesn't shrink as much as a 'Thinset Mortar') to fill in the gaps.

3. With tiling, it's not doing whatever is easier. It's doing the easiest thing that will give you a long-lasting installation. In many cases, your 80 grit sandpaper isn't going to do much to a tile unless you've got carbide grit. The idea is to either grind/scuff the tile so that the setting mortar can stick to it....or use a product that will both stick to the tile and allow new mortar to stick to it (like with the Mapei ECO Prim Grip). I don't think many folks would use both the scuffing and the product...though it wouldn't hurt anything.

4. That's likely to be a sticky combination that bonds well. But your trowel notch size is something that is determined by how flat the bottom of your tile is in relation to the substrate. Waves in the floor, or hollow-ness of the bottom of your tile have an affect on required trowel notch size. If you can prepare your substrate flat...then burn a coat of mortar onto the backs of your tile, you can start with a 1/2" x 1/2" trowel notch and see what percentage of mortar coverage you're getting after a few tiles are installed. To accurately assess your mortar coverage, you remove tiles from time-to-time during install and examine your coverage.

5. The minimum grout size should not be determined on desire alone. The minimum industry standard grout line is determined by measuring the difference between all the tiles and multiplying that figure times three. So, if you've got a difference in tile sizes of no more than 1/24th of an inch (yes, I know it's not a standard measurement) from the smallest to the largest tile, the minimum tile size would be 1/8". Now, I'm assuming you're going to use an off-set pattern. If this is the case, you really only need to measure and compare the narrow 6" widths of the tiles, as they are the only widths that will be adjacent to each other where the naked eye can pick up on undesirable visual differences. And while unsanded can technicalled be used for 1/8" grout width size, it shrinks a fair amount and I'd always go with sanded grout with anything 1/8" or larger if I can possibly help it.

6. Tiling over tile involves extra time and expense. And it leaves you with tile that's physically higher than if you had just removed it. With spending extra time and money: 1) scuffing the tile, 2) applying the ECO Prim Grip, and 3) using the expensive additive, have you considered trading those expenses and energies towards chipping the tile off and using a hand-held grinder outfitted with a diamond cup wheel and dust shroud to start with a bare slab? I'd ask myself these questions before proceeding with tiling over tile...
A) How large of a bathroom is this?
B) Have you checked how well the tile is bonded using something like the handle of a large screwdriver and tapped around in several locations on each and every tile?
C) Can you afford the extra height you'll gain by working atop the old?
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Unread 06-13-2020, 03:12 PM   #3
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When you have an exposed edge, removing tile often isn't all that tough of a job when you use a chisel to get underneath. Power tools for this make it faster, too. If well bonded, and you clean the floor well, then prep it with a primer, it should be fine over the existing, too. Depending on whether you want it 1/2" or so taller than what's outside the bathroom.

Being a basement, you might want to consider something like Ditra Heat Duo. This is a mat with some insulation on the bottom designed to hold electric heating wires to warm the floor and then the tile is bonded on top of it with thinset. The DItra is held in place with mortar on the bottom as well.
Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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Unread 06-14-2020, 09:52 AM   #4
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If you decide to remove the tile and want to use a power tool I recommend this hammer drill from Harbor Freight. I have used it to remove tile in two bathrooms and it has worked really well for not a lot of money.

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Unread 06-15-2020, 08:51 AM   #5
Jason Cantin Mapei Technical rep
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as a general rule Mapei recommends removing any flooring and get to the original substrate. That being said we understand that is not always possible. Which leads to new products. if the floor is well bonded and displays no concerns, the method you discuss is a viable option. as another general rule the TCNA recommends a minimum 1/8" grout joint on most installs. Good luck and you can always call our 800-992-6273 with any additional questions.
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