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I am preparing to do a kitchen remodel where I am replacing an existing tile floor with a new porcelain tile floor. The old floor that is being replaced has several areas where it is not level and some areas where the old tiles have cracked, possibly because the sub-floor is not stiff enough.

I am interviewing contractors to do this work and I really want a high-quality installation, even if I have to pay more for it. I have a book about remodeling by that TV guy Mike Holmes and he says that the proper way to prep a subfloor for tiles is:

First, make sure you have a 1 1/4-inch sub-floor before any tile goes down. That means if there's a 5/8-in layer of oriented strand board, then you need at least another 5/8-inches on top of that. Use oriented strand board, conventional plywood, or better yet, a waterproof product like WonderBoard or HardiBoard which are concrete-based products. Whatever you use should be set onto the existing sub-floor and glued and screwed to the joists underneath. Finally, I recommend a waterproof membrane on top of that such as Ditra from Schluter. It's a bright orange waffled plastic that's put down right before laying the tile. It helps with water-tightness and provides just a little bit of flex, which allows the tile to absorb impact.

I have prepared a scope of work describing that I want this done and sent it to some local tile laying companies for quotes. The responses I have received have indicated that this is not the way they would recommend prepare a floor for laying tile and they recommend alternatives. After I received those responses I went to a tile store and asked about this and they also said not to do this.

I have three questions

  1. Is the way described in the book a good way to prepare floor for laying tile?
  2. Why would he write this in his book if it wasn't a good/standard way of preparing the subfloor?
  3. If you were having a tile floor installed in your house what subfloor preparation would you want assuming you are looking for the highest quality installation?

I guess I am also wondering why I am getting pushback on this from the installers?

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Do you know what the existing subfloor material is, what layers (if more than one), and how thick? –  Comintern Mar 15 at 23:48
    
Yes indeed. The existing subfloor matters a lot. In the case of building up the underlayment, will that introduce a step down (even if only a half inch) to surrounding flooring? –  wallyk Mar 15 at 23:59
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..."why am I getting pushback" - you are telling people who's business is laying tile how to lay tile. You read a book, they have probably got 15 years experience. Ask to see their work, ask for references that have had it installed for a while, ask about their guarantee - but if you are going to tell them how to do their job, they are liable to put you in the "quote this one high, it will be a pain in the rear dealing with this customer" pile, or just not bother bidding if they have adequate work. –  Ecnerwal Mar 16 at 0:13
    
I hear what you guys are saying I guess I'm just wondering why Mike would make that recommendation if it wasn't a best practice. I'm going to go with whatever the tile contractor says is adequate for the job. –  Shane Wealti Mar 16 at 0:50
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3 Answers 3

up vote 2 down vote accepted

One comment on the advice you read is that I don't see a thing in there about deflection - evidently Mike Holmes is sure that if he merely has 1-1/4" of subfloor, all is good. Real tile specs tend to involve a concept called deflection, and that's the distance part of the floor system moves when loaded, relative to the span you measure over. L/360 is a typical spec - L/180 is twice as bad. (It's usually a lowercase L, but on the computer I'm using a capital to be clear that it's an L.) I happen to have a 3/4" subfloor that's L/587, so it really doesn't need another half-inch on that basis.

Consider what a tile manufacturer has to say... (I'm not going to cut and paste their extensive text here, or rewrite it all) but for one thing they specifically mention setting the backer board in a mortar bed, not "gluing" it down. I'd also venture to sneak in that if you really want "the best" (and will you pay for it these days) look at the "old fashoned mortar bed" (TCNA (Method F145-02)) mentioned at the end of their article. That, of course, means you need to establish an adequately rigid base 3/4" or 1-1/4" + tile thickness below the surrounding floor level, which is why it's not popular these days. But it was a VERY good method, as a lot of old houses demonstrate.

Kinsman Tile

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That sounds like overkill.

The Ditra underlayment is good for specific applications like tiling over a concrete slab on grade. It's pretty expensive and something like the paintable elastomeric membranes like RedGuard were cheaper per sq ft and do a similar job.

Most of the times I've seen floor tile crack it's been because the wrong tile was used. Tile is rated for hardness by the Porcelain Enamel Institute. Their rating is commonly called the PEI rating and a higher rating denotes a stronger, more robust tile. PEI 4 is good enough for floors, PEI 5 is better. Most porcelain tiles have at least a PEI 4 rating. I've seen tiles crack that were laid directly over plywood with thinset. They were replaced with tiles with a higher PEI rating without additional underlayment and they never cracked.

Plywood is stiffer than OSB. If you know you're going to be tiling over an area it's better to go with T&G plywood instead of OSB. If you put two layers of OSB, then the Ditra you're really building up the floor and can have issues where the floor levels don't match up.

For most installations you're good with just using a cement underlayment like Hardiboard that is secured to the subfloor with screws and laid on a bed of thinset mortar. Then thinset and tiles on top.

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What does PEI stand for? –  Patrick M Mar 16 at 8:37
    
@PatrickM I forget what it stands for. It's a rating scale for tiles indicating their strength from 1 to 5. 1 Being decorative wall tile (not very strong) 5 being suitable for commercial floors. Or something like that –  OrganicLawnDIY Mar 16 at 23:21
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The two more important concerns for laying tile is the deflection of the floor and how stable it is. The right way to deal with deflection is by measuring it, and that will tell the installer what is necessary to make the installation work. If the subfloor is solidly constructed, a typical installation would involved 1/4" cement board attached with thinset and screwed to the joists, with the joints taped and mortared. The tile then goes on top of that. The board might be thicker if the floor needs more stiffness or to match the height of adjoining floors.

Ditra can be useful if you have a large area and you need to provide a bit of flex and/or you need an area to be fully waterproof, but it's probably overkill for a kitchen.

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