What is a proper high-quality sub-floor preparation for porcelain tile floor installation

Question

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?

Answer 1

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.

New link that's just an illustration of TCNA F145.

Answer 2

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.

Answer 3

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.

Answer 4

Foremost, go with your contractor's advice. That being said I would not hire a contractor who thinks Mike Holmes' technique is inadequate. I think his show & his book are adequate for qualifying his technique. Also remember most of the advice you get when you talk to other people will give you a snapshot of how ignorant your locale is to quality standards and professionalism. In other words if the guy wants to throw mortar down on top of your floor and tile it's probably because he has been getting paid to do it for years. This is a huge problem in the construction industry today where unethical cheap work trumps professional quality and cost. Mike Holmes exposes this in his show in spectacular fashion. So make sure you're not listening to the money maker job taker and that your advice comes from a moral and concerned professional. Good luck sounds like your going to need it.

By the way: deflection is for architectural engineering something you would test on materials in a lab not on a job site. Also the step up step down argument is a planning consideration and should not compromise a quality standard such as having a subfloor of 1-1/4". Only after you met the minimum standard would you consider possible ways to make a flush step. In other words you wouldn't tile over a 1/2" subfloor just to make the tile flush. Again good luck and don't sweat it; most people can get inadequate work done without even noticing it. Out of sight is out of mind just not for a real professional.

Answer 5

3/4" OSB, then 1/2" plywood screwed down, thinset mortar first then screw down 1/4" cement board. Thinset mortar again then lay tiles, that's how I did my house

Answer 6

Ive also used 2x4 blocking with pl400 and nailed between joist. Every 8". Two have hardwood and tile flush. Ive been told once by a home owner i was trying to cut corners lol. The subloor was super solid ?