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shopvac 11-18-2009 08:16 PM

tile sub floor
I am preparing to remove carpeting and install tile (Porcelain?). My current sub floor is 3/4" (Plywood)). I have read many different publications and received many different recommendations.

I will avoid OSB. Do I even need to add additional plywood? I have been advised by some folks to have a minimum of 1", others say 3/4" is fine. I'm considering another 1/2", for a 1.25" total. If I do add plywood, what adhesive is best, in addition to screws?

Thanks in advance,

Nestor_Kelebay 11-18-2009 09:58 PM

I've installed ceramic wall tiles in 22 bathrooms so far, but I have yet to set my first floor tile. However, it's not possible to understand the gameplan in wall tiling without also being able to understand the gameplan when it comes to setting floor tiles. Here's my advice. See if it makes sense to you. Confirm everything I say with someone who's knowledge and advice you trust, and by comparing view points, you'll learn enough to walk away with your own informed opinions.

The purpose in putting down a thin layer of something over your plywood subfloor is so that you don't start spreading a mortar bed directly on your plywood subfloor. That's cuz the subfloor goes UNDER your walls, and so if you make a mess of your subfloor, you can seriously damage your house.

Here's how they build a floor:

1. IN THE BEGINNING, there were the floor joists
2. Over the floor joists, they nailed, screwed, glued and screwed (take your pick) a lumber or plywood subfloor.
3. On top of that subfloor, they built the walls out of 2X4's
4. Then, they nailed or stapled down thin plywood underlayment inside each room.
5. Then they drywalled and painted,
6. And then they installed the flooring in each room, and nailed the baseboards on.

If you don't have any underlayment under your carpet, then that plywood you're looking at is your subfloor, and you don't want to do anything that might wreck it, like trowel a really flexible (meaning it has lots and lots and lots of glue in it) thin set mortar onto it. (It's the large amount of "additive" (pronounced "adhesive") content that makes these high performance thin sets flexible, but it also makes it stick like chewing gum to the underside of a church pew, and much harder to remove.) You don't want to put ANYTHING on your subfloor that will put of a real fight to remove.

By using a thin plywood over your subfloor, you don't have to worry about getting the thin set off. You can pry up the thin plywood and get back to a clean plywood subfloor relatively easily should you want to. (That's one of the benefits of having underlayment in each room of your house. You can always avoid having to remove the glue that might be holding an old worn out flooring down by replacing the underlayment.)

The way people recommend prepping the floor is to spread thin set on the floor and lay cement board (I recommend Hardibacker Board for a floor) into that wet thin set. The purpose in doing this is to fill in any dips in the floor where the cement board otherwise wouldn't be supported. That way, there are no soft spots in the floor where the grout would otherwise crack. You leave a small gap between the cement board panels, but this gap is not to allow for expansion since the cement board panels are very dimensionally stable.

The reason you need the cement board is that wood swells and shrinks with changes in it's moisture content, and ceramic tile thin set or grout simply doesn't have the elasticity to accomodate movement of the substrate. So, by putting cement board down, you're thin setting the tile to a dimensionally stable surface. There may be lots of stress between the plywood subfloor and the cement board panels, but there won't be any stress between the tiling and the cement board panels underneath, and that's what matters because that's what puts stress on the grout joints. It's exactly like standing on the fault line the day before the earth quake. There may be plenty of stress in the ground a mile below your feet, but you don't feel anything at all cuz the ground directly below your feet isn't moving. Since the cement board panels don't expand or contract, the grout joints of your tiling don't feel any tension or compression, and therefore don't have any excuse to crack.

Then, you fill the joints between the cement board units with thin set and put fiberglass mesh over the joints. Alternatively, you can put the fiberglass mesh over the joints first and then fill them with thin set. The purpose in doing this is to bond the neighboring cement board panels together so that (as it was explained to me by Custom Building Products (who make Wonderboard) "the whole cement board assembly is locked together and acts as a single surface", which I presume means that there can't be any relative movement between cement board panels.

Then, if you're using a porous tile, you seal the tops of your tiles with a sealer.

Then you trowel thin set on the cement board panels and set your tiles. (I suppose you could wait to set the tiles before sealing them, but you're likely to have sealer dripping down the edges of the tile, and I don't know if that would prevent the grout from sticking to the edges of the tiles.)

Then you grout your tiles, relying on the sealer to prevent any grout from getting into the porous surface of the tile and discolouring it.

You wipe the excess grout off with damp sponges until all that's left is grout haze. Then, after a few hours when the tiling is dry, you wipe the grout haze off with a DRY towel. (And, since the grout will be hardening up while you're working, don't mix more grout than you can comfortably apply in half an hour or so. AND, keep a green or white Scotchbrite pad handy so that if the grout does start hardening up faster than you can get it off with a damp sponge, you can tear it off in a hurry with a Scotchbrite pad without harming the tile surface.)

Then you wait as long as possible and then seal your grout, the dryer the grout, the better. The kind of grout sealer you want to use depends on where your grout is. On a bathroom wall, I'd use a penetrating sealer. On a kitchen floor (and for most floors I expect) I'd use an ACRYLIC film forming sealer like Glaze N' Seal's "Grout Sealer":

Stay away from any grout sealer that contains any kind of "siloxane" in the list of contents on the back of the bottle. That word means that it's a silicone based grout sealer, which should be taken off the market. Silicone based grout sealers work well, but the problem is that nothing sticks well to silicone based products (grout sealer or caulk) and so you can't apply another coat of grout sealer over what you have when it starts to wear off. (You can apply more silicone based grout sealer, but the 2nd coat won't stick well to the first coat.) You're much better off getting an ACRYLIC grout sealer so that you can apply additional coats as necessary to better maintain the protection on your grout for many years (or as long as necessary).

You want to use a film forming grout sealer to prevent liquids spilled on the floor from causing any stains or soft food from getting "mooshed" into the porous grout and providing a food source for bacteria. With an acrylic film forming grout sealer, then if push comes to shove, you can always strip the grout sealer off the grout with some acetone, and paint on more grout sealer. If you moosh peanut butter into porous grout underfoot, then about all you can do is have at 'er with a toothbrush and hope you get it all out. Either that, or try removing the affected grout and regrouting that spot.

Hope I answered some of your questions.

shopvac 11-19-2009 09:55 AM

Thanks Nestor for your reply! You obviously put some time into your response, it's appreciated.

I never considered the addition of sub floor material to protect the original base. Good to know for future needs, should they arise. I merely thought the additional material added to the stability of the floor, thus reducing the risk of cracks in the grout and/or tile.

The carpeted rooms do not have an underlay, rooms with linoleum do. I will forge ahead with intents to add an underlay. Of concern is the transition height difference where existing carpeting will meet new tile, as well as a rigid base. I believe my floors are very firm however I could be wrong?

Is a 1/4" underlay material available or will I need to accept a thicker material?

Nestor_Kelebay 11-19-2009 03:17 PM

You should have no problem getting 1/4 inch plywood underlayment.

You should also know the difference between plywood, underlayment and plywood underlayment. Plywood can have voids in the interior plys larger than a certain size. Underlayment is anything that's meant to be put over a subfloor, and doesn't have to be be constructed like plywood. Plywood underlayment is constructed LIKE plywood, but any voids in the interior plys have to be smaller than a certain size, or filled with wood putty. If you use plywood AS underlayment, then you can have a soft spot in the floor where there's a large void in an interior ply, and this can cause problems if you install ceramic tiling or sheet vinyl over that soft spot.

There's lots of people going around saying that to prevent the grout on ceramic tile floors cracking, everything under the floor has to add up to 1 1/4 inches in thickness. This is as stupid as it sounds. What's needed is merely a strong enough wood floor system to limit the amount of flexing of the floor to a minimum AND a dimensionally stable material on top of the wood to set the tiles on. The wood floor system (the joists, blocking and subfloor) provide the rigidity needed to prevent the grout from cracking due to flexing of the floor when you walk on it or put something heavy on it. The cement board is there to be dimensionally stable so that any movement of the wood (swelling or shrinking caused by changes in it's moisture content due to seasonal changes in humidity) won't result in any tension or compression on the ceramic tile grout lines. THAT is all you need. To say that everything has to add up to 1 1/4 inches is like saying you can install the tiles directly over two 3/4 inch thick layers of particle board. If that particle board gets wet and swells, that's the end of the tiling despite the fact that you had more than 1 1/4 inch thickness of material under the tiling.

You need a strong enough wood floor system not to flex due to traffic, and a dimensionally stable material to isolate the ceramic tiling from the wood (which isn't dimensionally stable). There, I said it twice. Now you understand what's really needed. How you provide that for your tiling is up to you. I would talk to a ceramic tiling contractor to see if your joist size and spacing as well as the subfloor are strong enough for ceramic tiling without any strengthening. I would use Hardiebacker board on a floor to provide the dimensional stability for the tiling.

The best underlayment I know of is fir plywood underlayment. If soft spots weren't a concern, I'd use 6 mm Baltic Birch furniture grade plywood as your underlayment. It might have voids in the interior, but it's made of hardwood (Birch) and it comes from Russia, so it's surprisingly inexpensive. Please don't use a processed wood (particle board or OSB) underlayment. If they get wet, they swell up and lose all their strength, just like a quality breakfast cereal (Count Chocula).

Mortman 03-26-2010 06:50 AM

I am also not a professional tiler - but have learned quite a bit from this and other forums.
My comments based on what I have read:
- Do not use 1/4 plywood as underlayment - 1/2" min. (try to find that!) 15/32" is a minimum to get added strength
- After ensuring you have the structural strength with you joist (deflection control) run your plywood (only plywood) underlayment perpendicular to the joists and don't stop the seems on the joists - stop 25% of distance past the joist to the next joist (4" on a 16" center span) There are engineering article about this approach.
- Do not nail into the joists with the underlayment (another reason not to put your edges over joists) You are trying to create a self contained and stable underlayment that is not further stressed by alternative movement of the joists and subfloor - but still sell attached to minimize movement.
- Nail (ring shanked) or preferably screw at 1' intervals in center field of plywood and 6-8" at edges
- Leave 1/8" spacing between plywood sheets and 1/4" at room edges (wall)
- Use 1/4" cement backer or Hardieboard / Wonderboard (no one seems to have a problem with any of these) on the floors and 1/2" on the walls
Use thinset over the underlayment so its between the plywood and the cementboard or Hardieboard. I need to clarify as some say plain thinset and other like a modified thinset. Most say never any adhesives that come out of a premixed bucket!
- Whether you need to put mesh tape (alkaline stable special mesh for cement board) over the cement board is debated - seems to make sense to me. Since no movement of this material - curious why the want you to leave 1/8" between the cement boards if they don't move and then fill with thinset (?)
- There are special screws for the this upper layer (cement board or the like) and make sure screw heads are below the surface a bit. These screw should penetrate the the 1/4" cement board and underlayment - (15/32 + 1/4") - maybe someone can add whether these screws should penetrate the total floor - 3/4" + 1/2" + 1/4" = 1 1/2") I have notice the specialized screws are in two sizes - 1 5/8" and maybe 1 1/4"
- If you penetrate the three layers, once again, you would want to avoid the joists - so mark the walls while you can see them
- Hope this helps!

Bud Cline 03-26-2010 07:31 PM

The Tile Council of North America (TCNA) is the recognized overseer of techniques when it comes to tile installations in the U.S. Canada's authority is a clone of the TCNA.

The thinset used under the tile backer (cement board) should be unmodified unless the tile backer manufacturer says different. Unmodified thinset is used so that it WILL NOT permanently bond to the substrate. The thinset will release thereby allowing some slight lateral movement between the wood structure and the tile installation. Never mind the fact that the fasteners are attaching the two together, this doesn't seem to be an issue.

Later, if and when it is necessary to remove the tile installation for remodelling (or whatever) the thinset won't be an issue and only the fasteners will cause any concern.

Cement board of 1/4" thickness is suitable for a ceramic tile installation generally over a 3/4" plywood subfloor. OSB sub floors are also allowed. Plywood of 1/4" thickness is not suitable for use in any tile installation no matter what you call it. Test have proven that 3/8" plywood is the minimum acceptable and to be used. If additional underlayment is to be used it should be "exterior grade EXPOSURE 1", this is usually a BC material. Sheathing grades and CDX grades are forbidden.

The thickness of 1-1/4" (1-1/8") is an old standard typically used in what is known as mud-jobs and it has carried over and confused many. It is not necessary with today's products.

Cement boards are to be installed with a 1/8" gap and fastened according to the manufacturers nailing schedule. Coated alkali resistant screws with double hi-lo threads are the required norm. Alkaline resistant mesh tape is used to bridge the seams. This is not debatable and is in fact mandatory to maintain any warranties. The gap is filled with thinset as well as the tape to unitize the overall cement board substrate. The thinset in the gap will allow a monolithic seam to be formed further strengthening the installation.

Stone tile installations absolutely require a second course (layer) of plywood. This layer is not to be glued to the first and is not to be nailed to the joists. This second layer is to lap any previous seams by a minimum of 25% as stated above.

Wood floor structures intended for a ceramic tile installations require a deflection rating not to exceed L/360. A stone tile installation requires the second layer of plywood and a deflection rating not to exceed L/720. Twice the strength required for ceramic tile. Joist style and spacing can be an issue.

State your joist style and size, your joist spacing, and the free-span of the joists and we can give you your deflection rating as it stands now.:) It would be good to also know the wood species of the joists if known.

None of this is as complicated as it may sound but if the specs aren't followed very costly failures can occur.:)

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