How Much Thinset Has to Come off Old Plywood?

I have a mid-sized entryway that I ripped up all the old 10x10 ceramic tiles from, using a crowbar, hammer and chisel, lots of sweat, and the mouth of a sailor. The house is 20 yrs and I think the tile is original from the looks of the black grout that was originally white (bought it 2 yrs ago).

My goals was to pull up the tile & backerboard and re-do the whole application. To my surprise, it was tiled directly over plywood - no backerboard. [Now, as a side-note the interesting thing is that this defies a lot of what has been said on this forum about durability issues over plywood considering this job lasted 20 yrs, but I don't doubt this was a risky thing to do].

My problem now- I can't raise my floor up any higher as I am butting up to 2 rooms of hardwood. I plan on using Ditra after everything I've read here regarding movement control and thinness (golf clap appreciated for being adventurous) and I'll make sure to read the tutorial first. I'm not necessarily a newbie to tiling and this would be the 6th tile project I've tackled. Just new to Ditra.

My question is (because I'm getting very weary of this project) - How clean does the plywood have to be from the old thinset/mortar left behind before I can lay the Ditra down with a fresh application? I have taken a hammer and chisel and popped up all the elevated pieces of mortar. In some stubborn areas, I used a water bottle sprayer to moisten the mortar and it popped up much faster. There are areas where there are thin lines of mortar on the plywood that are not raised up and when I run a scraper over it, not much more will come up. I'm trying to avoid a belt sander because of the dust which affects my daughter.

In some areas, I even took a sponge mop that had a hard scrub-brush on the other side and I've run the mop over it and scrubbed up as much residue as possible (and promptly towel-dried the areas).

So how much is enough to ensure proper adhesion for the Ditra? For example, in a 12"x12" space, 85% of the plywood is clean and completely free from mortar - even better in some areas. I'm not trying to cut corners, but I'm certainly trying to reduce overkill if possible.

Any help is appreciated! Thanks! Laura

The substrate should be "plane". Stains from the previous thinset wouldn't be an issue but humps and bumps would be. The DITRA requires a flat surface, not necessarily clean to the eye but "flat".

The fact that the previous installation lasted as long as it did may well have to do with the fact that yesterday's plywood is not necessarily the same as today's plywood. We discourage installations directly to plywood simply because we have no way of seeing what it is a DIY'er is actually doing. Installations directly to plywood usually requires some special effort and products. It's not that it can not be done it is the fact that it can be a highly problematic installation.

In your case I would think you could go right back over the plywood if you use the proper setting materials.

Thank you Bud - I'm pleasantly surprised to hear this since I've seen so many of your posts caution against it, but then again, most of them involved the installation of new plywood, not tiling back over existing plywood that has already been installed with specs to support a tile floor. The floor is sound (no creaks) and level. Not only that, but I am only talking a 8x10 ft entry way.

Here's the next tricky question. I'd like to use 20x20 porcelain tiles. Previously there was 10x10 ceramic (fairly heavy stuff with red clay under the white glaze). So do you still think I could go back over the plywood, provided I use the right setting materials and do all my homework?

If so, I don't know what you mean by "stains" -do you mean the grey sandy residue left behind after I've scraped everything off? In most places, the clean grain of the wood prevales, but some spots there is still a light grey residue covering the 10x10 vacant tile spot. It's a flat plane for sure, but not 100% off the wood grain. Perhaps this is overkill because the majority of the plywood floor is bare.

I'm not trying to save money by doing it myself, I actually (call me crazy) love tiling as an art form almost and find it gratifying when done right. I'm a very hands-on person. I appreciate how tricky a job it is.

If 20x20 is not appropriate over plywood, is it better over Ditra? And will the Ditra stick to the plywood (provided I use the right ratio of mixture) even with some of that residue still behind?

Obviously, a new layer of plywood would solve the problem, but I have this huge issue with trying to maintain uniform plane butting up to the hardwood in the next room.

Thanks for your expertise. Laura

If there are no humps and bumps you should be OK. In this case I would go right over the plywood. Any small amount of old thinset residue will be taken-in with the new application of thinset and shouldn't cause any problems. The twenty inch tiles don't matter but keep in mind---the bigger the tile the flatter the substrate needs to be.

I would use unmodified thinset and mix it (myself) with the compatible additive to make it site-modified. This will give you a better bond for the plywood. In this case I would use Mapei's KeraBond thinset mixed with their Keralastic additive.

I see no need for the DITRA in this case even though I am a huge fan of DITRA.

I should also add that you want to be sure to use the proper notched trowel. In this case 1/4" X 3/8"(deep) notch will likely work but you'll also want to flat-skim the backs of each tile after you comb the thinset onto the substrate.

Comb the thinset evenly in one direction only then after skim coating the backs of the tiles place them in a manner that will cause the combed thinset peaks to collapse into the combed thinset valleys.

To be absolutely sure that is the correct trowel after you have set a few tiles, remove a couple of tiles to check for sufficient coverage. Eighty percent is generally acceptable for coverage but your target coverage should always be 100%.

[Edited by Bud Cline Tile Contr on Sun, Aug 24, 2008 at 9:31 pm CST]

Fabulous! Thanks Bud, and thank you for supplying the specific manufacture names for the products to use and trowel size. I will see if any of the trowels I have a match before buying a new one. I'm so excited to get this project wrapped up and get my new tile down.

I'm going to go around the area of the floor and make sure everything is level. Plus I bought one of those little circular tile levels (looks like a hocky puck with the bubble in the center) -not sure if it's just a gimmick or not but thought it might help if I check each tile as I go.

Ah-h-h-h-h! The ole "Tavy Puck". Good luck with that thing.

I saw this at HD when I was tiling and thought it might be useful for a person doing tile for the first time.

I passed because I could not figure out how this was going to get me a flat floor. I thought I would end up with all the tiles perfectly level but a different heights.

I almost think the best tools would "sticks" that have a perfectly straight edge. So the floor ends up flat.

JimR

In the past, I've simply used a 3 foot long level and laid it across several tiles at once and worked my way down the row with it and did the same process in the perpendicular direction. It worked pretty well but it's a tedious task. Then again, so is tiling in general I just thought the "puck" might also do the trick, but I see what you mean about it being level but at different heights. hmmm. Guess that was a waste of money then. But it looked so shiny, bright and pretty.

Bud, you mentioned Kerabond unmodified with an additive. Before posting to this site, the rep at Lowes had already set me up with White Spectraset (to be mixed onsite with water) that is recommended for large stone and porcelain tiles (I'm using 20x20 porcelain).

It claims to have an additive for extra bonding. Is this acceptable to go over my plywood or should I take the bags back and get the Kerabond as you suggested? thanks!

Not familiar with the product you mention. My recommendation would be KeraBond (powder) mixed with the compatible Keralastic (liquid). This will let you make your own modified thinset and it will be of a higher quality than some of the modified thinsets you get from the bag and add water to.

You must be careful mixing additives to already enhanced thinset products as this could wreck the chemistry. Some of the "attendants" at those stores don't really know what they are talking about. They are quick to make recommendations with no real knowledge of or experience in what they are suggesting. Don't assume those stores train their employees because they don't.

Go to Mapei's website and research any recommendations you get. Don't just take the word of anybody, including me.

Lowes sells Laticrete products, what she bought was one of the modified versions they sell.

The big box stores in our area sell different things, Menards sells Mapei (Buds suggestion), Lowes sells Laticrete and Home Depot sells Custom - all the above brands sell an unmodified thinset that can be modified with an additive.

If I remember right, the Lowes "version" is Flexbond and I can't remember what the liquid is called.

One better than the other? I dunno, but Bud wouldn't steer you wrong.

Thanks! I think my Lowes also carries Kerabond as I've used it before but not with the Keralastic additive. I checked the Mapei website and the Kerabond instructions says to add with water, but the Keralastic instructions seems ton indicate that it is to be mixed with Kerabond in place of water - does this sound right? I think I'll exchange for Kerabond. Thanks again for your help.

Kerabond is an unmodified thinset. When mixed with water it remains an unmodified thinset. When mixed with Keralastic it becomes a modified thinset.

There are modified thinsets available with the necessary additives already in the bag and all you do is mix them with water.

Using an unmodified thinset (Kerabond) and mixing it with the proper additive (Keralastic) you then have a modified thinset much stronger than what you would typically get using the already-modified-in-the-bag thinset.

Mixing an additive with an already modified thinset has the potential of wrecking the intended chemistry of the product and therefore rendering the altered product useless. This isn't to say there aren't modifieds that can be mixed with an additional additive but that is getting way to sophisticated for these purposes.

Okay, the pooch is screwed - after taking back the spectraset mortar to Lowes I come to learn they have completely ditched the Mapei line they once carried in Louisville (and also surrounding areas in Indiana)! On top of that I went to the Mapei website and called the local distributors on their list and they only sell in large volume quantity.

I know my area to cover is not large enough to support $100 = 5 GALLONS of Keralastic additive! I might as well go over and tile the 5 neighbors's houses down the street after I'm done with mine

So I think I'm stuck buying something at Lowes or Home Depot but I'll look for the unmodified/additive combo that you suggested to make it site modified.

Any further suggestions other than "good luck - hate to be you"? Thanks!

Could someone tell me why this job requires an unmodified thinset mixed with an additive? Would a premium modified thinset not be good enough?

JimR

Could someone tell me why this job requires an unmodified thinset mixed with an additive? Would a premium modified thinset not be good enough?

JimR, typically the general public doesn't have access to some of the "premium modified thinsets" you suggest. Making your own modified thinset usually produces a stronger bonding product than the modified thinsets that are available at the home centers.

I don't usually recommend DIY's install tile over plywood but in this case I did. (see first few posts).

OK Laura, TRY THIS!

At Home Depot you should be able to find the basic thinset "CustomBlend". To this product you can add "Thin-set Mortar Admix" or "Ultra-Strength Thinset Additive". I'm sure they will have one or the other polymer additives named.

IF NOT then technically any of the higher priced modified thinsets can be used as JimR suggests. They require exterior grade plywood. When I knew you had Mapei available I suggested the Kera products, seemed like the thing to do at the time.

Thanks to you both! I am indebted for the advice. Before your resopnse came in, I purchased the Lowes Latricrete Megabond unmodified and the suggested additive of the same brand. Man, it was like spreading bubble gum! My arms are sore.

But those tiles are now solidly in place -- those suckers aren't going anywhere! I had to work quickly and since I was laying 20x20 I only spread mortar for 2 at a time. That kept it supple enough to squish the tiles into place. And there wasn't much "squishing" as the product is very very thick.

I cut *all* the pieces prior to mixing up the mortar, so every square inch of the floor was already laid out in a "dry run" before I even mixed the mortar. Otherwise I would have surely wasted product.

They all laid very evenly though and I checked with a 3ft level to ensure the height was consistent. Bud, the trowel you recommended did the trick! That really made the difference in ensuring I had consistent coverage on each tile and also took the guess-work out. When I finish with grouting, I'll poste my final product.

Glad it worked out for you.

You are correct - mixing unmodified with your own additive can create a product that will test your stamina. I probably should have mentioned that!