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Question DetailsAsked on 5/11/2017

Thoughts on laying new porcelain plank tiles over existing thinset mortar?

Removed tile was travertine marble. The thinset has holes, and cracks

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1 Answer


Obviously you have reservations about this, and with good cause. While I guess putting new tile over old mortar bed or thinset can work out OK sometimes, I doubt "sometimes" gives you a warm fuzzy feeling about this, particularly since you presumably intend this floor to last decades with minimal maintenance. Clearly it leaves you with some unknowns and doubts.

Yeah, going to be some cracks and "holes" in the thinset if you used an impact method to remove the tile. Obviously, if the "holes" broke cleanly from the substrate instead of cracking through the thinset (which is pretty brittle) and leaving some well-bonded thinset on it or removing some of the wood when it came up, that clean break indicates the thinset was not fully bonded to the substrate so a cause for concern in that case. And of course the more cracks in the thinset the more chance pieces will detach and result in tile debonding from the subfloor.

On the upside of leaving the thinset in place:

1) save maybe an hour of labor (on floors) per 10-25 SF (manually removing) or per 50-150SF or so (using power tools) on DIY removal of the thinset, or about maybe $1.50-3/SF (likely $300-500 minimum charge) if a contractor is doing it with carbide or diamond grinder. Here are a couple of links on general costs and methods:

2) avoids the dust of breaking up or grinding off the thinset

3) moves job along faster

On the downside:

4) I would guess 9 out of 10 top-notch tile contractors would not touch such an approach because they would not know how well the thinset is bonded to the underlayment and to itself (with the cracks), and whether the underlayment is securely screwed down or is going to start flexing and popping up and crack the tile surface.

5) your finish floor will be higher by the thickness of the existing thinset, relative to adjacent flooring and cabinets and such, so might make for an objectionable transition height.

6) you have to grind (power disc grinder, circular disc floor grinder, or by hand with scouring block) the surface relatively flat, both so you do not end up with alternating very thin and very thick thinset areas on the new install, and also so you knock off all the high points and ridges so you do not have stress points under the new flooring which can induce cracking as the new flooring "bridges" over the high points during installation and grouting - and of course larger or long tile like plank flooring will be worse for this than smaller tiles. And yes, the grinding is messy.

7) with porcelain planks you have to worry even more than usual about variations in the surface level or you can get very visible "waves and wobbles" in it - it is tough enough to get it right with that product without messing with existing variations in the surface the thinset is going on.

8) thinset is called that for a reason - it works best when thin and tends to debond when thick because it does not set correctly when placed in thick layers, so that is a problem if the existing thinset is not ground fairly flat before placement.

9) leaving existing thinset down undoubtedly violates the manufacturer instructions for tile placement, so manufacturer warranty will be useless in the event of tile cracking (and a lot of plank tile flooring is having this problem because the product is basically a violation of common sense in using porcelain or clay based products) - and if done by a contractor he might limit or void his warranty too.

10) after removing the tile (assuming it was hammered/chiseled off) you don't know how well bonded the thinset is to the substrate and to itself - so you could have a problem in the future with it popping free and flexing free ofthe substrate. When it does that bits or thinset and/or splinters get in under it, effectively jacking it up bit by bit as it flexes - so the detached areas will progressively bulge up more and more similar to frost heaving of poles and posts.

11) leaving thinset down is not likely to leave a good result if the floor surface needs levelling before putting the tile down - I would not count on proper bonding of some floor levelling compounds to thinset, though cementatious ones should work fine unless an epoxy thinset.

12) why not do it right and remove everything down to the substrate, make sure the substrate is correctly prepped and fastened down (and levelled if necessary) so you are doing the job "right". Generally taking shortcuts is a mistake, and if you have a failure because of it consider how you will probably kick yourself. I can tell you I learned early on that your bottom gets tired of that real fast. Granted, the removal is pricey or a lot of work, depending on how it is put down and how well it bonded. I have seen thinset on sealed / unprepped concrete or which was put down on dry subfloor pop up in several foot sheets when removed with a power chisel or even manually with a heavy duty ice scraper - though if well-bonded if can also be a real pain to take up. One thing to note too - if this has a mudbed under the thinset, except by grinding with a floor maintenance machine, you will not be able to remove just the thinset - impact methods will break them up together if they were done right, so you will be down to the substrate anyway in that case.


Generally I have found (on well-bonded thinset which does not just pop up easily with a hammer or ice scraper) that a wide flat but moderately sharpened chisel in a rotary impact hammer (using impact mode only) works best on thinset over wood substrate or concrete backer board - preceded by a thorough "thumping" of the entire surface with a heavy hand sledge or blunt-tipped hammer head on an impact hammer to pre-crack it. On concrete slab for an area of any size I go for the electric jackhammer with broad (6-10 inch) dull chisel blade like this -

The idea in each case is to get it started (by intensive hammering on one tile) in under the thinseet, then use the chisel prying action to pop it up, "sliding" the chisel blade along the substrate at the interface with the thinset at about a 30 degree angle from horizontal. Can also be done by hand with a 6-8 inch hand flat chisel and 20-48 ounce hammer, but that gets old pretty quick unless you are looking to build armm muscles like popeye. (Wide chisel to minimize gouging into the subfloor). Actually, an hour or two of that and you will be best friends with the Excedrin bottle and a cold compress for a couple of days, and might even be lucky enough to get carpal tunnel syndrome or "tennis/pitcher's elbow" at no extra charge.

Epoxy and sanded thinset (latter is pretty rare on newer floors) comes up harder than straight cement thinset by maybe 25% or so lower production by chiseling or hammering, maybe 50% slower and burns through the grinding disc MUCH faster on disc floor maintenance machines - can triple or quadruple the disc cost because of the burning-on (epoxy) and faster wear (sanded thinset). [Discs cost $50-100 typically and a machine can have from one nine depending on size, so can get pricey]

Answered 3 years ago by LCD

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