Thinset and Luan [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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07-12-2019, 06:24 AM
Hello. My son bought a 180 year old house (against my advice!!!), and I am helping him rehab the house. As expected, nothing is plumb, and nothing is level. In what is going to be his kitchen, we had to pack up the floor in areas with 1/4" luan, to get the floor close to flat and level. We are ready to have the floor tiled. One of the tilesetters that he called in tells him that, instead of using cement board under the tile he would be better off to use 3/8" plywood. His reasoning was that thinset does not adhere well to luan, and will crack and crumble over time. I had never heard that before. Is this true? Are there other thinsets that will adhere?

Thanks in advance.

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07-12-2019, 07:22 AM
Luan is not an approved substrate for any thinset. When it gets wet from the mortar, it'll buckle, and yes, it can crumble. It's one step up from cardboard. If the flood needs leveling, then the only industry approved method is pouring a self-leveling underlayment. Get a hold of the manufacturer's tech data sheet and follow the directions to a T.

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07-12-2019, 07:40 AM
Thanks for the quick reply.

We used floor leveler (not self leveling) to feather out the areas where different materials met. We then covered it all with luan and stapled it down heavily. It would be virtually impossible to remove that luan.

What is/are our best path forward? Is 3/8" plywood over the luan and no cement board a recommended path?

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07-12-2019, 11:35 AM
With Luan in the system I don't think there's any industry-standard that you could follow. I don't know why that guy said don't use cement board. Plywood it's not going to make a difference, aside from the fact that you typically thinset the cement board in prior to screwing it down. At this point I think your best bet would be to use cement board and to screw it down very well. Use the manufacturer recommended schedule and add a few for good measure.

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07-12-2019, 12:28 PM
I believe his reasoning was that thins etc would adhere to plywood but not to luan.

Is adhering tile to plywood rather than cement board okay? If we use 3/8" plywood, the total floor thickness before the tile is installed will be about 1 5/8".

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07-12-2019, 03:58 PM
Truthfully, tile should NOT be adhered directly to plywood. A surface covering is required. It could be Ditra, Greenskin or a thin, cement board.:dance:

07-12-2019, 06:01 PM
Understood, but the luang is already down. The question is, if I install cement board, it is supposed to be installed with thinset underneath it. Supposedly, from what I have learned, I should not apply thinset to luan.

Therefore, it seems to me that I have no other choice but to lay down 3/8" plywood, and the thinset and then tile.

Which is less desirable?

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07-12-2019, 06:13 PM
Hi Jeff! :wave:

The thinset over Luan isn’t what folk are objecting too, it’s the luan itself.

It’s your choice, but I’d remove it given that the voids in the luan can cause failure of the tile. If you want to keep the luan, you should put a minimum a 1/2” BC grade plywood over it and then a decoupling layer such as Ditra or 1/4” CBU.

07-12-2019, 07:44 PM
We understand that the luan is already down. That's your mistake. Continuing on that path because it's already down is like saying you're continuing to smoke because you already opened the pack.

07-12-2019, 08:00 PM
Luan tends to be soft and has some oils in it that prevent thinset from bonding well. It can work well under sheet goods (linoleum, vinyl, etc). Not so much tile.

FWIW, the thinset underneath cbu isn't to hold it down, it's to fill any voids so that the board is fully supported. The anchors (screws or appropriate nails) hold it in place, but the moisture in the thinset can delaminate the luan.

Some floor levelers don't really like to have fasteners through it, breaking the bond and potentially cracking. When you do use one, you want one compatible with the next layer as well.

07-14-2019, 12:41 PM
I think the tilesetter with the plywood suggestion is trying to essentially salvage the project done wrong. Because of what you read about the luan here, it's not good for tile. And cement boards technically do not add strength to the floor (which imo is sort of debatable, but no manufacturer will advertise a cement board as adding floor strength to already bad floors...) whereas a layer of 3/8" plywood will add strength to the floor and lessen deflection of the floor, because you could likely walk across a sheet of 3/8" plywood without it breaking held end to end, whereas you likely could not do that with a cement board. The install would not be "industry standard" due to the luan messup, but it would be the closest to such without removing the luan and everything else. You'd get no material warranty from any mortar manufacturer/etc over such an install, and the installer for any warranty probably should waive any liability on his end for the installation not being industry standard in the contract.

There are thinset mortars that will adhere direct to plywood, but you need to wet the plywood first with a sponge and water or else adhesion will be poor, due to the plywood sucking moisture out of the thinset. Cement boards are mainly used on floors as it's a better adhering surface than plywood, but the mortar manufacturers will warranty their mortars being used directly over plywood. You'd want to use a mortar that's at least ANSI A118.11 or A118.15 rated, ideally 15 in this scenario as that is the strongest tile mortar standard and this install would be pretty on edge. So A118.15 would be something like Custom Prolite or Flexbond. You'd also want to mix it exactly to the manufacturer directions including slake times. You could as well add a 1/4" cement board for a better bonding base, on top of the 3/8" ply, but this would raise your floor height more.

As far as how badly you messed up with the luan, it depends. Nobody really knows for sure, as this site doesn't really discuss things in the realm of "well, maybe it could work, because my friend did this and..." it discusses industry standards. The tile industry has standards and entire books related to standards and exact instructions for installs that is valid in court. So if you decide to go forward with your already done luan work, nobody really knows how successful it could be or not, as it's not a valid industry standard. That said, I did work in a house with tile over 1-2 layers of luan in bathrooms and kitchens, tile looked to be about 15 years old, from the early or mid 2000s. (Saw boxes in the garage...) The only cracked tiles I saw were in the kitchen from water damage. Again, the site doesn't like to discuss things in the realm of "Maybe it will work" it only likes to discuss and advocate proven standards.

07-14-2019, 01:09 PM
Thanks for your insight!

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clifton clowers
07-15-2019, 12:54 AM
Fwiw, I live in a 120 year old house (also bought against my parents' advice). When I gutted the kitchen floor tile, I found the tile was installed directly to lauan that had been nailed and glued to the original hardwood. Everything about that installation is wrong, but it had lasted for well over 50 years without a crack. It was miserable to take out. (it was replaced with 3/4" advantech + 3/4" exterior ply + liquid backerboard).

My suggestion - take some of the lauan scraps and soak them in a bucket for an hour or three to see if they come apart

07-16-2019, 01:59 AM

What is "liquid backerboard"?

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07-16-2019, 09:03 PM
What is "liquid backerboard"?

A product by Ardex.

Tool Guy - Kg
07-16-2019, 09:54 PM
We are happy to talk about 'what if's?' on this forum. But realize that we don't want to be full of conjecture for our visiting guests that are doing this for the first time and don't have a keen ability to know which one of us typists to believe or not. For that reason, we generally like to have that type of discussion in the Pro's Hangout ( The thought behind this practice is that most pros know when they are treading on thin ice and can more readily know what advice may be good.....or stupid bad for them. But don't think you *have* to be a pro to visit the Pro's Hangout. All are welcome. It's a perfect place to ask wild and crazy questions or get into a discusion about doing things outside the 'manufacturer's instructions'.

But for the Tile Forum/Advice Board that your thread is currently in, we like to keep it to industry standards or manufacturer's instructions to keep anyone from being fooled accidentally. Tile projects can get expensive and this is a good way to protect our visitors investments in time and money. If we veer outside "da rules", we ask that folks make it plainly obvious.

Getting back to what some of the other folks have said about lauan. If this helps shed light on how some projects with lauan last a long time and some a very short time, it's noteworthy to understand that that there is more than 50 different grades of lauan alone. Some interior and some exterior. The stuff you get at the box stores is typically interior with non-waterproof glue. Between that and the thin outer skins over a thick interior core, it's not good to get it wet, as it's dying to delaminate. With the amount of moisture that is in mortar and the length of time that the lauan would stay wet, it's a practice to avoid.

Could you do something outside the industry standards like coat all the lauan with RedGard so that it stays dry...then install your cement board and tile? I guess you could. But, like mentioned, we don't know if that would be enough to make your project last. Testing is a drawn out, relatively expensive endevour. And no manufacturer can give a definitive answer on such a unique method like this. Once in a blue moon, a chemist or manufacturer will offer "probably" advice that is based on years of working in the field. But I wouldn't hold your breath for a confirmation on my unconventional exploration of such an untested method.