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Other than doing a wire-reinforced mud job, tile setters seem to agree that a cement backer board is the best underlayment for a tile floor. I want to know why. It seems to me that the additional stiffness that an equivelant layer of plywood underlayment would add to a floor would outweigh any benefit offered by cement board.
The only answer that I’ve heard to this question is that the cement board is not subject to water damage. This seems spurious to me because the cement board is not water impervious and must be supported by plywood (or OSB) that can be damaged by water.
What gives?
Andy
Replies
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Andy, the cement board has similar characteristics to that which is applied to it....Think of it as an engineering transition layer....Both bonding and expansion rates as well as the effects of moisture upon the the complete system will have a more durable and lasting inherent quality being that the substrate is closer matched to the final strata.
Near the stream,
aj
*What Jack said,i I think!MDMy feeling is that you get a better bond between cemboard and tile than with ply, (perhaps because of wetness added to plywood in the first place) so far no tile I've laid over cement board has cracked or came loose; good enough to keep using it.
*Andy, I have installed a zillion sq.ft. of tile over plywood and have never had trouble with it. There is no doubt that a laminated plywood floor is much stronger than one with cement board. I don't think cement board adds any appreciable strength to the floor. That said....I am so fed up with bad plywood that I don't use it anymore. Even if it doesn't get wet, I'm always finding problems with missing plys or delaminations etc. I try not to use plywood anywhere in a home.
*Mo' better bond to thin-set I should think."Tile guy" is way down on my card of things I do, but I can't see ever using mortar on plywood. I'd think it would pull the moisture right out and you'd be left with junk. Kinda like setting old dry brick with out giving them a bath first.... and dura-rock smells nice too!BeWell, PB
*Andy, Having just torn up a 20 year tile floor installed over a plywood substrate, I can assure you that properly installed, it is in no way an inferior installation, at least for floors. Could it be that the the cement board allows for a sloppier installation?Is laying the cement board over thinset overkill?Is there anything wrong with hardibacker and the like?Tom
*Mr Engel,
View Image © 1999-2000"He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss the abyss also gazes into you." Friedrich W. Nietzsche
*Had an interesting experience trying to set slate on plywood in the main bathroom. I used a Quikset brand mortar and the next day i could pull all the slates up by hand, using a scraper to shovel the mortar off. Went to a masonry supply and got a product called Spec Mix, re-laid the slate as before except with the new product, grouted it with same, and it is totally not coming up six years later. Maybe the CB is more forgiving of a bad product or poor mixing?
*Wood is cellulose. It may organically decay when repeatedly exposed to a moist environment. Doesn't take much, just enough for the bonding medium (usually thinset) to lose its tooth.Your right that cement board isn't structural...but it is inorganic. When exposed to moisture, the moisture may indeed wick through and wet the cement board...but the cement board won't decay from the wetting and the bonding medium won't release in the process. Same with a full mud job. The mud can get wet, but will eventually dry...all the time holding the tile.AJ's right on about it being a good "meat" in the structural tile sandwich...it can be thinset and screwed to the bottom piece (subfloor), and it accepts thinset to bond it to the upper piece (tile). It's a good transition between wood and tile.Cement board is not a cure-all, and problems invariably arise when installers try to use it as such. Used judiciously, it's a "good thing."
*G.LaLonde,So if you don't use plywood anywhere in your house, I'm assuming you use OSB and that you aren't having problems with delamination there?I have had clients specifically ask for plywood, and I, too, have had problems in the past with delamination after just one rain before the roof was on. Haven't had that with OSB - just wanting to see if you are using something I should look at.Peggy
*Peggy, This subject has been discussed many times here, so maybe you want to check back in the archives for more chat. 25 years ago, I used fir plywood everywhere on a home and rarely had trouble with it. Now all we can get is SYP junk that is poorly made and very susceptable to swelling and delamination if it gets wet. I have heard people mention that the company "stands behind it" when this happens and will replace it....just what I want when I'm in the final stages of framing a home....start cutting your floor out!I use mostly LP Top Notch and am about to try Advantech because it has just become available here. I have never had OSB come apart. The worst case scenario is that it swells at the joints so I give it a lick with the floor sander along the joints.If someone insists on plywood, I warn them of the potential problems and make it clear that we will do all we can to close the place in as fast as possible, but they will pay for any costs resulting from plywood problems either before or after the home is finished.The final straw for me was when I had to go tear up a hardwood floor over plywood. The plywood hadn't gotten one drop of rain on it, but still delaminated just from poor production work at the factory. (I have had the most problems with the brand that advertises plywood as the "material of choice" )
*There's some sense to those answers. I have to say, though, that I've installed tile in about 15 houses over 3/4-in. T&G DF subfloor, topped with a perpendicular layer of 1/2-in. DF CDX that's glued, edge and joist nailed, and screwed in the field, with nary a problem. The 16-in. O.C. 2x10 DF joists rarely spanned more than about 13ft. 4in. though, so the floors were pretty stiff.Still, I'm thinking an extra layer of HardiBacker might not be a bad idea so that the expansion and contraction can take place between the cement board and the plywood.Andy
*Andy...I installed 6" tiles over the same set up years ago...The homeowner found like 3 hairline cracked tiles during my warranty period(out of a thousand)...I think it did pretty well too. My above post was just an answer to your question...There are many ways to skin a cat.near the stream,aj
*AJ, I think that we all know that homebuilding is always a compromise between budget and quality for a given project. How much is enough? You might say that the pyramids approached the ultimate in quality construction based on their longevity. Were they a good use of resources? I doubt it.The upshot of this is that where people draw the line of compromise interests me. Tile underlayment seemed like a prime example of a situation where the paradigm of what constitutes a workmanlike job might be higher than it needs to be. Andy
*Currently I use cement board and thinset, but in my commercial building that I finished 12 years ago, I used mastic over plywood supported by 25 foot or so trus-joists and mastic over cement in the lower level. No cracks, no loose tiles. Yeah, now I do it right, thinset, CB, thinset but last time committed the ultimate sin - used dry-wall screws instead of powder coated screws. Shower hasn't fallen apart yet, but it is only about one year. So, do you use thinset from a bucket, or thinset from a bag? I asked someone on a different forum that question (here he was referred to as the tile god), and I was told I should read the instructions. Is this what moderation means?Dennis
*>You might say that the pyramids approached the ultimate in quality >construction based on their longevity. Were they a good use of >resources? I doubt it. Actually, it's only the last three (Khufu, Khafre, and Menakure) that were any good at all. They had a few failures before they learned to use a series of stress relieving chambers stacked inside. There's the Red Pyramid which is basically a pile of rubble, and the Bent Pyramid, which started out too steep, and they had to change the angle half way up. BTW, the architect, engineer, and general contractor who finally got it right was a guy called Imhotep. There's only one known representation of him, a little statue about 3" tall, at the Metropolitan Museum in New York. Sheesh -- that's the thanks he gets for all that work?Why bother to mention that? Because it shows that what you're trying to do with this thread -- learn from experience and do better next time -- has been a part of building since, well, the pyramids. ;-)As for being a good use of resources, you're right. They were all plundered within a couple hundred years of completion, so in that sense, they were a total failure. The Egyptians went to hidden tombs after that, which didn't work a whole lot better except for Tutankhamun.-- J.S.
*Mr. Loeffler,
View Image © 1999-2000"He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss the abyss also gazes into you." Friedrich W. Nietzsche
*Andy- I would have to agree with Aj and Mad Dog- cement board expands and contracts at a rate much closer to that of the tile and or bonding material. Wood is considerably more active than ceramic or masonry. The backer board is an excellent buffer with good "tooth". As Joe suggests the key to staying trouble free is in the framing...
*Were the pyramids a good use of resources?Well all I can say is that I cheched on the three at Giza this summer personally and there in pretty good shape and drawing a steady stream. You really have to stand next to the big one for a while for it to sink in that you looking at one dam ambitious project, and after you do a bit of climbing on it it's clear that in another 3000 years, people or no people, IT is still going to be there sitting in the sand soaking up the sun.If people are coming to visit any of my jobs in 5001a.d., I will consider them an even greater success than I already do. Wood is good, but stone really sticks around.BeWell, PB.
*My understanding is that Denshield is waterproof, answering the problem of water wicking through to a wood substrate. I've never used it...is there a reason no one has mentioned it? Cost?
*Isn't Denshield a gypsum board? Is it a tile substrate?I've used Huber's Advantech and liked it very much. No end swelling after considerable exposure. The Tile Council of America doesn't recommend any OSB under tile, though, last time I checked. And Huber isn't a member of the APA, so there's no help there.Bonsal (NC) makes a waterproofing membrane (paint or roll on) that is rubbery and they tell me you can put it on plywood, then lay your tile on thinset and you should have a good installation. Looking into that one.
*> The Egyptians went to hidden tombs after that, which didn't work a whole lot better except for Tutankhamun.Tutankhamun's just worked a little longer than the others, but it, too, was plundered in the end. The one's that really work we'll never know about.Rich Beckman
*3 Reasons:1. Wood expands more than mortar and cement board;2. It is dificult to waterproof plywood3. The thinset is not designed to bond against wood very well, although one could use expoxy which is messy.Your point is well taken that plywood will stiffen a floor better than cement board, but that is compared apples to oranges. Before one lays tile, one assumes that the substrate is sufficiently stiff; if not, one stiffens it up, usually with more plywood. Then you lay the tile. The cement board is only a setting substrate.
*The other thing I didn't add is that in the time I worked as a carpenter and a builder, I never saw anyone use cement board underlayment. Tile was always set on plywood or on a mud job (and darn few mud jobs). As Gabe says though, that doesn't make it right, just regional.If you read the latex-modified Thinset bag, I think you'll find that it's approved for use on plywood.Andy
*Actually, there are no more undiscovered Pharaohnic tombs. We have a complete list of the Pharaohs, and all have been accounted for. That in part is how Carter found Tut. There may be undiscovered tombs of lesser royalty, though. -- J.S.
*I have been using Denshield for about three years, walls and floors. I find it to be a great product, much easier to work with than cementboard. Cuts like drywall and a good third lighter.Bryon Kuntz
*I can tell you this much, I used to lay tile on 1/2 plywood screwed to 3/4 osb every 4-6inches, used the modified mortar and had cracked grout and tiles but it always got guaranteed thru the supplier because the TCA said it was approved. Now the TCA says minimum 5/16 cement board layed on mortar bed and mechanically fastened. I now use 1/2 cement board on mortar bed and have very few problems and the supplier still guarantees it. Other than those reasons, I can't help...........
*We usally attach chickenwire to the floor if we're laying over plywood. And use that acrylic "milk" when mixing the thinset. It's got really good grab, and haven't had many callbacks because of grout cracks. I don't know if this is any better than cement board tho. Any ideas?
*What comes to mind from strength of materials class, is "stress riser." Any surface is more likely to crack at a stress riser, which is simply any irregularity that tends to concentrate stress in one area. I would think this would be at the subfloor joints. My understanding of the correct application of cement board, is the joints are taped and covered with thinset. If properly done, this pretty well eliminates joints as stress risers. Assuming the floor is sufficiently stiff, there is no place for a crack to start.Does this sound right?
*GHessssss I cant belive this is still going on!!!Anything behind tile is going to be fine,,as long as water( you know the stuff we shower with??) dosent get to it, and Cement Bd, wonder Bd. the 3'X5' sheets( why dont they make that stuff 4X8??) cannot be afected by water at all, you can submerge it in the lake for a year or so with no effect,,so use it behind tile not sheet rock,,unless you want to lay up 1" of concret like they used to . Jmihcael22@aol.cpm
*GHessssss I cant belive this is still going on!!!Anything behind tile is going to be fine,,as long as water( you know the stuff we shower with??) dosent get to it, and Cement Bd, wonder Bd. the 3'X5' sheets( why dont they make that stuff 4X8??) cannot be afected by water at all, you can submerge it in the lake for a year or so with no effect,,so use it behind tile not sheet rock,,unless you want to lay up 1" of concret like they used to .Plywood would go as well, Jmihcael22@aol.com
*One more comment...I use 5/16" Durock which comes in 4x8 sheets. As I said before, I don't think it makes one bit of difference if you use 5/16" or 1/2" on a floor (neither does Durock). It is cheaper, lighter , easier to handle and is a common size that covers more area per board. If you are depending on the cement board for your floor strength and rigidity, you are already in trouble.
*I have always been told by the *experts that Durock is used because it does not flex as much as wood and is dimentionally more stable than plywood with moisture. Grout doesn't have as many cracks if it is stable. * DDefinitionof an expert: Ex is a has been and a spurt is a drip under pressure.
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Other than doing a wire-reinforced mud job, tile setters seem to agree that a cement backer board is the best underlayment for a tile floor. I want to know why. It seems to me that the additional stiffness that an equivelant layer of plywood underlayment would add to a floor would outweigh any benefit offered by cement board.
The only answer that I've heard to this question is that the cement board is not subject to water damage. This seems spurious to me because the cement board is not water impervious and must be supported by plywood (or OSB) that can be damaged by water.
What gives?
Andy