I do very little tile work but occasionally I get a request for it….FYI.
I need to install 1/4″ Hardibacker over 3/4″ Advantech. The Hardi mfgr says the board is to be EMBEDDED in thinset mortar using 1/4″ notches. I always thought mortar barely held to plywood of any kind so this kinda confuses me. If the product must be embedded in thinset mortar then what is the best mortar for this and can it be the exact same mortar I’ll use to set the tiles? I have a 1/2 bag of mortar that’s about a year old I’d like to get rid of. Can I use that to set the boards?
Or, as long as l’m sure the floor won’t move (deflect at all) can I just screw down the board with concrete screws and not set it in mortar? I ask that because this floor was built over top an old concrete porch. The bottoms of the joists rest on the concrete, if there is any deflection I’d be surprised.
Also since this is Advantech flooring, if I must use mortar will it adhere well to Advantech?
Replies
Maybe overkill, but I always use a premium, modified latex thinset. Multiflex is stuff I used most recently. I would think you would want the extra stick of modified or go with an additive and maybe rough up the Advantech...it is slippery stuff.
Well....what exactly is MODIFIED thinset anyways? I mean, what kinds of thinsets are there? And...can I use this same modified latex thinset for setting the tiles as well?If at first you don't succeed, try using a hammer next time...everything needs some extra persuasion from time to time. -ME
YOu can buy modified thinset, ready to mix with water. You can use it both for the board and the tile.
If nothing else, I think thinset is about the cheapest adhesive for the board to the plywood. Also it spreads a lot easier than adhesive or mastic.
Latex modified thinset is not a bad idea- you could also buy a gallon jug of latex admix, and use your dryset mortar. Mapei sells the latex modifier, as well as a lot of good mortars.
The point of mortar under the hardibacker is to ensure even bedding, and good support of the hardibacker. It's not really there to stick it down, although it doesn't hurt. You'll still need to screw it down with deck screws (1" or 1 1/4", and avoid the joists) or nail it down with roofing nails.
zak
"so it goes"
maybe this is stupid, but couldn't you just use construction adhesive and screws?The adhesive will allow for minimal flex and is weather proof.Not being a tile guy, I don't know if that's too cheap and easy?
You don't want to use construction adhesive. You need near 100% contact between the hardibacker and the underlying plywood, or the hardibacker will flex and cause your tiles to pop or crack. As mentioned before, the thinset fills the small gaps between plywood and cement board. Don't cheap-out; buy a quality thinset. You can use regular thinset with a separate latex additive, or a polymer-modified thinset (just add water). These are stickier and a little more flexible than regular thinset with water. Yes, you can use the same thinset for installing your tiles. I've always used cement screws to hold cement board to plywood; I think the manufacturers recommend them for floor installations. Roofing nails are ok for walls, but if memory serves me, not acceptable for floors.
Conrad
why not a spreadable adhesive and notch trowel?seen it done for hardwood on concrete.not trying to argue just trying to learn.
The backerboard folks specifically specify thinset under their products. While I doubt adhesive or mastic would deteriorate the backerboard, if you are going to go to all the trouble of troweling it on, it's just as easy to use thinset. If something went wrong with the installation and it wasn't done to manufacturer's specs, you can be sure you will take the blame. Why take the chance; thinset is easy to use and you will get the most rigid installation possible that way.
Conrad
Just an added question... does anyone do anything to the plywood before the thinset? I've had an easier job if i mist the wood with just a spray bottle of water... seems to keep the wood from sucking the water out of the thinset as fast.... now this is when it's 90plus outside and hotter inside... so... am i hurting anything doing this?
thanks
p
Ponyti,Dampening the ply before you apply the thinset is just fine. You want a wet mix under there anyhow, since the Hardi sheets are absorbent themselves, and represent a really big tile to bed in the thinset.The thinset under the Hardi is just to resist compressive loads, so even unmodified is okay under the Hardibacker. Bill
Not too wet or the plywood would de-lam.
You might also consider one of those PVA bonding agents, or mix some aliphatic resin glue in your water. It would help with bonding the thinset to the ply.
-- J.S.
you are certainly right about that, best to cover your butt and follow specs, because when the time comes you don't want to start the explanation by saying " well I just figgered. . ." thanx for the info!
Do yourself a favor and get the hi-lo screws specifically for CBU. Roofing nails are OK but certainly have a lot less holding power, and deck screws don't have big enough heads.
In this case "embed" means "to lay in a bed of". The purpose of the thinset under the backer board is to provide full support under the board. It needs to adhere some in order to do that, but adhesion is not the most critical factor, support is.
Without it, there will be areas where the backer board does not quite rest firmly on the substrate below. If tile were installed over such a base then when someone walked on it and stepped in just the right spot over a hollow the backer board would flex. Over time the repeated flexing could crack the grout joints and even the tile.
It is for this reason that construction adhesive is even worse than nothing at all. It would guarantee the creation of hollow spots between the beads because it is not fluid enough to flow completely under and uniformly support the backer board.
I disagree . The adhesive , in its plasticene state, would have to be rigid and hard enough to withstand the force of the screw, an inclined plane, most likely propelled by an impact driver. I envision the bead of adhesive compressing and the sub-floor and backer-board joining as one. It has even been tested. In an attempt to break the bond, the failure was in the plywood not the adhesive. Plus you don't have to introduce any water.
You'll get a strong bond, but the adhesive will not spread out and form a continuous bed under the backer board. It is not the proper method for installing a tile system.
for my backer setting bed I buy the cheapest thinset I can find and mix it a bit loose.
trowel with the 1/4 trowel and slap then down.
I prefer roofing nails to the screws ... for both walls and floors.
the TSet under the concrete boards is to simply provide support ... not adhesion.
Jeff
Buck Construction
Artistry In Carpentry
Pittsburgh Pa
Binding to plywood with adhesive is not a good idea. Plywood expands & contracts while backerboard doesn't. Nor does tile. The thinset isolates the two substrates while creating a dimensionally stable (3 dimensions) surface under the backerboard. Flat surface + dimensional stability = well set tiles.