install hardi backer on a floor

I am having a lisenced tile contractor do a tile floor installation for me. Not Knowing how to do the job, I asked him about the installation method. He told me the backer board was to be pasted down directly to the wood floor. I asked if there would be a problem with the moisture forming on the bottom of the cement product against the wood floor. I don't know much but I do know that concrete is not to be in direct contact with wood, especially when the room is warm and the floor is coldthere could be a condensate problem. I'm not sure of his experience level, but he said that he would just glue the board down to the wood floor with construction adhesive. I don't know the best way, but I thought maybe just a vapor barrier of some sort. He is using 1/2" board over a 3/4" subfloor. Any thoughts or suggestions? Joe

Joe - I am only a DIYer. Using construction adhesive to 'glue' the hardibacker to the subfloor is not a proper installation method. The hardibacker should be thinsetted to the subfloor with unmodified thinset and screwed or nailed down.

If it were my place, I would be adding 1/2" of plywood (not screwed into the joists) to increase the strength of the floor before thinsetting the 1/4" cement backer board.

If possible, respond back with your joist size, joist spacing and the unsupported length of your joists. Also are you planning to use ceramic or stone (ie. marble) tiles? This will help to determine if the floor is strong enough.

Not sure about the condensation issue, but think that you don't need to worry about it.

Thanks for your input. I am using 9 1/4" I joists spaces @ 16" O. C. w/ 3/4" osb t&g sub floor. I believe there is very little deflection due to the strenth of the I joists. Our biggest consern was that we would not exceed 3/4" in height due to the abutting wood floor.
I am starting to think that we should have used 1/4" backer board over a thick bed of thinset. As far as the moisture barrier goes, the floor insulation probabally would not let condensate form at the concrete to wood transition. Although, I see that this is a typical installation over existing vinyl floors. So I suppose if a person was conserned about the moisture issue, you could simply put down some building paper beneath the thinset morter bed.

You don't need to be concerend about the moisture issue with this. What I would be concerened with though is that this guy wants to use construction adheasive.
The way it needs to be done is thinset under the C. B. U and then nailed or screwed every 4 to 6" in the field and then every 2" on the seams. Then the seams taped with an alkalie resistant mesh tape and the thinset over that (flat troweled).
You want to use the 1/2 C. B. U with this cause you need at least 1 1/8 of subfloor for tile intsallation.
The only other thing I would want to know is the span of the joists between the beam

Oh find another tile guy too

There's a few things I'd like to clear up, but first, I should say Hi, as this is the first time Im posting on this forum.
Hmm. well. Hi.

Alright, first off, the reason thinset is called that is because it's meant to be, well, thin. A thick bed of thinset as Joe mentioned is not a good idea. Next, the recommendation of another layer of plywood is to strengthen the area between the joists. Joe, your 9 1/4 I-Joists may more than capable of handling the load, but the 3/4" OSB can flex quite a bit between the joists. The plywood helps limit that deflection. I agree with the 1/2" ply and 1/4" hardi (or durock, or wonderboard) advice. The backerboard doesnt add anything structurally, (or very little and you're not supposed to count it) so dont expect the thicker backerboard to help strengthen anything.

The construction adhesive thing is really bad. Please tell that tileguy to pick up a book before he takes on any new work. Backerboards should be set in a bed of thinset. I usually use a 3/16" notch trowel with the pookey a little on the runny side.

Alex

Joe what are you saying about vinyl?

Paul- I believe the suggested unmodified grout would not adhere to the vinyl, so I'm guessing the floor would be basically free floating, instead of being bonded to the sub floor as one solid unit. So what would the difference be between vinyl opposed to building paper beneath the backer board? Either one would not allow a bond.

Joe, reread Floorman's advice in reply #4, seriously

You need, not should, but need to embed cement boards into a bed of thinset. It fills in the little voids between the subfloor and the backer. You should use unmodified thinset under it because you don't want to adhere it to the floor, you just want it supported. The unmodified thinset will shear from the subfloor durring seasonal adjustment.
The difference between roofing felt and vinyl is vinyl and it's underlayments compress, shrink, delaminate, the vinyl can have pockets of air under it, some underlayments will swell with very little moistrue and the most often used ones are too thin and unstable to have under tile, even with a cement board. Roofing felt can also trap air under the tile if used under a cement board. Roofing felt should only be used wither on walls behind the backer in wet areas or under a full mud job as a cleavage membrane, but not under the backer. The voids it could leave behind would jsut invite verticle movement. As for what you see about tileing over existing vinyl floors, it should not be done, period.
Ok, so you see and read on bags of thinset or backer board instructions about tile over vinyl. Read on and you will find an exclusion about tile over cushioned vinyl. So what kind of vinyl does that leave to tile over? VCT, and even that is a crap shoot.
So where are you that the tile isntaller is a licensed tile installer. He might be a licensed contractor but most areas do not require a license for tile. Even if he had a license specific to tile, he is obviously without a clue when it comes to how to install.

Thanks to all of you. My installer and I were able to straiten things out thanks to the tips you gave me. Joe

Hi Joe - unfortunately, sometimes it is unavoidable to have the surface of the tile slightly higher than the surface of the hardwood in order to ensure a good install. I had the exact same issue at my place. A transition (either metal or wood) is required in these cases. Schluter makes some metal ones like the Reno U that I used in the photo below.

http://www.schluter.com/english/products/2002/sectiona/overview-a/section-a.html

Yeah, the schluter looks like just the ticket. My carpet guy had metioned a this transition, but he couldn't remember how to pronounce it. Thanks again

All - I've read this trail of advice and still am a bit confused. Based on what I think your suggestions are this is what I'm thinking of doing. I'm putting ceramic tile in my kitchen, the floor has 10" (maybe 12') joist, with 1x6 boards laid diagonally. There is tar paper, plywood, and carpeting on top. Everything down to the 1x6 has to go. I plan to put tarpaper or a thick plastic shield down, then 1/2" plywood or OSB (which one I need advice on), the a thin layer of thinset, then 1'4" of backer board screwed in, then mortar and ceramic tile. My question is do I have the sequence right, is OSB ok, tarpaper or plastic, and if I screw the backer board does it need to be set in thinset?

No tar paper or any barrier, just put the plywood or the OSB on the plank flooring.

Use Schluter Ditra instead of cement/backer board and you will avoid two things:

1) No height issue because ditra is 1/8" thick.
2) You won't have to install another layer of plywood because ditra is good with 5/8" or greater ply or osb subfloor.

It's also easier (and lighter) that backer board and not much more expensive. Actually if you add the cost of the extra plywood you need and backer board it's CHEAPER and a lot less labor!

Never use unmodified thinset for hardybacker rooky moove

Say what?

All the rookies please take one step forward.