Home Improvement → How Clean Does Subfloor Need To Be For Hardibacker?

Hey folks,

I'm replacing my tile floor in my bathroom. After removing the old tile, I'm now prepping to lay hardibacker board with a layer of thinset in between. But the old tile left this paper material glued to the subfloor in a crisscross pattern. I've been sanding and scraping and power sanding trying to get as much of it up as I can, but I wonder if I'm wasting my time. How clean does the subfloor really need to be before laying the hardiboard? Just pretty good or extremely good?

Thanks

Did you have the paper material tested for asbestos? I would not be sanding on it at all without doing that.

Really? Was asbestos an old tile backer? At least I was wearing a high quality mask.

Alright so let's assume it's asbestos paper backing. Can i just put a layer of thinset on top and then the hardibacker board or does the hardibacker board need a really clean subfloor?

I would wipe it down with water and a sponge and then use a modified thinset to bed the hardibacker. What are you fastening the hardibacker with?

Thanks. The special hardibacker screws.

If you are using fasteners and the thinset is just being used to fill unevenness, then it really does not matter what is left on the sub floor as long as the hardy-board lays flat. If you are using the thinset as the only fastener, don't.

As for the asbestos, you really should get it tested. If you were doing what you said you were, and if it is asbestos everyone in the house is at risk of breathing that shit in for much longer then the project will last.

The most important aspect of using hardibacker is to make sure that it is screwed in accordance with the manufacturers recommendations. Typically every 6" around the edge, and every 6" to 8" inches across the board. The stuff that I have used comes marked with ink-stamped locations for the screws so you don't have to measure for each screw.

How thick is the existing subfloor? You want to make sure that there is very little flex, otherwise, the ceramic tile will crack. For the 12"x12" tiles that I used in our master bath, the recommended allowed flex was pretty stiff - if I recall correctly, it was something like 1/300" of an inch over 36" inches. Since it was a relatively small bathroom, adding 1/4" inch of plywood over the existing 5/8" inch plywood, plus the additional 1/4" inch of hardibacker gave me a floor that didn't have any perceptible flex.

Welp the good news is that it is a 5x5 bathroom so its pretty contained. Also I had wet it in an effort to help the paper come off easier so there wasn't any noticeable dust created. I'll see about testing. All those tile removal tutorials I read and not one mentioned possible asbestos. Never occurred to me.

The subfloor is either 1/2 or 5/8. Didn't have a great spot to get an accurate measurement. No flex or squeaks though. Tile is 1" x 1" marble.

What kind of tile was on the floor?

There is no difference between fiberglass, mineral wool, and asbestos except patent royalties. All are non-organic, non-metal, and all will cause cancer. »en.wikipedia.org/wiki/Hi ··· ion_wool . The only reason why people aren't dropping like flys from fiberglass is modern industrial hygiene code (aka wear respirators and use a shop vac). IANAL.

It was small ceramic or porcelien. Ugly as sin.

OK thanks everyone. I washed it down and wiped with a large sponge. Then cut out my backerboard and mortared it and screwed it down. I'll make some calls tomorrow and see about getting it tested and if it's necessary or not. There was literally no noticeable dust created so I'm not too concerned but I'll see what they say. Thanks

If that is the case, I doubt that there was any asbestos involved.

That's good to hear. Thanks

the other part about asbestos is that it's generally best practice whenever possible and or practical to encapsulate rather than remove. So aside from the sanding you've already done you did the best thing which is to cover it 'permanently'

If you want to mail out a sample of the glue and/or tile for asbestos testing, try this place: »www.asbestos-test.com

I sent a chunk of ceiling tile to this place for testing. They really do run the test and send you the results the same day they receive the sample (I sent mine overnight express mail). They also have the best price I've ever seen at $30 for the first sample test, and $20 for each additional sample.