Uneven Backerboard

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Old 11-11-08, 09:54 AM
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Uneven Backerboard

Removed existing carpet from 5' x 5' bathroom. Plywood subfloor, not sure how thick. Put down 1/4" Hardibacker with thinset and screws. Unfortunately the slabs of Hardibacker are not quite even where they meet with one slab being maybe 1/8" higher than the adjacent one. Probably had different amount of thinset under the Hardibacker. Can I smooth out the unevenness with further thinset before putting on mosaic marble tile or do I need to take up Hardibacker and relay again to get it even? Help!!
 
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Old 11-11-08, 11:19 AM
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Something is not making sense here. Was the floor flat prior to installing the cement board? You did use screws to secure the cement board to the plywood right? If you drive the screws all the way in, the cement board should follow the contour of the plywood subfloor.
 
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Old 11-11-08, 01:07 PM
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Not sure how flat the floor was as I didn't measuure it. I did use screws to secure the cement board but I also used a lot more thinset underneath some slabs of backerboard than others.
 
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Old 11-11-08, 01:28 PM
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Did you just do this in the last few hours? If so, you may be able to get the cement board up and scrape the thinset off. You should have used a 1/4" notch trowel to consistantly apply the thinset to the substrate. That assures that your cement board will follow the contour of the subfloor. You will need a flat surface to set mosaic tile, and you wont get that by trying to screed thinset to plane the surface.

You mentioned that you didnt really check the subfloor before installing the cement board. You need to do that first. If the subfloor isnt flat, you may need to do something to make it flat first.
 
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Old 11-11-08, 01:42 PM
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The thinset was put in over 24 hours ago so by now its probably pretty hard. How difficult is it to remove the backerboard and thinset once it has set? Is there any other way to rectify than removing the backerboard and thinset, getting the subfloor level and starting over again?
 
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Old 11-11-08, 01:45 PM
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If the floor is solid, you could pour self leveling compound (slc) over the cement board. You would be adding at a minimum, 3/8" to 1/2" to the floor height, and its really expense stuff, but it'll do the job.

It will be a pia to get the board up, and the board probably will not be salvagable.
 
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Old 11-11-08, 01:50 PM
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Just to be clear on what I mean by the floor being solid. There can be no voids or air pockets under the cement board, that is the screws draw the board into the thinset such that there is full thinset contact with the subfloor and cement board. There can be no voids.
 
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Old 11-11-08, 03:00 PM
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How can I be sure there are no voids or air pockets under the cement board? I put down thinset (maybe not enough in some places) and screwed down the backerboard so how am I to tell?
 
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Old 11-11-08, 03:04 PM
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What did you apply the thinset with?
 
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Old 11-11-08, 03:15 PM
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I put some thinset on the floor, smoothed it out with the smooth side of the trowel and then used the 1/4" side to create ridges, then put on the backerboard.
 
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Old 11-11-08, 03:52 PM
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Lets get back to some basics. Do you at least have 5/8" t&g plywood or osb as a subfloor? When kind and size screws did you use for the cement board? Something is still not making sense here. Did you set the screws flush with the top of the cement board? Did the screws appear to be stripping out when you set them?
 
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Old 11-12-08, 04:05 PM
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The subfloor is plywood but I'm not sure how thick. How do I measure this?
I used 1 1/4" backerboard screws. Some went in quite well while others began to strip and could not get flush with the backerboard and are sticking out a little.
 
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Old 11-12-08, 04:11 PM
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You may have mixed the thinset a bit too dry to conform with the subfloor. You will have to either drive the proud screws on in or remove them for the tile to lay flat. Hell, if you can I'd pull up the sheets of backerboard and scrape up the thinset if its not yet too hard and do it over. In a 5x5 bathroom that shouldn't be that big a deal. Your other choice is butter the low spots with more thinset before tiling. Your floor will end up a bit uneven but thats the price you pay when you mess up.
 

Last edited by dhamblet; 11-12-08 at 04:12 PM. Reason: forgot
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Old 11-13-08, 07:02 AM
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Id not trust what you have there. From your description, its likely you have voids under the cement board that will be a future problem. I think you need to bite the bullet and start over.
 
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Old 11-13-08, 09:48 AM
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Thanks. What is the best way to get off the dried thinset from the subfloor?
 
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