I am installing a new marble tile floor (12×12) in the entryway of my home. The base is 1.5″ mortar over a 2X6″ T&G sub-floor with a proper vapor barrier.
After removing the old tile I needed to smooth out the mortar base and make up a little height so I am installing 1/4 inch Hardi-backer board before laying the tile.
I am installing the the backer-board with thinset mortar. Here’s where I made a rookie mistake. I mixed enough thinset (hopefully thoroughly) and started applying it to the floor. Of course this took longer than expected and by the time I was ready to install the fourth piece, the mortar was a little stiff but I forged ahead. Now the corners of the backer-board are popping up a little. Can I fix this with construction adhesive and a few screws? Also, I still need to install two more pieces of backer-board. I had not initially planned to use any screws because I did not want to have to screw into the concrete.
If necessary, I can correct this by ripping it all out and starting over. Obviously I would like to avoid this. However, I would rather sacrifice $50-worth of backer-board & a day’s labor than risk the tile I’ll be installing over it.
Thank you in advance for your advice
Replies
If I understand correctly, the mud base was existing under other tile when you got there, and you are adding hardibacker on top of it. I think that's an acceptable application, but I'm concerned about how you're attaching the hardibacker. Sounds like you are simply laying it in thinset with no screws or nails, and I don't think that will work--it may not stick well and it won't be flat. Hardi is not designed for that application. The dot pattern on hardi shows you where they want the fasteners (a lot of 'em). Anyway, it sounds like the stuff is not laying flat and I'm pretty sure you need to tear out and start over.
I'd find a fastener to install the hardi with, so that I could do a standard thinset application and lay my tile flat on the hardi. I've always used hi-lo rock screws but you may be able to use 16d galv box nails. Read the hardi spec on acceptable fasteners. Precut all the hardi and lay it dry on the floor where it goes. When you have all the cuts made, mix your thinset and reset each piece and nail it down.
A quarter inch is not a lot of height to add. You could also consider a mortar that will work at the 1/4" thickness you need. If you don't have a pro tile shop already, find one and go talk to them about an acceptable mortar for a build up. It would be quite a bit harder to keep things flat with a thick adhesive layer like that. Pros do it, but it might not be a good thing for your project.
Also--make sure your thinset is appropriate for the marble. I've set marble once and used a sack of 'marbleset' for it. Not sure what the chemical difference is, but that was some very creamy and sticky mud, very nice to use.
I remember being a novice and understand why you want such a nice, clean, smooth, even, surface to apply your marble tiles. However, it's is not necessary. Yes, the prep will be really pretty but that is not what counts the most in the final result. What you have done thus far has a good possibility for failure so why risk it? Whenever you lay any sheet goods over a floor they must be "glued" AND screwed.
Rip up the Hadribacker. Scrape the floor with a stiff 3"-4" scraper/ putty knife to knock down the high spots. Use a leveling material such as Levelastic ($18) to fill in the low spots. The stuff is really soupy and with a bit of coaxing, will fill any voids. It also can go to a feather finish.
Normally I would not do this last step and just compensate during tile installation. After setting your first 6 tiles you will realize that you will adding and subtracting thinset thickness (even if it's ever so slight) to make the surface of the tiles flush. Filling those voids then won't be that much extra work.
How big of a floor is this?
Wear knee pads, or as I prefer, rest your knees on a 12" x 24" foam pad. The knee pads I have used pinch behind my knees and get uncomfortable pretty quick.
Protect the floors of the entry to the adjacent rooms. Thinset is like sandpaper.
Lots o luck!
F.
thinset under backerboard isn't there as an adhesive ....
it's a leveling agent.
if I read this right ..... sounds like a case for a rip up and redo.
Jeff
Buck Construction Pittsburgh,PA
Artistry in Carpentry
Yep, rip up and re-do. The thinset won't cure for 2-3 days anyway, and I usually can easily remove tiles as late as the next day, so your in the clear. Scrape off the old thinset, called cut back.
If you want to make up some space, I'd consider using either SLC. A Quarter inch mud bed would be impossible to float. If you are only shy a quarter, you might try shifting to a deeper knotch on the trowel. Or use a threshold.
Regards,
Boris
"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1934