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Can’t see any good reason for the cement board.
The only problem the old mastic could cause you is to disturb the bond between the thinset and the concrete floor itself.
I’d clean off as much old mastic as possible, start with a stiff putty knife or scraper (it is probably soluble with paint thinner, but be careful of open flames, and provide good ventilation).
After cleaning, make sure the floor is good and dry, then mix up a little thinset, and trowel some on the floor (maybe a sq. ft. or so) and leave it until the next day.
After it’s set, go at it with that putty knife or scraper and see how well it bonded. If it bonded well, you’re all set. If not, clean a little better.
If that doesn’t work, before I installed cement board (which is available in 1/4″, IF you have to use it) I’d pour the whole floor with self-leveling underlayment which will add minimal height, and will probably bond to the old floor better than even thinset.
Good luck.
Jay
Replies
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Can't see any good reason for the cement board.
The only problem the old mastic could cause you is to disturb the bond between the thinset and the concrete floor itself.
I'd clean off as much old mastic as possible, start with a stiff putty knife or scraper (it is probably soluble with paint thinner, but be careful of open flames, and provide good ventilation).
After cleaning, make sure the floor is good and dry, then mix up a little thinset, and trowel some on the floor (maybe a sq. ft. or so) and leave it until the next day.
After it's set, go at it with that putty knife or scraper and see how well it bonded. If it bonded well, you're all set. If not, clean a little better.
If that doesn't work, before I installed cement board (which is available in 1/4", IF you have to use it) I'd pour the whole floor with self-leveling underlayment which will add minimal height, and will probably bond to the old floor better than even thinset.
Good luck.
Jay
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I have a powder room which has tile set in black mastic. I would like to redo the floor with ceramic set in thinset,but I need suggestions in proper preparation. I know I can remove all of the mastic but for what is in the pores of the concrete. Any suggestions where I should go from there? The local home center said to put down 1/4" wonderboard and then tile, but this will raise the floor at least 5/8". Any help appreciated!
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Backerboard over a slab??? It's not needed. (Lesson: discard information gotten from a home center)
Yes, remove all the black mastic. Scrape, scrape, scrape. Waitng only one day to see if the thinset has bonded is not a good idea, the thinset is still curing. It'll come up quite easily, therefore you will be discouraged. Better to wait minimum 3 days, longer if ambient temp is below 50F. Or, if the slab is really cold--read freezing conditions.
Use acrylic admixture instead of water for mixing thinset, creates stronger bond. It's mfg. by the same company that makes the thinset.