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I need to tile a floor in my house (concrete slab) where there was some vinyl tiles that were put down with some type of yellow mastic. Can I mortar and tile over this mastic or does the floor need to have this stuff scraped or stripped off first?

The holes/divits in the picture are from me removing carpet tack strips from the concrete that were put over the old flooring when the bathroom was carpeted (ick!).

Old floor with yellow mastic

  • Watch out for asbestos! – Tyson Oct 31 '16 at 1:55
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You should scrape as much off as you can to prevent any interference with the mortar bond and to ensure the tiles will lay flat. I use a razor scraper like this:

razor scraper

P.S.- invest in a few packs of replacement blades.

  • I agree with jimmy That old glue is a real PITB and I have tried to tile over it in the bast with not so good results. The easiest way is to rent a Shot blaster machine that blasts steel shot on the floor and it cleans the surface but it is very expensive. Scraping will take elbow grease but will remove the sections that are loose and provide a better surface for the tile to adhere to than not removing as much as possible. – Ed Beal Oct 31 '16 at 13:07
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You should wet down the area you want to prep for tile. Take up what you can with a razor scraper then clean up area with a bucket of water and a sponge. Don't be concerned with getting everything down to the concrete, the important thing is using a good thinset that will bond. Any kind multi-purpose thinset, Ardex 5 and Magniflex have been my two personal favorites that I have been very impressed with. Also using a bonding agent prior to setting will definitely help.

  • I saw water being suggested on a different site. That appears to have softened up the old mastic quite nicely to aid in scraping it up. – Jim Nov 3 '16 at 13:28

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