Published on October 10th, 2013 | by Henri


Replacing tiles over old adhesive

Q.  I enjoy your column and blog, and hope you can provide information on my current situation. My laundry room had 2 layers of vinyl tiles on top of a concrete slab. After removing both layers, there is what looks to be a black mastic adhesive left on the concrete slab.

Tests were done and no asbestos was found. The vinyl was probably installed 30 years ago.

What do I have to do to prep the floor in order to install ceramic tiles, using a mortar mix to set the tiles? Does the adhesive have to be removed? And, if so, what with? Or can I tile directly over the adhesive?

A.    The 30-year-old black adhesive is most likely cut-back asphalt commonly used at that time. It cannot easily be removed chemically or mechanically. The only way may be by grinding or shot-blasting — a very messy method that few will be willing to tackle due to the small size of the project.

However, you may set tile with one of the specialty mortars made by Mapei, designed for use over cut-back mastic. Please keep in mind that the bond strength between the tiles and the concrete, in spite of the strength of the thinset mortar used, will only be as strong as the bond between the concrete and the cut-back adhesive.

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