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Carpet Glue Residue

Can I bond tile to the black "cutback" residue left when old vinyl tiles were removed?

Whether or not a person should ever set thinset and tile to the glue left behind when non-asbestos vinyl tile was removed depends on the type of glue left behind.

Physically removing all coatings is the most reliable way to make sure you have an appropriate surface to which to bond tile but the old asphaltic (black) adhesives (often called "cut-back") can be very difficult to remove unless you use shot-blasting equipment. Fortunately, most grout and mortar manufacturers make a thinset which will bond to "cut back" adhesive.

Typically, the "cut back" should be scraped smooth so the tile is not sitting on ridges of adhesive - although it does not have to be absolutely flat. Note, not all thinsets will bond to cutback so it is important to make sure the thinset used is made for that purpose.

Also, the adhesive must be true "cut back." That means it was made from asphaltic material, is not water sensitive, and will not soften when exposed to water. There are also black-colored latex-based vinyl adhesives that will not hold up under ceramic tile if they get wet.

If the adhesive is softened by water, then it should be removed by shot-blasting. Generally, chemical removal is not a good idea because the solvents used to remove the adhesive can drive "bond-breakers" deeper into the concrete.

It should also be remembered that the removal of asbestos-based resilient tile requires very different handling and the comments above do not apply to such installations.

Can I set tile on the residue of carpet adhesive?

The adhesive (often called carpet mastic) used under most glue-down carpet is usually water-soluble and tends to prevent thinset from properly adhering. Should the tile get wet, the mastic may soften, negatively affecting the bond of the tile to the substrate. Thinset manufacturers recommend removing all traces of carpet adhesive if you plan to bond the tile directly to a surface that previously had glue-down carpeting.