I' am about to install ceramic tile on a concrete floor that had carpet glued down. Can I install over glue? Or do I have to remove glue?

 

Remove the glue, but try to stay away from chemicals. They can potentially cause bond issues with the tile. Most carpet is glued down with multi purpose adhesive. Use a four inch razor scraper and lots of elbow grease to get the worst of it off. This is one of those hands and knees jobs. Then scrub the floor with hot soapy water and a good stiff scrub brush and let the stuff soak a while. The water will loosen the remaining glue. When it's all off, rinse the floor good to get off any soap residue and then allow it to dry a few days. You don't want anything on there that may cause the tile to not adhere well. It's sort of like gluing something to a floor with a layer of dirt on it. When you're done, you'll be solidly glued to dirt, but not the floor. A bit of an exaggeration, but the general idea.

 

If you get the majority off, just clean it well and then you can do the thinset right over it. That adhesive ain't going anywhere. If you have foundation cracks or possible shifts/movement in concrete, there's some underlayment (roll) at Home Depot & others that you put under the thinset that will allow floating ---alleviates cracking/grout and joint problems and definitely worth the expense.