Tile installed over concrete on grade in a converted warehouse, has brownish deposits coming up between the joints. What could be the cause?
Most likely, the concrete had a metallic hardener used and due to moisture coming up through the slab, the metallic flakes are rusting. The adhesive, which probably softened due to the presence of moisture, mixes with the rust, and bleeds to the surface of the tile at the joints as a brownish deposit.
Can tile be installed over roofing felt or scribing felt?
No. Adhesion and staining are both concerns.