In October 2020 a tile contractor installed more than 800 square feet of tile on our southwest facing patio. We live in Oregon -- the rainy side. Shortly after install we started getting what I thought was efflorescence. This would occur after a rain, when the tile was once again dry and hot. Now, almost two years later, we continue to have the problem. But now, the grout is leaching what looks like a white pigment with oil. What is this and what should I do? The tile contractor says he did everything right -- he used the following:

3/6” Megalight Thin-set over a bare concrete slab (Daltile rep shared with the owner that Megalite RS should have been used)

• Daltile TS Membrane (recommended for interior but not exterior use?)

• 1/2” x 1/2” notched trowel

• Back-buttered large format tile (Emil America’s “Elegance” selected to match the large-format slate tile in the adjacent room, large format tile has a bow (warp), was set with 1/2 offsets, water pooling observed after rains, some tiles noted by the client have a hollow sound (implies not adhered) when a chain was pulled across them by the Daltile rep)

• Prism Grout (the GC said that it was sealed)