said by PSWired:
Supposedly it'll just stick. I believe it, given the stickyiess of the thinset I'm using. Still, I've got visions of 18" tiles falling onto my head during installation.
One internet guy says to backbutter the tiles and use the zigzag side of the trowel to make circular marks on the tile like a bullseye. Supposedly that makes the thinset act like a suction cup and hold the tile up better while the mortar cures.
That's interesting on the bullseye concept. In the worse case scenario where you find they really do need pressure, sandwiching plywood over an area and supporting it with tees (or a drywall lift if you rented one) until it dries is my only idea. I really hope it just sticks for you. I've never tiled with anything bigger than 4x4, so 18 inches sounds pretty heavy.--
nohup rm -fr /&