I have a concrete cellar, and the roof is essentially our outdoor patio. We are looking at getting it tiled with a porcelain outdoor tile (needs to be a thin tile due to levels etc, otherwise would have used a 20mm stone). What I'm worried about is waterproofing. The tiler has stated that putting a membrane down prior to tiling is not necessary. I want to be 100% sure water won't seep through into the room. Any tilers out there agree with not needing to put a waterproofing membrane down first? And if you would do so, what would you put down? I'm in South Australia. Cheers!