I have a covered patio that is currently painted 2x6 boards. I'd like to convert it to tile to make it waterproof under the deck in my storage area, as well as easier to clean. Is it acceptable to slap mortar right on on the 2x6s? Maybe with a 3/4" trowel. They are flat and solid. It complicated things if I need to lay down cement board or plywood because of the door height. The door sills are roughly 1" above the 2x6s.
No way that would ever be waterproof. First of all, your 2x6 deck flexes when you walk on it and flexes a lot when you have a few others up there walking around. If it was covered with tile, the mortar joints and maybe the tiles themselves would crack on day one.
As
bdamico said, tile alone is not waterproof. If you put down 3/4 inch plywood, moisture barrier membrane, cement board, thinset morter and then tile, it might work. You would need to establish a constant slope of at least an inch every 10 feet for drainage as well.
You of course would have to rip out the door sill, trim the door bottom, and build a new sill. Or tear out the whole door, rough in a higher sill, and install a new pre framed door.
Sounds like WAY too much work.
I have a waterproof deck over my 2 car garage. First layer is 1 x8 diagonal lumber. Next is 2 layers of torch down rolled roofing. Then deck boards are layed down over flat 2x4 pressure treated sleepers. Still hard to clean but it is waterproof. Deck is sloped 2 inches in 20 feet and drains through scupper drains through the parapet wall.