I have a small pool and large hot tub on the roof of a four-storey building. The pool is about 3200 US Gallons (6'x20') and the hot tub around 1500 US gallons (8'x8'). These pools are concrete and completely tiled.

I've always wondered what the best thing to do with them in the winter is - leave them empty, drain to below returns, or fill them right back up?

The last few years, the winterization method has been to drain them completely (using the circ pumps), blow out all the lines and fill them with antifreeze, and put plugs in EVERYTHING (main drains, skimmers, returns, hot tub jets, etc.). Half-full antifreeze jugs are left in the skimmers and in each of the drain sumps (two 8x8" or 12x12" square drains in each) as ice compensators. The pool and hot tub are then left empty for the winter, uncovered, other than whatever rain/snow ends up in them. By spring there is typically 2-6 inches of water in each.

The lines are blown out and have antifreeze added despite all of them going beneath the concrete deck in what should be warm space, and the equipment is all drained and winterized even though it is all in heated rooms.

Everyone always figured it was safer to leave these ROOF TOP CONCRETE POOLS empty, which I would agree with, but I wonder what frost might be doing to the tiles and grout?