So we're converting an existing bedroom into a bathroom. Still in the planning phase, but I want to do a waterfall overhead shower head. The problem is that above the ceiling is attic space. If I run a pipe from the valve into the ceiling to the shower head, I'm going through uninsulated space. Not a problem if this was a tub/shower combo and water would drain out through the tub spout when the valve is turned off. But this is a shower only. Water will be trapped in the horizontal run in the ceiling and pose a freeze hazard.

What are some solutions to this? I saw one example of the same scenario where the plumber ran the pipe (pex) about 2' up into the attic space then looped it back down to the shower head. the idea was that after the valve is shut, gravity/siphon will draw the trapped water back down the shower head. Is this feasible?

(I'm planning on doing the work myself and I'm not a pro)

lower the ceiling in the shower area a few inches and run the pipe below the existing ceiling.
you are going to have to figure out what to do with the vent fan as well.

If you make a dead space above or below the ceiling for the water line you will want to add a layer of insulation in that dead space, that will help to retain heat in the bathroom during the winter.

Or you can run the shower head water line in the attic above the ceiling and add a thick layer of insulation on top of it. As long as you keep the bathroom reasonably warm the heat that permeates the ceiling will keep the pipe from freezing with that insulation on top of it. No matter how you heat that added space the insulation will pay off. Might want to wrap the water line in pipe insulation tubing like used for hot water pipes, that will help to reduce sweating on the outside of the water line that would drip onto ceiling or dampen the insulation.

They also make nice looking chrome water pipe, you could run it in view along the top of the ceiling and attach to the shower head. It is made to be seen, can even be run up the wall from the water valves if you want to. Pipes don't always have to be hidden behind walls, floors etc.

A "siphon" will NOT work, unless air can get into the other end of the pipe, which will NOT happen in your situation.

You can do a rain shower type installation without the pipe being above the ceiling. They have extra long shower arm that mounts from the wall with an anchor at a higher spot or from the ceiling.

If your valve has a plugged port on the bottom, I would use a toe tester - a diverter tub spout. Besides letting the riser drain it also lets you adjust the temperature before you step in.

Great answers, I've got a lot to consider now thanks