Using a power washer to unclog a roof drain?

A sewer jetter is a machine designed to clean out pipes and lines, and it is really just a pressure washer with a pulsing valve and special hose/nozzles. You can even buy a jetter valve for your pressure washer to turn it into a sewer jetter. I will include a link in my sources to an article on hose size vs. pipe size. I also explained sewer jetter's in a recent Yahoo Answers question, so I'll link it as well. Bottom line, it is worth a shot, and if it doesn't work, you may be able to clear it out by getting some sewer jetter hose and nozzles that are designed for this kind of think.

I'm almost positive that would not work. If the pipe is above ground and you can see the bend in the pipe I'm not sure why a cable wouldn't go through it? unless it is a small diameter cable lacking in tensil strength. Your best bet would be to try a "blow Bag" this is a ballon like device that you put on the end of your garden hose then put it into your drain. The ballon expands to the size of the pipe and forces water down the drain. it works best if there is already water backed up in the drain so if not then fill up the drain before putting the blow bag in place. If that doesn't work then call a plumber to cable it out for you.

Roof Drain Pipe

It might work. Stuff towels in the pipe then push the tip of your nozzle past the towels. Just anyway you can stop the water from coming back.