I had to laugh when I observed someone doing just that...........standing on the tower with the powerwasher directed UPWARDS - driving that water under the tiles! Feeling charitable, I did point it out to him. Relaying the sorry saga to our friendly local roofer, he said that it is a common mistake and he is frequently called out to investigate a leak after someone has "cleaned" their roof!If the roofer is using a tower he/she can wheel it around from one hose to another was my thinking.
Anyway a few options to explore!
It was prompted by another nearby house having some scaffolding put up. To my surprise the owner was up on it power washing the tiles - I thought my OCD was bad!
Ian