Here is another video of us cleaning a roof. This one is tile and I started the video after we had already hit the right side of the roof. So, you can see the clean part, compared to the algae blackened part on the left and then watch that part come clean.

http://www.tubechop.com/watch/474113

We cleaned this roof in South Gulf Cove. A large development that falls within the 33981 zip code for Port Charlotte, Florida
It's right down the street from me, I am in Gulf Cove.

This was originally a 10 minute video, I cut it to 2 minutes.
The 10 minute version is on YouTube. If you want to see just how much black algae discoloration there was, you should watch at least the 1st 10 seconds of the 10 minute video below.
It has some close ups.