Powerwashing Lanai

You can rent a powerwasher. A rental will be more powerful than a $150 unit. Check the rental department at your Home Depot or independent rental center. You don't need any chemicals to clean off the algae. The plain water at high pressure will do the job. The chlorine won't hurt asphalt shingles but you could cause a lot a problem by directing a high pressure stream of water under the shingles. Therefore, I would avoid using a PW on the roof. You can get an oxygenated cleaner for roofs designed to clean off algae.

It's not that I want to clean the roof. It's the fact that there is siding leading up to the roof. Will that be affeced?

I would use a detergent-and-bleach solution, applied from a garden sprayer. Use 1 cup of household bleach to one gallon warm water with a small amount of dishwashing soap. A stiff-bristle brush on a pole may help. You mentioned Home Depot--they have anti-algae products you may want to try.

I use Clorox and TSP to wash down my deck and siding. It gets rid of the mildew and algae. However, I have also used a pressure washer with plain water.