[PRP]

Downstream with a 10:1 proportioner a 1:1 mixture of 12.5% bleach and water with surfactants and sodium metasilicate. If you don't want to mix your own then Ben Moore 318 cleaner is a good one but spendy.

Apply, let dwell and rinse. No pressure needed. Let the chems do the work.

This is rough cedar w/ solid stain house we cleaned prior to painting. We do shake homes all the time it works the same.

We can reach up to 3 stories from the ground so no ladders needed either. The tip he has on is for high work a different one is used for low.

This is as close as it gets to a magic bullet. This place was filthy so it took a bit longer than usual about 3 hrs. Which is slow by pro standards because our washer is 4 gpm they use 8-10 gpm and do it in 1-1.5 hrs. If you need more info PM me, glad to help.

 

[PRP]

Nothing first hand but there are a few videos that may offer something promising.

https://www.youtube.com/watch?v=XkSeOh9NFRQ

Although he was able to clean the debris off he took quite a chance with the pressure washer even at low pressure. The apply and hose rinse method he used was much safer.

 

[PRP]

The chemical Sodium Hydroxide is used to clean algae off asphalt shingles.

Sodium hydroxide is lye and is great for stripping decks and boasting the cleaning power of house wash solutions but I've never heard of it being used by itself to clean roofs. Have you tried it?
Sodium hypochlorite (bleach) is used for roof cleaning all the time.