Yes, they are a nightmare LOL. You have to walk them and wash every inch of them with low pressure. Often times they are brittle and crumblng and covered with some type of mold. Very fun when wet.
I bid over $20,000 (clean, ph balance, seal with two coats of WT) on one earlier this season and the customer took it. Now I am dreading the project. This is the house. Keep in mind I am sitting in my truck 300 feet away.
Ken, I went to school at University Of Washington in Seattle, and worked my way through schools doing roofing work.
Wood shake roofs like these are cleaned every day in the Pacific Northwest by spraying first with a Sodium Hydrooxide based cleaner.
Percarbonate MAY be better, but we never used it.
You spray the Sodium Hydroxide, wait 20 to 30 minutes, and rinse off with low pressure.
On a house like this, I might use a water broom type of contraption.
Though it isn't the right way, I knew plenty of Seattle area companies that simply sprayed them with strong Chlorine, then rinsed with a mild Acid/water solution to neutralize!
They would then wait for a good rain, and come back and stain them.
Quick and dirty, but it worked for them.
I do big roofs like this every now and then, but Tile or shingle, not shake, We do not see a lot of Shake here in Florida.
I just sold a 7800 sq ft Tile one, in an exclusive part of Tampa, just waiting for this tropical depression to leave ....