An algaecide and a regular garden hose should do it.  Far too many people destroy their roof trying to beat the fungus off when all they need to do it to kill it so that the roots will release and it can be rinsed off.

No, you shouldn't pressure wash or brush asphalt shingles. There are several spray on rinse off products available these days. A basic garden hose should be all the pressure you need. Products are available for cleaning wood, slate, and terra cotta as well. For biological growth and stains, D2 will work. Other surfaces,use  a product like 1 Restore , but more precautions are needed when using that product. A mixture of baking soda and water will help neutralize acid runoff after cleaning.

What the hell is on that roof?

We mostly have shingled roofs and the standard for cleaning them is a mild pressure washing using an antifungal solution.

Good question but I would be asking a reputable roofing company since I don't really know the right answer. Lots of tile here and you don't want to tke cleaning that on by yourself!

Fred,

I have no idea!  I would talk to a roofer.  We have a lot of terra cotta tiles here!  A

A great question for a roofer.

powerwashing can easily damage the roof by removing sand granules from the shingles.... but a light powerwash does the trick.... 

It looks like you've already got some strong answers. Different products for different rooves, don't pressure wash.

On a side note, does anyone say "rooves" any more?

When it comes to roof questions, I consult the advice of a qualified roofing professional.   

The same goes for information regarding the high tension wires in the background of the photo.  

Jeff Dowler nailed this one!

Down here where we mostly have terracotta or cement tile roofs pressure wash is perfect. I have never had chemicals added and it comes out beautiful. Asphalt or "shingles" is a big no-no as the pressure will destroy them.

Soft wash with specialized cleaning material.

I would ask a roofer but, I've always heard that you never pressure wash roofs, windows or wood!

Fred Griffin good answers from .Bob Crane, Jeff Pearl 

Fred Griffin Power washing can damage a roof and reduce its overall life.

Depends on the roof and who you ask for advice.

It can be pressure washed, but at a soft setting, not high pressure with an algaecide.

It does matter and asphalt shingles should not be pressure washed or brushed

No idea.

We have a cedar shake roof and would never use a pressure washer.

I think pressure washing could actually damage the roof.

No clue... I've never had a roof cleaned.

That is above my pay grade!

Never force anything and let the gentle approach be your default

Interesting, I have never had a rood cleaned in 27 years of real estate.

Hose it. If you are afraid of falling off get a roofer. They have special shoes and know what not to step on. I am surpised the guy on the roof has not tripped yet.

Since it is usually dust and some leaves ( no rain in SoCal), a regular hosing or brushing/blowing is usually sufficient. 

Wow, never had it done so do not know

Have never cleaned my roof, just the gutters to remove leaves and debris.

I have no clue.  Don't think I have "washed" or cleaned a roof in any home I have owned.  

Definitely not a CA thing.  Maybe a garden hose to move the leaves around.

Soft wash only

I am no authority on roofs or cleaning them.  My sense is that the type of roof does matter. 

Roofers love to see homeowners pressure wash their shingle roofs

Never pressure wash! What should be done is a shampoo type of cleaning. When you start to see many granules from the roof shingles,this is a sign that the shingles are starting to deteriorate,so why quicken the process. Just shampoo or use a mild cleaner made for roofs,and hose off.