Don't pressurewash your roof!!
 

If you've got regular shingles on your roof, pressurewashing will take off 50% of the granules in the shingles, therefore wearing them out and making their life-expectancy shorter. We bought a house a couple years ago and the previous owners said they recently within the past 2 years put on a new roof. Well, when we got the house inspected, the inspector noticed that half of the granules were missing and some shingles were loose. He said that only happens when it is pressurewashed.
Who knows, it could also be cheap shingles, but it's not smart to pressure-wash. If you got moss on your roof, you should sprinkle it with Tide before it rains so it loosens the moss so that you can brush it off. (Tide also works on shaded driveways.)

wow, never heard of anyone trying that, I'll make a mental note not to try stupid stuff with a pressure washer though :)

You can also install zinc metal strips at strategic areas to help stop mold growth. The zinc needs to be replaced every couple of years though.

Thankfully, I've never been bored enough to want to climb a ladder and pressure wash my roof.
Laziness is its own reward.

Hmmm...I think I'll file this one next to "Don't pressure wash my testicles."

don't pressure wash your car either, as my next door neighbor discovered this past summer. Stripped the paint right off his 'vette.

Also, if the roof has flashing for dormers, chimneys, etc. a lot of water could get forced into places you don't really want it.

I'd love to see Oshnsouls house in a rainstorm - foam pouring off his roof :)

Ouch man, hurts to hear that!
Ill definatly keep that in mind.

I had my shingles washed with a brush and a weak bleach solution that made it look like new, but with little erosions of the surface of the shingles. It required a guy on the roof gently scraping, but I'll guess it extended the life of the shingles about 10 years over a pressure washer. The only thing I've seen pressure washers successful with was a concrete surface