Bob wrote in message ...
I had ugly black stains on my roof shingles. I read up on it and first
used oxalic acid. That didn't work and I tried household bleach. The
black stains disappeared but left an ugly light brown stain in its
place. I tried bleach twice more. The last time I used nearly full
strength bleach. Before the bleach could dry (I did this in the early
morning before the sun hit the roof), I washed the bleach off with a
hose. The brown is still there; looks worse than the black mold because
the light brown really stands out against the dark shingles. It makes
the whole house look shabby.

Now I'm getting really frustrated. I didn't want to use a power washer
but I feel that I'm left with no choices. I use a 40 degree tip (the
widest tip) on the power washer angling down on the shingles (never
pointing up the roof).
The power washer is extremely powerful so I don't want to get too close
and damage the shingles. The power washer does reduce the brown stains
somewhat. In the higher elevations of the roof where I didn't want to
spray up and get water under the shingles, the brown looks positively
awful.

I believe that my original mistake was not rinsing the bleach off on the
first pass and I caused the brown stain to become permanent.

Does anyone have any suggestions on how to proceed from here?

quoting my post from last month in thread about power washers:

I live in Houston, a swamp of algae and mildew, so my 'new' house has
some algae on the bricks. I wanted to try chemical removal first before
pressure washing. I'm not certain if bleach will have any effect on algae
(as it does on mildew). I browsed at Lowe's and found several spray-on
cleaners for houses. Then I found this stuff at Ace Hardwa
www.sprayandforget.com specifically for algae. It was expensive per gallon
but it seems to work well at killing the stuff, plus it leaves a residue
that keeps the stuff from growing back. It doesn't have the immediate effect
that power washing would but then power washing can damage siding, etc.
(update: this is cool stuff, it's working well).

Check out their website, they show some machine to spray roofs. Don't know
about the bleaching effect from the bleach, could be permanent.