It depends on the condition and type of brick, Is the house clear of
trees, then cleaning may do hardly anything. Darkness on N side might
just be mold which bleach will kill. If tree covered it could be mold, a
bleach soap solution would be first followed by easy powerwashing. for
real problems check out Detricth [ spelling may be wrong] Chemicals in I
think Minnesota or Wisconsin. Buildings like the Wrigly building in Chgo
get cheaned every year with their chemicals. Ive cleaned many brick
houses, just powerwashing is often of little worth on a sun exposed
wall. You dont want to blast it apart just remove dirt, so you must know
how to keep a distance.