thanks for all the replies. i just now saw them. for some reason, i didn't get an e-mail telling me there were responses. the "quat" i'm refering to is mentioned in a couple of articles on deck cleaning in powerwash magazine(if that's the name). supposed to kill the spores, which i'm told are generally not eliminated by chlorine or sh. i think it's intended for use after another chemical(bleach, sh) has killed the mildew or fungus growth.
in any event, i've learned that i was misinformed about the composition of the shingles. they are actually asbestos shingles, which i assume are no longer used in new construction but are apparently available for restoration of pre-existing asbestos roofs.
the house is literally in the woods about 75' uphill from a creek(which, incidently, is home to the vermillian darter, an endangered species). wild hydrangas(sp?) and dogwood trees, among other indiginous vegitation, surround the house and would be directly hit by the run-off from the roof.
i'm aware of the environmental issues (which is why i'm going to try the quat first) and the on-going debate about which chemical is less environmentally hostile. what i don't know is anything about asbestos shingles--whether they might be damaged by certain chemicals, how much pressure they can withstand, whether the process of cleaning them release asbestos into the air in dangerous amounts, and so forth. some of the shingles have been replaced recently, but most are original to the house, which was built in the late 1930's.
i've told the owner that i would do the job only if i satisfied myself--after consulting with my experts (the members of this bullitin bourd)--that i didn't run any risk of damaging the roof or creating a dangerous situation.
i look forward to getting yall's thoughts.
richard
ps another problem, and the subject of a different thread, is that the water supply available is only about 3.5 gpm's, and the manual on my 4 gpm washer says it requires 5 gpm's.