Hi a theoretic question I have a 4000sq ft white barrel tile ( steep ) I use 50 gallons of 50/50 mix with 32 fl oz`s of dawn ( obviously I refill mix as needed ), I cover the roof and I leave it for around 30-40 minutes then I rinse off with a garden hose ( should I use powerwasher from ladder to rinse instead ) I`m doing this because there`s around 30 galls of product applied to one side of this large roof and I`m guessing that with morning dew and steep roof product is going to fall on grass / plants during the night or early morning?
What system do you have Simon and is it a 2 story roof? you need to get some ammonx- lo to thicken your mix to reduce the run-off, there is an option of getting a lift if need be. if you look through some of the tile pics in the open side, it will give you some idea of how others are tackling jobs that may be similiar to your project in relation to size / height.
As far as the runoff issue you had, like Gary said rinse the bottom 4ft. or watch the weather and try to tailor your job to that as well it does help white tile to have the ph of the rain rinse it than hose water, believe it or not, there is some chemistry with in that. I had a tile roof that was black and hit it 3x to get it to an oyster looking color, I was stumped.. think this is not what I expected at all. about a week later w/ a good rain, drove by the job, and Bam it was sparkling white!
If you want some pics of the process, I have them. Let me know.