Many are under the mistaken impression that simply cleaning a roof with TSP/Chlorine/Water is "treating" the roof, and somehow "preventing" the algae from growing back.
Lets take a quick peek at what really happens ?
The Sodium Hypochlorite releases Chlorine, it cleans the roof, and most of it is evaporated in 24 hours, sooner if it is hot outside.
What is left behind is sodium chloride, commonly called Table Salt.
Believe it or not, common Salt is a Biostat.
A Biostat prevents plant growth, as opposed to a biocide that kills a plant.
But, soon the salt is scattered by the wind, or washed off by the rain, or the cleaning contractor.
BORAX on the other hand is much longer lasting.
Ever heard of Boric Acid ?
Before we started using what we do now, a chemist we hired to do consulting for us suggested it's use.
You do not need much, 1/2 8 ounce cup per total gallon of mix.
It seemed to make it clean better too.
I have also field tested with, and w/o Borax, and found about a 3 to 6 month improvement in roofs staying clean with it.
It's cheap like TSP, it works, and will give you an "edge" on your competition.
The picture below is me spraying a roof.
This is an OLD picture, and we were using Borax then, and of course, TSP.
See how wide that pattern is ?
This is because the Gun we use places a nozzle farther away from the roof, giving it more time to develop it's pattern.
Here is the truck powering that gun/hose combo.
Yep, it's a BOX TRUCK .....
Notice those Titan Stainless Reels, and the AG hose?
BTW, that is "ONLY" a 25 degree nozzle I am using.
25 degrees is plenty, IF it has enough time to fan out.