That ratio depends on shingle vs. Tile roof.
If it is tile so may want it a little stronger.
Here is something from Hanson Tile ( Pioneer )
HANSON ROOF TILE
TECHNICAL BULLETIN NO. 119
July 30, 2001
CLEANING OF CONCRETE ROOF TILE
As part of a normal roof maintenance program, concrete tile is often cleaned to remove
dirt, grease, algae, fungus and molds. The skill of the individual doing the cleaning and
the method employed will determine the ultimate appearance of the roof.
Roof cleaning requires an experienced individual who understands what he or she is
doing. When done by a professional the roof can look new, but when done by an amateur
a roof can be irreparably damaged. Furthermore, if the individual is inexperienced in
walking on a roof he or she may break tile or fall and suffer serious injury.
The method used in cleaning the roof is dependent on what needs to be cleaned off. If
the problem is algae, fungus and mold a dilute solution of bleach is recommended. The
bleach is diluted to 5 to 7% and applied by low-pressure spray. The area is allowed to sit
for a few minutes until the algae, fungus or mold is killed. The plant materials will visibly
lighten in the bleaching process. The roof is then washed off removing any remaining
dirt.
On roofs where kitchen vents have left a layer of grease a solution of TSP and detergent
is used. Sometimes the roof needs to be scrubbed with a soft brush if the grease is thick.
A scraper is used to remove the thick areas before cleaning.
In summary, roof cleaning is a job for professionals experienced with roof tile and
cleaning roofs. Allowing an in-experienced individual on your roof is a risk not worth
taking.