The Roof Cleaning Institute Of America Training & Certification Forum
"Find A Certified Roof Cleaner Directory"

No you cannot. Number one you are in training to be a professional roof cleaner and number two, bleach is bleach and sodium hypochlorite 12% is just that.
Household bleach is ridiculously weaker than SH bought from your chem supplier.

If you have a 55 then you may want to keep it vented anyway. Some one gallon containers come straight from the manufacturer with a vented cap on there, so why are you worried about opening it?
The worst thing I believe to weaken the SH is the sun and to much bouncing around and to mix it with something that will eat it up. So you should be ok; or at least for a little while.

How did you purchase one 55 by the way? I guess I should say how did you lift it off over 500 lbs of your truck. Try smaller quantities, it helps us a lot to work with small amounts.

Doing your own roof for practice is an excellent idea though. You really have to get a feel for your equipment and mixes and develop you own system and what works best for you. Go for it and take your time and learn.
Good Luck

Line up a few houses to do (preferably some forgiving friends and or relatives) and practice using the entire 55. Depending on the size of the roofs, you will probably only get a few roofs out of the mix (search apple cider and apple sauce mix on this site). The Pro's here love it.