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Injecting only gives you a mix that is strong enough to clean exteriors and even then some spots on exteriors are so black that you need to apply a stronger mix. Injecting mixes about 10% give or take depending on the injector you will not get any more than 20% at best which is not sufficient enough to cleana roof efficiently however it will clean after 7 or 8 coats and 8 hours of work on a 2500 sqft roof which should only take 30 minutes to spray and clean completely, of course rinsing adds about another 30 minutes.

You'll learn as you go and find ways to be more efficient. Here is a video of a 40% mix on a tile roof I made. Not the best tutorial since I did it on a whim and made it to battle this guy who uses a surface cleaner to clean tile roofs who says his way is faster than mine.

Here is the link: http://youtu.be/pJgICdjzRbY

Nice video...short and to the point. Perfect. Was Dirty, Now cleaned. Love it.