That's the problem so many window cleaners are running into. A good waiver, while not being a substitute for insurance, will protect you from liability from defective tempered glass. If a builder or homeowner will not sign a waiver, I won't use a scraper, and probably won't even do the job. The International Window Cleaners Association has great information on Fabricating Debris(even a sample waiver), as well as the newly formed Association of United Window Cleaners. Gary Mauer and Dan Fields, tempered glass experts and window cleaners, both have websites; scratched-glass.net and stopscratchedglass.com. They have kept many window cleaners out of hot water, including me.