I offered a similar warranty when I was in the service end of the business. If you do a search for warranty verbiage, you will find FTC web pages that spell out exactly what you can say. I warranted my deck cleaning and sealing service with a 3-year pro-rated warranty (like a tire dealer gives you). It allowed me (actually forced me, if you want to say it that way) to make my customers happy. I got a chance to come back and do maintenance work (easy work, low cost, and the customer got a fantastic price). The end result was that my customers were always happy, remembered who I was, and always called me back.
The guy who wants to beat you up with a warranty claim simply has to pay for what he already used up. After a year, a job that was $600 originally would now only be $200. After two years, it would be $400. After 2 years and 11 months, the job priced out at $583. Everybody was happy. I guess I don't understand why this would be a problem for anybody. It is a marketing tool that can be used wisely. It worked for me.
By the way, I cleaned my own roof about four years ago. (Shortly before I fell off that very same roof doing another honey-do task :frown
. I live in a heavily wooded area with tons of shade and mold, and the moss was about 1" thick at the time (north side). Not a single sign of moss or roof stain has re-appeared yet. It must be clean livin' and the warranty I gave myself that's keeping the problem away.