I'm not a lawyer, speak to a lawyer. But, since no money is changing hands a good waiver should be enough. They would most likely have to prove gross negligents. ie, there was a 6 inch spike coming out of the wall that you new about and didn't fix. Like Tony said, waivers are hard to legally enforce once it makes it to an official leagal proceedure. It does make people think twice about trying. Remember, people can sue you for anything at anytime, it will cost you big money to fight it just to get it thrown out. I was involved in a lawsuit, where the company I worked for was named in the suit because someone "thought" it was our product" It was not even our stuff. It cost us $8000 plus a ton of our time and energy to get us removed. And there was a signed waiver involved. A real cost of about $15k to our business and we had NOTHING to do with it. If you can afford insurance get some. And never,ever let someone onto the mat with out a waiver. Even if they are a "good friend". Cheap and easy baseline protection.