I'll let a plumber answer your main question, but I know whenever I have put a laundry on a first or second floor I have installed a pan under the washer/dryer to catch and drain the water away in case of an overflow or other washer/plumbing failure. Everyone, sooner or later, has one of those supply hoses behind your washer fail and cause a major leak. I have always had my plumber make it up out of copper with a drain connected to the bottom and a curb that would allow at least 4" of standing water in case of a bad failure. I think you can even buy these pans made from plastic all pre-made now. This would be a lot cheaper.