I have an 80 year old house that has two layers of soft wood for the kitchen floor. THere is no plywood for the sub-floor. First off, is a 2 layer floor typical in OLDER homes with soft wood? And 2nd, if I am laying backerboard, do I definetly need to rip up one layer of the soft wood and put down 3\4 inch exterior plywood?? We are laying tiles; hence the reason for backerboard.