Guidance please, I want to lay a solid porcelain tiled floor in the kitchen, which is 1930's floorboards. As the kitchen units are only 3 years old, they have to stay. Overboarding with ply thicker than 6mm would mean no head clearance for washing machine or fridge/freezer. My local tile shop has suggested the Schluter -DITRA membrane directly onto the floorboards. i.e. no ply needed. This seems too good to be true. Any experiences would be helpful as a floor that cracked within a short time will be an expensive nightmare to sort out.