Will my mattress sit on top of the bed frame or inside the frame? Updated June 12, 2019 16:44 The slats of our platform beds are at frame level so mattresses sit on top of and not inside the bed frame. Related articles What is the difference between the Deluxe, Elite and Premium SmartBase? Do I need a box spring with a Zinus Smartbase? Can I attach a headboard and a footboard to my SmartBase? Do I need a box spring with a Zinus platform bed frame? What is the weight limit for a Zinus bed frame?