Polylok's 24"x12" risers can be stacked on top of each other. There are screw holes on the inside of the riser to attach it to a Polylok adapter ring, another Polylok riser, and Polylok distribution boxes or catch basins. There are two Polylok 24"x12" riser models, one with a round wall design and one with wall flats. Please note that the Polylok 24"x12" riser with flats will not accept Polylok riser safety screens.