A method of retrofitting a drainage pipe is disclosed. The method provides removing a pre-existing bathtub drain of a pre-existing bathtub drainage system to expose the concrete-encased drainage pipe, attaching a gasket to an exposed end of the concrete-encased drainage pipe, attaching a drain body to the gasket, attaching a strainer to the drain body, and attaching the drain body to the concrete-encased drainage pipe via the gasket. The drain body and gasket create a water-tight seal with and between the concrete-encased drainage pipe when attached. The method enables a user to convert a pre-existing bathtub drainage pipe to a shower drainage pipe without the added necessity of digging into the concrete surrounding the pre-existing drainage pipe below the pre-existing bathtub drain in order to place a small piece of modern piping material and a secure fastener below the body of the shower drain.

A central position of a water storage tank is provided with a drain opening; a water return bowl is in a short box shape, and is made of silicone; the bottom end of a skirt of the water return bowl is in direct contact with a bottom plane of the water storage tank to form a closed surface; a top plane of the water return bowl can attach to a grate, and the skirt of the water return bowl can be folded inwardly to form a curved flaky structure under drainage pressure; drain holes of the grate are provided on the edge of the water storage tank. Supports for supporting the grate are provided in the water storage tank. A gas buffer device is provided on the top surface of the flat-top water return bowl.