A method of constructing in-floor drains includes measuring a vertical distance from a component in the floor to an in-floor drain pipe, and ordering a customized floor drain flasher adapter membrane having an outer ring for engaging the floor component, such as a waterproof membrane, and having an inner ring for positioning adjacent an entrance to the drain pipe, the outer and inner rings being spaced apart a dimension correlated to the measured vertical distance. The method includes installing the customized floor drain flasher adapter membrane to connect to the in-floor drain in a way that reduces leaks and that prevents unnatural and uneven folding, bending or stretching of the membranes. Preferably, an inventory of different types of pre-manufactured floor drain flasher adapter membranes are pre-manufactured for just-in-time shipment. The adapter membrane can be attached to a pre-formed waterproof membrane, or to a liquid-formed membrane formed on the construction site.