A floor system includes a clamping ring drain assembly for funneling water to an opening of in-floor drain plumbing. The drain assembly includes clamping rings and a cover for the input opening. A waterproof membrane has marginal material defining a hole, with the marginal material being located outboard of the upper clamping ring. A customized floor drain flasher adapter membrane has an outer first ring bonded to the marginal material of the waterproof membrane, creating a double thick ring outboard of the clamping rings. The adapter membrane further includes a ring of transition material sloping to the clamping rings. The adapter membrane has a preformed shape selected to match a vertical dimension at the construction site, so that the adapter membrane is not bent, stretched, folded or wrinkled during construction/installation, hence preventing water leaks, puckering of the membrane, and membrane cracking.