Fiberglass shower stall
You probably hurt Kansaz's feelings by not asking him. Your next step is to remove the drywall and see how the drain is put together. There should be a shower trap that extends up to the drain opening. With any luck you will be able to see if it is threaded into the drain pipe. If it is, remove the drain screen and begin unthreading the tailpiece (remember: righty tighty, lefty loosey- unless you are looking at it upside down). Once it is disengaged go back upstairs and pull it out. Go to the store and purchase a new piece. Since it is black plastic it is ABS. The white is PVC and should never be mixed. While there, also buy a small bottle of pipe joint compound (never use teflon tape) to apply on the threads before you screw it back on.
If you were unlucky and it was not threaded, you are going to have to cut the plastic pipe after the trap. Take this to the store and buy the replacement parts- also remember to by a union to piece the two sections back together. Measure carefully before you put it together with the glue- it is rather hard to seperate. If you need more detail write back and let us know what is unclear.........
I sure didn't intend to hurt anyones feelings; I am new at this computer stuff, but I sure appreciate any and all of the input from anyone about the fiberglass shower drain that I'm having a problem with. I think my questions have been answered and I'm going to give it a try to fix it myself. All your suggestions and input sounds reasonable and makes sense. Soooo here goes.
Also, It wouldn't hurt to try to break apart the "rotted" plastic from above first if you are replacing it anyway.
Before you rip the ceiling apart, let's attempt what Kansaz is refering to. Remove the cover screen from the drain (usually one screw holds it in the center- sometimes two on the ends). Now take a large common plier (channellocks are ideal) and insert it into the drain with the handles down. Straddle the web inside evenly. Take a large screwdriver or metal bar, place it between the handles and try to loosen the tailpiece. The larger the bar the easier it is for you. If it is threaded, it should loosen after a few #@%*&@
If this does not work then you may have to tear the ceiling. Hope I caught you in time.
WELL----I got the replacement part @ Ace Hardware for my shower drain and I saw where the metal "wrench" was missing in the drain of my old gasket. I got the hole cut in the basement ceiling and believe it or not, ended up right under the shower drainpipe. I cut the old plastic PVC pipe with a hacksaw and got the old "rotten" gasket out and put the new one (the bottom part) on the pvc pipe and slid it up under the shower hole. I measured and cut the pvc pipe the correct lenght and pieced it to the old pvc pipe with the adapter piece they sold me. I put the top half of the gasket in the shower drain hole and putting all seals where they showed in the directions, and using silicone and plumbers putty, I turned the gasket in the shower drainhole with a large screwdriver like you said to do and IT WORKS AND NO LEAKS. THANKS SOOOO MUCH FOR ALL YOUR HELP1 Sally