Leaking shower drain

This morning the dining room ceiling started to drip (thru the drywall/stucco finish) underneath the shower. Before tearing too much of the wrong stuff up, I thought I'd ask here for suggestions.
Good quality acrylic shower enclosure, including acrylic molded base. Installation about 20 years old. ABS DWV.
Upon investigation, I'm just about 100% convinced that the seal between the drain/flange and the shower floor has cracked/perished. Doesn't leak when you run the shower. If you step on the shower floor near the drain and "work it", it leaks like crazy.
Shower base appears in just fine shape - no cracks. I don't think there's any issue with the shower enclosure itself.
Appears to me that I have to get the drain "tail" out, clean off the mating surfaces, replace seals etc, reseal and reinstall.
Tho, it could be that a tightening nut has corroded out.
Is it possible to pull these out from inside the shower, or do you have to loosen something from below? I assume most of the work has to be done underneath. Hopefully there's enough clearance through the subfloor.
Any other hints? Do you use plumbers goop on this sort of thing?
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
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I had the same thing happen and solved the problem for about $10. On our shower, the drain unscrews (I would imagine that this is a standard set up). You can use a needle nose plyers, but for about $8 I got a tool designed for the job. It is a special wrench used to remove the drain. It has prongs that slip in between the cross members in the drain. Insert the wrench and twist. The drain will unscrew. Then, I removed all the old plumbers putty and replaced it with new goop. Don't worry about using too much as the extra will ooze out as you tighten the drain back into place. Just remove the extra putty with your fingers and throw away (DON'T let it wash down the drain!).
One other thing, with the drain removed, it is a great opportunity to remove any clogs that might be restricting water flow. I saw some hairs and started pulling. The resulting wad that came out was huge. I didn't even relize that the drain was slow!
Good luck.
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You might luck out and be able to get the drain out as noted below. However, my drain system was a piece that was installed through the base and locked onto the bottom of the base with a nut--similar to how a drain gets attached to a sink. The drain tail then extended into the drain line where the two pieces were cemented together (all PVC). My old shower had a lead seal connecting the shower drain to the drain pipe-copper lines. I hope that you're not faced with having to tear out the whole shower. MLD

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Chris Lewis wrote:

More than likely on a molded shower base, the drain looks like this: http://www.oatey.com/apps/catalog/showskus.asp?ctg &subctg=4&prodgrpid3
You can only loosen it from underneath. They come with a rubber gasket which should provide adequate sealing. If you can loosen it enough, you could inject silicone sealant between the top of the shower base and the upper part of the drain.
The shower base should have been set in a cement bed to prevent movement.
If there is a ~2" section of the ABS pipe which could be sawed out, insert a rubber "Mission" coupling there. That's the style of coupling which does not have the stainless band, just 2 hose clamps. It will take up some movement and help prevent leaks in future.
Jim
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Thanks one and all for their replies.

The drain is similar to that, but instead of solvent-weld, the 2" coupler is a tight rubber sleeve with a spiral ridge which you slide over the end of the 2" DWV pipe.
The body of the drain is chrome-plated brass. The replacement units I saw at HD all seemed to be PVC versions of the same thing.
Once I got the nut off of the drain, I managed to get the drain off the pipe by tapping on the bottom end of the drain (steel rod driven by gentle hammer taps - the joists, ceiling lathe and other pipes nearby make the work area quite awkward to work in).
The scary part is that they had to notch a joist to only 3/4" thick, and chew off a chunk of the top to permit the pipe and drain nut to fit. The joist is at least 14' long...
How do you turn these stupid nuts in tight quarters anyway? The nut is round except for four tiny little tabs at 90 degree intervals. No way to get any kind of wrench in there.
Took me half an hour to loosen the nut with gentle hammer taps on a piece of metal against the tabs.

Right you are.

As you surmise, the leak was between the drain flange and the shower base.
I took the drain unit into the local HD to buy replacement parts (at least for the seals), and we were in luck, the guy with 30 years experience as a master plumber was in.
So:
1) The original installation was wrong. The rubber gasket is supposed to be _below_ the shower base (opposite side from drain flange), not above it. The drain and rubber gasket was perfectly fine and didn't need to be replaced.
2) The fiber washer was shot, but as HD had none in stock, he suggested using cardboard (ie: thick paper board, not corrugated, ie: detergent box cardboard).
3) The original installation was installed _without_ putty or silicone. I'm going to use GE silicone II this time around.
4) There is no support under the shower base. He suggested shims, and then concurred with my idea of using spray foam instead ("some of my plumber friends swear by it"). The foam is curing now.
5) The DWV is held _very_ rigid underneath, and the end of the pipe is perhaps 3/8" too long (interferes with snap-in screen). I'm going to try to free up the pipe a hair (to permit what movement the foam will allow), and possibly cut off a bit of pipe.
6) Hopefully a smear of vaseline will get the drain slid onto the DWV easier than it was to take off.
The original owner of the house was a plumber. He did the plumbing. 1, 3 and 4 are depressing in that light.

Thanks for the suggestions.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
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