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I had a water leak dripping under my shower onto the ceiling below. I replaced the drain (especially the rubber gasket), and now the leak seems to be much less.

It looks like the fiberglass making up the bottom of the shower pan is pourous and water is seeping into it. After shooting the water at the drain or clogging the drain and letting water build up a bit, the fiberglass around the drain appears wet from the bottom.

During my last test, no water actually leaked (dripped) at all, but the fiberglass appeared wet. Is this acceptable?

Pictures of the bottom of the shower pan:

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    You say shower but if the shower is actually a bath tub you take showers in, it could be the overflow drain cover is loose. And the drain could be fine and the leak could be from the supply lines. Jan 12, 2021 at 5:51
  • Thank you for inquiring. It's a shower, and the leak is not from the supply lines. Jan 12, 2021 at 19:50

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I had a similar problem with my shower drain. It seems to be caused by the floor of the shower not having enough support near the drain - so the shower floor can flex if weight is put too near the drain. The flex of the fiberglass floor disturbs the seal between the fiberglass and the outer edge of the drain assembly.

My shower drain has a large lockring on the underside of the fiberglass floor which clamps the shower drain and the fiberglass floor together. The outer diameter of the ring is about 4 1/4 inch (measure yours). Support needs to be added to prevent flex of the fiberglass floor near the drain - not to the drain itself, because that would not prevent the fiberglass floor itself from flexing.

I used a 4 inch PVC straight socket connector because its inside diameter is about 4 1/2 inches and fits nicely around the lockring and presses onto the bottom of the fiberglass around the lockring - you'll need to saw a piece off the socket so it will fit around the drain pipe and trap.

Install some wood pieces between the floor joists near both sides of the trap and use more wood and shims to support the bottom of the PVC - it doesn't need to press the fiberglass floor upward, just have enough contact to prevent the floor from flexing downward around the drain. I used construction screws to attach the wood to the joists - so they could be easily removed and to give access to the drain and trap if further work is needed. If the supports are nailed into place, then future removal would be a pain.

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  • Some formatting to help find the useful bits in here would really help.
    – FreeMan
    Dec 13, 2023 at 14:25
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Even a small amount of moisture is not acceptable beneath your shower pan. If the drain is properly connected you shouldn't have any seepage. If you're still getting moisture you need to determine the source. You imply that the fiberglass pan may be porous. Fiberglass isn't porous so it shouldn't be seeping but it can crack. If it's a tile shower you could also be getting leakage around the seam between the shower base and the tile. Check and caulk all of the corners and seams.
You indicated the pan looked wet underneath. Looking wet doesn't mean it is wet but you need to find out. Just take a dry paper towel and wipe the bottom of the pan. If it's wet at all the paper towel will absorb any moisture. If you're still not sure, you can purchase a moisture sensor meter online or at a hardware store that will sense moisture levels in drywall, wood and other homebuilding materials.

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  • (Edited my question to add pictures) You said fiberglass isn't porous, but I'm not sure why you think that. I mean, on the top where I stand, of course it's smooth and not porous, but the bottom is made of glued fibers, so of course it has gaps and voids. I understand that the glue and fibers aren't porous themselves, but I mean the whole surface with its voids, etc. has many places where water could seep. Do you see what I mean from my pictures? Jan 12, 2021 at 21:44
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    In your pics I can only see a small portion of the shower pan around the drain - not enough to tell anything. I do see the subfloor which appears to be osb that is under the shower pan but it doesn't appear damp in the pics. When material is porous, such as osb it will permit water to permeate it. So if the osb is wet it means you have a leak somewhere in the shower or the drain assembly. A properly manufactured fibeglass shower pan is not porous but, again, it could be cracked.
    – HoneyDo
    Jan 13, 2021 at 0:24
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    I just looked at the pics again and, although the osb doesn't look wet I can see some very slight discoloration where it may have been wet at some point - but hard to tell. It may have been from the drain leak. You're the only one who can verify if there is currently moisture present - you have to be on-site.
    – HoneyDo
    Jan 13, 2021 at 0:33
  • Thank you for looking. The bottom of the shower pan is black, but closer to the drain, it's tan-colored (perhaps because of gunk from where it has been getting wet?). Jan 13, 2021 at 1:06
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    I've encountered several fiberglass shower pans which were full of voids and channels, and one specifically which was dripping from near the drain opening from water which was entering from a crack where the flange had been nailed to framing.
    – kreemoweet
    Sep 18, 2023 at 3:43

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