I want to fix a leaking shower pan. The plumber forgot the sand when he installed the shower and my idea was to drill a hole into the PVC pan and through this hole try and spray building foam to fill the space that was supposed to be filled by sand. Is this a viable idea?

Building foam has air bubbles that can compress so that's not a good idea.

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I want to fix a leaking shower pan.

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Lets fix the leak first. My bet's on the seal of the drain lip. (see image) This is caused by the shower floor not being bedded properly . The shower floor will flex when someone steps into it rupturing the drain sea in timel. If you still have a leak when you close off the drain and put a inch or so of water in the base then the first thing I would do is open up the ceiling, loosen the large retaining nut and running a small rope of plumbers putty under the drain lip. Now tighten the nut up, remove the excess putty that squeezed out and test again. Bet the leak's gone now
Can you take or open anything up to get under the shower floor and bed it with cement or mortar,( we don't use sand) to bed the floor? Let me know, Tom

stagepic, I would call the plumber back to do the job right if I were you. Good luck

We have tried to seal the lip, but as you (Speedball1) say, with all the stepping into the shower the seal raptured again. That's why I want to try and stabilise the pan. Unfortunately I can't get under the shower floor. I was hoping the foam would harden enough and that the air bubbles would not be a problem. Is there a liquid that I could poor through a drilled hole to fill the space that would harden solid?

Calling back the plumber is not such a good option as he would just screw it up even more. We learned the hard way...

Stage, a big problem with that foam is that it not only fills the space, but expands pretty aggressively. Could end up with a bulge upward. At any rate, the stuff is not terribly hard when set and I think it would just collapse over time.

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I can't get under the shower floor.

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Is it because the shower's on a slab?
You can try the foam but I'm afraid that over time the weight of people stepping in the shower would force the air in the foam out and the foam will sink down under it. If you do interject foam load it up around the drain. Good luck, Tom

Thanks for all the input.

Please let me know how you make out, Tom