What caused the leak?

Can anyone help me with this problem?
We have a spa bath in a first floor bathroom of our 2 year old home. For the first 18 months (and about 8 uses) there were no plumbing problems.
Then, after the spa was used one night, bucket loads of water started dripping through a light fitting in the ground floor ceiling soon after the spa began draining. A plumber was called immediately. After drilling a series of 5" holes into the plaster ceiling, he was unable to find the source of the leak - although it was obvious there had been a big one. After filling the spa and emptying it twice he was unable to dublicate the problem. In the six months since, we have used the spa another 6 or 8 times hoping to repeat the problem ... but to no avail. Dry as a bone!!!
We want to have the ceiling repaired but until we work out what went wrong we cannot proceed with surity. Any ideas what the hell happened and what we can do to be sure that it won't recurr???
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It's possible the spa leaked for a time before water came through the ceiling. Which means the leak may be related not to the draining of the tub but something that happened when the tub was full. I would run the pump with the tub full and try all the various combinations of water and air settings as well as pump operations in attempt to reproduce the leak. I'd also block water discharge jets during pump operation to see if that causes water to blow out from a joint outside the spa shell.
MM

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you might have filled the tub up with to much water.and when person or persons got into the tub the water level went above the overflow plate on the side of the tub.which half the time is installed with poor workmanship(no spongewasher)
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"jim donovan"
My first thought was that they didn't use dielectric unions on the tub.
My next thought was exactly as you expressed. The tub is first filled to the overflow then when someone gets in, utilizing Archimedes Principle of Fluid Displacement, the water level reaches the overflow plate that wasn't securely fastened.
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Good point. Some of these spa tubs are so thick that the screws for the overflow will hardly reach. It wouldn't be the first time a plumber threw out the sponge gasket just to get the screw started.
MM
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If its not the Overflow (you know, 2 in the tub, makin waves) then I would check the Dielectrics. You might check the caulk around the tub.
--
Dale
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When I was a wee one I used to like to slide back and forth in the tub and try to get a big wave going; untill it would actually spill out onto the floor.

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What did the plumber charge for drilling those holes?

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that is a very good question, what did he charge?

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