Problem that stumped plumbers

We've lived in our 90-year-old home for 4 years. We have a basement bathroom with shower, toilet, sink. The bathroom is on an 18-inch platform.
Last New Years Eve, water flooded the basement from underneath the bathroom, probably 20-30 gallons. We suctioned it out, no further problem. A few weeks later, it did it again. A few weeks later, again. At this point we called plumbers out to take a look, and they could find nothing wrong, and had no explanation for what was happening.
A few points:
1) The water is cold, not hot. My first thought was the water heater (which is also in the basement, in another area) had spilled, but that's not it. 2) The water is clean. There is no smell or debris in the water. (No toilet overflowing, etc.) 3) It floods all at once. We checked for some kind of leak that might be building up, but there was none. 4) The flooding happens at night (or very very early morning), so the water is not in use at the time it floods. 5) The flooding is irregular; that is, we can't predict when it's going to happen. 6) We had not had the problem for 3 1/2 years....why did it start now? No construction or anything like that going on.
We need some advice on how to stop this flooding; it stumped our two plumbers (who seemed competent in all other ways.) Thanks for any help.
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Buy a flood alarm and put on floor where flooding occurs. This way you can watch when it happens again.

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You say cold, not hot; but do you really mean cold (as in fresh from the tap) or simply "not hot" (i.e. room temperature)? If the latter, are you certain you have found the flooding soon after it happened, or is it possible you're finding hot water that cooled down?
Any correlation with weather (e.g. rain)?
Any correlation with the use of the facilities (shower, toilet)? For example, any chance there's some shower overspray that's collecting somewhere to dump when it reaches a certain level? (I had a really weird & hard to trace leak problem that turned out to be something similar. The shower leaked when someone was using it, but not when no one was in the tub. That one took me a couple weeks to find. <g>)
Have you actually witnessed the flooding?
Is 20-30 gallons perhaps an overestimation? Any chance it's the same amount of water that's normally in the toilet tank? Does the toilet run during or after the flooding incident? Does it happen quickly (say, in a minute) or does it take a few minutes or hours?
Do you have young kids who might know they're doing something to cause this but are afraid to fess up?
I'm mostly stumped too, but I have to think the toilet is suspect #1 if it's coming from the bathroom platform. I don't know what might cause it, though.
Joe F.
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Several good responses, I appreciate all of them. Let me address some of the points, expand a bit on the original post.
1) The water is "cold" to touch, but bare feet are not good thermometers. We have two sons whose bedrooms are in the basement, and they caught the water mid-flood a couple of months ago. The water was flowing from under the bathroom platform, and the water was cold, not hot. The water heater was the first thought because of the pressure release valve, but it's located in another room 25 feet away. Some water seeps into that room, but looked like it was water spreading from the point of origin in the bathroom area.
2) The amount of water was based on a half dozen or so dumpings of the shop vac used to suck up the water; i.e., 3-5 gallons per load x 6 loads = 20-30 gallons of water. This amount has been consistent.
3) No correspondence with weather at all. New Years Eve night was cold and dry here. Another time it had rained the night before.
4) No correspondence with water usage, other than it's when water is NOT in use anywhere in the house, very early in the morning. The plumbers checked for leaks from the shower, and around the toilet (which I had already done.) The flooding appears to happen very quickly, within a few minutes. I'm also pretty confident it's not kid-related. Our kids are teenagers, and the boys, who have best access to the bathroom, are the ones who clean it up every time. (We even keep the shop vac downstairs for them. LOL)
5) Have already thought of the flood alarm, but the chlorine tester might be a good idea.
Thanks for the brainpower guys, I've run out of ideas, other than overhauling the entire bathroom. I really believe that the sudden onset of this problem holds the solution. Just wish the pros had been able to figure it out.
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Do you have a water pressure gauge? Often city water is at VERY high pressure during times of low usage and valves, joints, etc leak. If so you need a water pressure regulator.
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So, you have a bathroom IN the basement, not above it, which is built up on a platform underneath which the water flows. What is under the platform? Do you have a pump to move waste water UP into the sewer line? What exactly is the situation with the bathroom setup in the basement? --Phil
wordkyle wrote:

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wordkyle wrote:

It sounds like probably water is backing up and coming out the lowest drain in the house. Based on the timing and the volume of water, a couple possibilities are: (a) sump pump discharge, or (b) water softener discharge. The water softener in particular would probably be set to cycle during the wee hours. If you have either of those things you might want to have them checked out. Note that the problem may be that your house drain is blocked somewhere downstream and this is just the event that shows it because water is put in the system faster than it can handle. Although I would think that draining a bathtub full of water, say, or washing machine discharge would do the same thing. -- H
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Heathcliff wrote:

First off, recommend not going into unknown source water with bare feet.
Is there any water on top of the platform when the flood happens? If so, I would be looking for the source at or above that level and in the vicinity if the platform.
I like the sump pump idea, is there a sump pump and where does it drain?
Randy
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wordkyle wrote:

Get a chlorine test kit and the next time it floods check the water for Cl. if it extremely low compared to normal tap water the source is most likely condensation or maybe rain. In my case it was condensation. Not as big a problem as yours but big enough. The water would suddenly come out from aound a pipe in the ceiling of the garage which was under the house. Old vinyl floori covering was creating a kind of valve that would allow the water to build up then suddenly empty. toilet and shower were on a platform. I guess this was so you could get the car in the garage. Problem was cured when bathroom was overhauled and garage was dug lower. The platform was removed and the plumbing was run in the garage. Rguardless of what is wrong you will probably have to rip out at least part of the platform to fix it.
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Sounds like a pressure relief valve letting go. There is one on the water heater and may be others in your system.
Look for where it drains.

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That is a real mystery. Any chance the city main sewer is clogged or overflowing occasionally and backs up your toilet? We had a sewer problem that showed up in our condo first because we are the closest to the city main sewer line. Both toilets began backing up and initially there was sewage in the puddle but then it seemed cleaner. First thing I did was to ask other neighbors if they had problems, and initially they said no, but everything was coming into our unit so they didn't see any backup. Then I called the city (Sunday evening) to see if there were problems downline. Nope. Then we got a plumber to ream out all the way to the city main, about 300'. Nothing of note pulled out. About a week later, the city was excavating right where the sewer line from the condo joins the city's main sewer line. There are also pumping stations for sewage on the city side to get it from here to town.
Maybe you just have a bad seal and if the city's lines are overloaded you get some back pressure occasionally?
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Perhaps what you are seeing is the result of a sump pump with a deteriorated out put hose. The reason it doesn't happen all the time is that the pump would only come on when the water in the sump reached a certain level. This might also account for the 20-30 gallons, as most sumps that I have seen are not much larger than a small trashcan (32 gallon).
Interesting problem. Please keep the group informed.
.

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