How to Find a Leak That Only Happens Sometimes
Sep 18, 2026A stain appears on the ceiling, then dries. A patch of water shows up on the laundry room floor, and a week later it is back. You call somebody out, they look around, everything is dry, and there is nothing to find.
The short answer: an intermittent leak is the hardest kind to find, because by the time anyone arrives there is nothing happening. The way to catch one is to stop waiting for it and make it happen on demand — by running, one at a time, the things in your house that move water only when switched on.
Why intermittent leaks go unreported for months
They are small, they stop on their own, and there is nothing to connect them to. So people stop noticing. On a job recently the homeowner had not mentioned anything at all — we were there for a routine check. When water appeared, he said:
“It’s weird, because I’ve been noticing water on the floor every once in a while. I never knew what it was, and I didn’t really think twice about it.”
That is not carelessness. That is what intermittent does to people. A problem with no pattern becomes background noise, and it stays that way until something forces it to happen while somebody is standing there.
The logic that narrows it down before anyone opens a wall
Continuous cause, continuous symptom
If something runs all the time, its symptoms show up all the time. If a symptom comes and goes, the cause comes and goes with it.
So the question stops being “where is the leak” and becomes “what in this house runs only sometimes, and does the water appear when it does?”
That reframing matters, because it turns a search into a test. And a test can be repeated.
What you can switch on and off in an average house
This is the useful part, and it applies whether or not you own any of our equipment. Run these one at a time, and watch the spot where the water shows up:
| Run this | What it tells you |
|---|---|
| Water softener regeneration cycle | sends a large volume down a drain line that often runs through an attic or wall. One of the few things you can trigger deliberately |
| Dishwasher, on a drain cycle | drain hose, air gap, and the connection under the sink |
| Washing machine, on a spin and drain | standpipe and the drain behind it — a very common source of intermittent floor water |
| Each irrigation zone, one at a time | isolates a single underground line instead of all of them |
| Both showers, then tubs, separately | pan liners and shower valves leak only while running |
| Water heater relief valve discharge | discharges occasionally, which is exactly why it gets missed |
Do them one at a time and wait between each. Running everything at once tells you only that something in the house leaks, which you already knew.
What happened on a job in Apollo Beach
We were at a house in Apollo Beach for a routine system check. The homeowner had reported nothing. As part of checking the system properly, we put it into a regeneration cycle — and water began dripping from the ceiling.
On that build, the softener’s drain line runs up through the attic, across, and down into the laundry room to an air gap over the laundry drain. That is a standard arrangement here.
The builder came out and suggested the air conditioner. It is a fair first guess — attic air handler, condensate line, water in a ceiling. But it did not survive one question:
An air conditioner runs constantly. If it were the AC, there would be water constantly. The water appeared when the system was put into regeneration, and stopped when it finished. It could be turned on and off.
That put it on the drain line — our system’s drain line. So that is what we told him, and we told him to get somebody up there.
The cause was a nail. It had been driven through the drain line during unrelated work in the attic. The builder repaired the pipe and patched the ceiling. Nothing to do with the water treatment system at all — the drain line simply happened to be where the nail went.
Why we are usually the first thing blamed
When there is water somewhere it should not be, the water treatment equipment is the first thing people look at. It is the newest thing in the garage, it has tanks and hoses, and it is easy to point at.
We have got used to it, and it has made us good at proving what is actually going on. Sometimes the answer is our equipment, and when it is, we say so — on the job above, the evidence pointed straight at our own drain line and we told the homeowner exactly that before we knew why.
More often it is something else, and the only way to know is to test rather than argue.
What to do if this is happening at your house
1. Photograph it, with the date. Intermittent problems are dismissed because nobody ever sees them. A photo with a timestamp is evidence.
2. Note what was running. Laundry? Dishwasher? Sprinklers? Had the softener just regenerated overnight? The pattern is usually in there.
3. Run the list above, one item at a time. Watch the spot. You are trying to make it happen, not avoid it.
4. Once you can reproduce it, call the right trade. A drain line in an attic is a plumber. A roof penetration is a roofer. Knowing which one saves you paying two.
Not sure whether it is your water system?
We will come out, run the system properly, and tell you whether it is ours or not. If it is not, we will tell you what we think it is and who to call.
Chasing a high water bill instead? Here is how to tell a leak from a meter problem →
Frequently asked questions
How do you find a leak that only happens sometimes?
Stop waiting for it and try to cause it. Run the things in your house that move water only when switched on — the washing machine, the dishwasher, a softener regeneration, each irrigation zone — one at a time, and watch the spot where the water appears.
Water only appears on my floor sometimes. What causes that?
Something that runs intermittently. Common sources are a washing machine standpipe, a dishwasher drain connection, a water heater relief valve, or a water softener drain line. A continuously running appliance would leave water continuously.
Could water on my ceiling be the air conditioner?
It can be, but an air conditioner runs most of the time in Florida, so it usually produces water most of the time. If the water appears only occasionally, look for something that also runs only occasionally.
Where does a water softener drain line run?
It commonly runs from the system up through the attic and down to a laundry drain or standpipe, with an air gap at the end. Because much of it is out of sight, a leak on that line can show up in a ceiling far from the equipment.
Does a water softener only use water when it regenerates?
A softener sends water down its drain line during a regeneration cycle, which is typically once every few days. That is why a leak on the drain line appears intermittently rather than constantly.
Should I call a plumber or the company that installed my water system?
If you can show that the water appears when the system runs, start with whoever installed it. If it appears at other times, a plumber is the right call. Being able to reproduce it tells you which.
