Moisture / Flood Sensor


A few years ago, the drain in the floor of the walk-up entrance to my basement door became blocked, and water started to enter into the house.  I did not discover this problem until much of the basement carpet was soaked (several hours later).  After the cleanup, I decided to install some sort of flooding/water intrusion detection.


The sensor is simply a standard $5 battery powered smoke detector that has a pair of wires soldered across the "test" button.  If the other end of the wire pair gets moist or wet, the smoke alarm will sound-off.  This home-brew or do-it-yourself sensor is inexpensive, yet effective, as it already has a very loud sound emitter.

Photo of the installed water intrusion alarm.  Note the pink colored wire pair wired
across the test button.  If the other end of the wire pair gets wet, the alarm will sound.
Note the edge of the blue note paper under the alarm with instructions if the alarm goes off.


I fastened the unit to a two-by-four stud in my mechanical room, and ran the wire pair to three locations: my basement walk-up entrance (near the floor), my sump pump (in case the unit dies), and near my water meter (in case the pipe from the street bursts).  I also attached a small piece of paper dating the work, and with instructions on what sites to check if the alarm sounds.  Over the years, numerous flooding incidents have been averted with this sensor.  When the alarm sounds (sounds very different than the smoke alarms that we have), we know to clean the drain in the walk-up entrance.  In addition, the unit gives us a backup smoke sensor in the mechanical room, where the furnace and water heater are located.

Long Term Update

Hello Ed; recently I spent a couple of days searching the web for a reliable water/leak sensor for our washing machine, however, the reviews for those I found were not very good. Well I was just about to give up when I came upon your terrific idea using a smoke detector for the task. I am always delighted when I find a way to re-purpose an existing item, while saving money and learning; and your's is a really good one.

After updating our smoke and CO detectors I had saved the older one for some future project and so I decided to give it a try and it worked just great! Because the bottom of the washer has a metal pan with large recesses in it I wanted to locate it there. Of course the metal was a problem so I had to come up with something that would serve as a dielectric and still work well in a leaking situation. What I finally came up with was a (dry) kitchen sponge(3"x4"x1/2"); the kind you find in the dollar stores. Anyway, I stripped about one inch of insulation from the wires, tinned them and inserted them about 1/2" apart into one end and set it on the bottom of the pan. When I wet that end of the sponge with about a tablespoon of water the alarm responded immediately. I am telling you this in the hope that it may be helpful to you or someone else in the future just as your idea helped me.

In any event, thanks for sharing your ideas and for the rest of your site which I enjoyed immensely. Reading about your work, vacations, the family album and the garden, one of my personal favorites, was quite pleasurable as I enjoyed it immensely.

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