My neighbor asked me for help with his flat roof (I think it's EPDM membrane), which sometimes has problems with water pooling on it during prolonged heavy rain. When enough water has pooled, it leaks through the membrane seams (despite being freshly resealed), and ruins the ceiling below.

There are two issues that cause the pooling:

  1. Leaves from nearby trees clog the scuppers and block water from going down the downspout.
  2. The building has settled over time, so some portions of the roof no longer slope toward the scuppers.

Solving the first problem is easier - we are setting up automated calendar reminders for someone to go up there and clean out the leaves.

The second problem can really only be fixed by completely ripping out the roof and redoing it, which is not a financially viable option right now. However, with light rain (or heavy rain for only a short period of time), the water typically evaporates quickly enough to not cause any damage, and storms big enough to leave puddles usually happen only once or twice a year. For the last few years, he has been sending someone up there after heavy rain equipped with a shovel, broom, or floor squeegee to push the puddle off the roof.

Right now, the only way to know if a particular storm was heavy enough to leave a puddle is to go out there and check, or to wait for the ceiling to start dripping (which is already too late).

I suggested that we hook up water sensors in the low spots, connected to his Elk M1 Gold home automation/alarm system to alert when water is detected. (Right now I'm looking at using either WaterBug or GRI sensors, but I don't think the specific sensor model will matter for this question.)

I need a way to mount the sensors on the roof without drilling holes. The sensor also needs to be above the surface so that it doesn't alert on lighter rain that just gets the surface wet, only when the water actually starts to pool. Ideally, I'd like to be able to adjust the height a few times after initial installation so that I can fine-tune it to avoid false alarms. It would also be nice if whatever mounting is movable so that the person who goes up there to remove the water can move it out of the way and then put it back (I'd paint a mark for the correct spot.)

The flat roof is about 25 feet wide and 80 feet wide. Along the west side, there is the wall of the next story of the house (stucco over Styrofoam over plywood sheathing), and along the east side there is a parapet (metal flashing over stucco over wood). The puddle is closer to the parapet, but the sensor cable will have to run across the roof back to the wall of the higher part.

The three main types of options I'm considering are:

  1. Mount to the wall of the higher part. The cable can run along the mount, and the mount can hinge out of the way, but the mount would have to stick out up to 17 feet from the wall, which is unwieldy, a lot of material, and probably difficult to adjust to a specific height.
  2. Mount to the parapet. The mount would be closer to 3-4 feet (at most), but the cable would have to run across the other direction, and that would be a tripping hazard to anyone doing cleaning up there because the cable would still be stretched across the roof even with the mount swiveled to the side.
  3. Build some kind of weighted holder. I'm thinking this might be the easiest way, but I'm concerned about leaving a bunch of weight sitting in one spot all the time.

I am planning to put two or three of these in various places on the roof. How can I make sure they stay at the correct locations, heights, and orientations to make sure they do their job?

  • As an alternative to a sensor, consider a camera mounted to an existing pipe or chimney. Cellular game camera set to take a picture every X hours. Outdoor wifi camera with solar panel. PoE security camera. CCTV camera. He can take a look after it rains. Just throwin' it out there.
    – MTA
    Commented Jun 1, 2023 at 18:54
  • There is already a CCTV camera that can see that area, and there is also a window in a higher story that can see it. The problem is that someone needs to remember to look at it, which the occupants are not good at doing.
    – Moshe Katz
    Commented Jun 1, 2023 at 19:33
  • i would power it with solar, even a modified garden light would work, so long as your sensor switch turns the radio on and off, instead of just providing an input to an always-on monitor. That uses zero power at rest, and won't turn on very often, so the solar cell and battery can be tiny. You can make the sensor with a microswitch above a chunk of styrofoam; it floats and pushed up, which activates the microswitch, which turns on the broadcaster or alarm or blinky. Since you have a camera, try just training openCV to detect the flood/blinky; that's even simpler than physical sensors.
    – dandavis
    Commented Jun 2, 2023 at 4:08
  • Do the scuppers have raised grates? I had a stairwell drain that would get clogged with leaves, but once I put a raised grate on it (in all honestly, my ghetto grate was a stainless steel strainer placed upside down), there was never a problem again. If the water got high enough to be a ponding problem, the leaves would float, allowing water to flow beneath. If you simply must install water sensors, is there anything wrong with a sticky mastic?
    – Huesmann
    Commented Jun 2, 2023 at 15:08
  • @Huesmann The leaves don't just clog the scupper itself, they also clog the hole in the parapet that leads to the scupper, and a grate would make that worse, not better. The two problems with mastic are that I want it to be raised so it isn't triggered by light rain and because ideally I want it to be able to be moved out of the way (or big enough to be a durable obstacle) so it isn't broken by someone using a broom or a shovel.
    – Moshe Katz
    Commented Jun 2, 2023 at 16:33

1 Answer 1


Mount the sensor to a smallish (but big enough to stabilize the sensor even in high winds) concrete paver. For a small light sensor, possibly even a brick. Set the concrete paver on the roof surface - set, don't drag or slide. If the sensor is not large and prone to catching wind, a paver adequate to stabilize it should not be an excessive weight.

Fixing the puddle (if the roof framing hasn't rotted out yet) should only require removing (or abandoning) the membrane, creating a new correct slope with rigid foam (XPS is typical) and putting new membrane over that. Not "ripping out the whole roof." If the roof structure has rotted, it will just get worse.

There are pumps that are optimized for such puddles. Seem to be called "puddle pumps," not very shockingly. Putting one or three up there to automatically remove water down to 1/8-1/4" / 3-6mm or so would be far preferable to getting someone to go up and push water around after too much has collected. In a very quick look (not making this a shopping answer) I find one that claims to start at 3/8" / 9mm and pump down to 1/8" / 3mm automatically. I would prioritize a pump to eliminate the problem above a sensor solution. The pump housing could be a place to mount a sensor to make sure the pump is working, though.

  • Most of the pumps I've found that require some manual intervention to start and stop them, which would mean someone has to climb onto the roof in the pouring rain. All but one of the "automatic" pumps I've found so far will only start pumping when the water is 1.5-2" deep (the exception is this one that has a repositionable sensor on a short cord, and the reviews are mixed). Having such a hard time finding a pump is why I focused on sensors in the question.
    – Moshe Katz
    Commented Jun 1, 2023 at 20:37
  • More context on auto vs. manual pumps in this question: diy.stackexchange.com/questions/212159/…
    – Moshe Katz
    Commented Jun 1, 2023 at 20:43
  • If your sensor triggers either a relay to automatically turn on the pumps, or an indicator for a human to switch on power to the pumps, no roof-clambering should be required. Providing switched power to rain-tight outlets with in-use covers would be required for that, of course.
    – Ecnerwal
    Commented Jun 1, 2023 at 21:30

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