Automate pumping water off a flat roof

Question

[Update 2022] We sold this house, so I am no longer seeking guidance on this topic. Thanks to all who answered for the suggestions.

[Original post] I have a 1950s home with a flat roof. The drainage is not great, so we use a water pump (currently a Wayne WaterBug Multi-Flo) with a hose for the output, to pump the water off the roof and away from the house.

The great thing about the WaterBug Multi-Flo is that it can pump down to 1/16" of water; and it can be activated with only 3/8" of water. Our roof ponds mostly in one place, and we don't like to have much standing water up there, so the 3/8" -> 1/16" is ideal.

We have two difficulties: automatically turning it off, and automatically turning it on.

We've had a couple pumps wear out from running dry, when we weren't able to manually turn it off in good time. And if we ever go out of town it would be really nice to know that it can turn itself on when appropriate, as well.

There are numerous products on the market that do have auto on/off capabilities, but everything we've found won't activate until around 1-3/4" of water or more; we'd really like something that will activate with 1/2" of water. (For example the Wayne WaterBug Auto pumps from 1-5/8" down to 1/2")

I see there is a new "WaterBug Glow" unit that has an auto-off capability. However it appears to work through thermal detection -- it takes 10 minutes to switch off at 70 degrees F, and longer for lower temperatures. In the Seattle area where we live, it's typically 40-45 degrees when raining in the winter, so I'm concerned about burning out the motor waiting for the auto-off to engage in the low temperature.

What I'd really like to find is: some kind of water flow meter, that can be configured to cut power when the flow falls below a certain amount. Would some kind of leak detection work? I'm not sure. I definitely don't want an alarm though. And then how to reactivate? I don't want to have to climb up on the roof to reactivate it.

If the WaterBug Glow works reasonably well for auto-off, perhaps I can put the thing on a timer, so it turns itself off and on once every 6 hours?

Interested to hear any ideas -- maybe there's a perfect pump sump pump out there with the features we need? Or some other devices I can pair together and configure for a good solution? We've spent quite a bit of time researching and are currently stumped. Thanks for your help!

EDIT: yes, the correct solution will be to redesign + replace our roof and drain system. A flat roof could work with good drainage but the drains are not good-- flooding water right next to the house rather than some ways away. And they are very difficult to keep unclogged. So we are looking for a short-term solution to serve us until we can afford to replace the roof and drainage system.

Answer 1

You may be trying to do something that could lead to a major failure by overloading the roof structure with the weight of the standing water if the pump does not operate properly AND violates the Building Code by allowing too much standing water to accumulate on the roof.

There are many circumstances where a pump could fail, but is even more dangerous with an automatically controlled pump.

In addition, the Building Code does not allow more than 2” standing water on a roof before it can self drain through a roof scupper. (See ICC R903.4.1)

Standing water is not a good design idea. If the roofing develops a pinhole leak, the entire roof will drain through that hole. It’s better to change the design of the “flat” roof to a sloping roof.

I’d spend my time and money eliminating the low spot and forget finding an automatic pump.

Answer 2

Attach a Watchdog float switch unit to your Wayne WaterBug and it will automatically turn on and off without burning up your motor. Have used this combination for years with great success. Have purchased mine at Lowe’s

Answer 3

Late to the game, but it may help someone else:

A Tsurumi Pump Surface Level Switch is designed to control a "puddle pump" with turn on at 3/8" and off at 1/8". (It can be adjusted higher with included screws.) It has a built-in 10 second delay turning off so that ripples don't cause short cycling.

It's submersible, but not intended for prolonged outdoor use. You would need to secure it so that wind, squirrels, ... don't cause it to shift around the roof.

I'd set the pump at the lowest point and the switch a tad higher. That should reduce the chance of the pump running dry.