Smart / Wi-fi /3G connected devices to catch a roof leak early

[Guido Jaramillo]

I currently manage 9 properties and I live about hour and half from the properties.  Over the last two years, I have had major issues with the roof in a duplex.  The original builder did a crappy job.  Last year, I had to replace most of the roof (flat roof) and the ceiling as water had been trickling in over the last 5 years since it was built.  I have been looking at many connected devices that let you know, if there is a leak or if the sump pump is malfunctioning.  I wonder what I can use to track the ceiling that is below the roof to catch a leak before it becomes a major issue.  There is no attic, it is a flat roof with a roof deck on it.  For example, if I put a device that monitors humidity above the drywall in the ceiling, would that be of any use? 

I have thought about putting one of the leak devices that you can buy to put next to the washer and install it above the dry wall in the ceiling, but if that requires for the actual water to touch it, it may not be very useful as probably by that time, I will already see damage or spotting in the ceiling.  Any ideas?