A flat roof must provide a way for water to go.
If not, water sits, and finds any small opening it can.
Since we know gravity works, if water can go down, it will. On a flat roof surface, that means eventual destruction.
This is a 17 year old EPDM roof surface. 
EPDM is a synthetic rubber surface, manufactured by Firestone, and is an excellent product when installed correctly. If it is not failure is imminent.
The bubble on the edge means the glue is giving away. The staining on the wood post trim indicates rot. And notice how the post is sinking into the roof surface. Can you guess why?
This view shows the same post from the other side.
Can you tell more of what is causing this problem?
The photo was taken by my reaching around and through the guardrail stiles.
I did NOT want to get onto the roof!
From that same vantage point I could see and photograph the other side of the roof, and its opposite post and railing.
This is the best view of all to see the problem.
Look at how high the water level can get during rain storms! And notice how the roof is sinking downward toward the left edge.
The railing was placed directly onto the EPDM surface and nowhere was provided for water to go.
So it sits. And makes its way where it can. Where does some of it go before it evaporates?
Down. The moisture meter under the first post registered about 28% moisture.
See how the colored gage only goes to 30%? It measures wood moisture content.
That is because wood is considered to be saturated at 28%, so above that is not much more information.
Actually the entire underside of the roof surface registered 28% so in my opinion the structure of this roof has been compromised.
Why? Because there water was never given the opportunity to leave the roof and it has done what water does best - it saturated the roofing.
My recommendation: flat roofs need careful examination. Often the people installing the roofing surface, which is typically some form of membrane material, and are not entirely trained on how to install it. And failure happens. Certainly when the roof fails, and someone buys the house without knowledge that the roof is failing, they inherit an expensive problem. Get a home inspection!
Jay Markanich Real Estate Inspections, LLC 
Based in Bristow, serving all of Northern Virginia.
Office (703) 330-6388 Cell (703) 585-7560
Jay Markanich, thank you for such a clear explanation and pictorial of problems with flat roof installation and wood saturation. Too often, flat roofing is installed improperly which reduces its life span.
Good morning, Jay Markanich well, how many options are there when installing a flat roof?? pitching it just enough to let gravity work in the proper direction?? then a gutter and downspout system would be needed to divert that water away from the house??
Good morning Jay. Flat roofs have always been a problem to me. No more, I won't have them.
Dave - so true. Installing most everything incorrectly reduces its lifespan!
Barbara - or simply create a big bird bath. Sheesh, what's wrong with a bird bath? And, by the way, you are looking younger every day.
I have heard that from many people, Sheila. Usually life experience is enough to teach people the do nots in life.
We don't have lots of flat roofs in our part of the world...nothing for snow to slide down !
We don't do many flat roof homes in Oklahoma City but a few builders that do make is slightly slanted for water runoff.
We don't have too many homes with flat roofs here in Sacramento. I can see why there could be issues.
Jay- those flat roofs remind me of the 50s style ranch homes. A friend was considering buying a home with just such a roof and asked for Larry's opinion. They did not buy the home.
S&D - it snows there?
Of course, Joe, and that's a must. Still, even with a slope you can't block the water with a railing!
Myrl - hence the lesson in today's post.
Probably sound advice, Kathy. Flat roofs can be problems waiting to happen.
Thanks so much, Jay, for sharing your experience and your photographs.
Thank you Roy. I try to contribute here, but it seems I am getting fewer and fewer viewers and comments on what I think are very instructional posts. So I have been posting less. The time spent on AR seems less valuable, so I am spending it elsewhere.
It's rare here to find a flat roof on a house here, esp w/ all the snow. But, I have noticed plenty of them here downtown in the city, so I think they have a lot of issues.
I've heard that from a lot of those up north Debbie, and all for a good reason. But you're right, in older structures flat roofs were more common.
Not very common here...or desirable...Santa prefers his sled on an angle !
Jay, just had to deal with one on a porch...gutters at the end made a difference because it has a itty bitty pitch to it but I have always seen these roofs as a red flag.
S&D - I would think not!
Ginny - so do I. In every case I am anxious to see what I can see.
Hi Jay - I would not have a flat roof. We have one on a building and it is always causing problems with repairs.
I agree, Grant. They often prove problematic. And necessarily.