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!
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