What is the best way to protect a flat Derbigum roof ?

Hi,
I have a flat roof on my house extension approx. 20ft x 20ft
Top coating is Derbigum which has a silver colored paint over it.
It is now 25 years old and the silver paint is completely worn off on a small 3ft. x 2ft. section. This section looks vulnerable to accelerated deterioration and i would appreciate advice on maintaining my leak free roof.
Thanks in advance.
Eugene.