<< My bet is still for built-up with gravel ballast ( need a to specify
kettle man).  I PM some roofs that are 75 years plus. >>

Please clarify, at least this section.

sign me,  Ignoramus Igitur

****  As Ralph was noting in his roof reply:  Hot applied built-ups are the
best, most durable.  "I PM" means "roofs that I have Preventative
Maintenance contracts for inspecting and repairing.  "specify kettle man" is
just that:  Many of the 20 year systems for built-ups now available do not
use coal tar. The temperatures at the kettle and the roof are critical for
all applications.  They vary and once the felts are mopped in and the
ballast is in place that's your roof.  As Ralph suggests: experienced crew.
To that end I require bidders to specify workers by name as a five man crew
with kettle man.  I require the name of two alternates and the backup kettle
man.  If they can't do that then they are not in my mind qualified to apply
a hot roof.      As built-ups went out of vogue in the seventies and
eighties, there were few opportunities for training these crews.  Odor,
set-up issues and level of skill for application, all work against these
systems.
Best,
Leland