Determine current age of BUR?

How you guys go about determining the useful life remaining for BUR systems. I know it will vary by state/region.

We have a client that asked for an overall assessment and wondering how much longer the roof may last. I do not have any permit history on the property.

Metal deck, 1.5 lightweight, 4-ply. 2’ parapet walls are smooth sfc cap sheet showing pretty severe wear. Building was constructed in 1976

If the roof was done in 1976 I would say it probably has reached its reasonable lifetime. I don’t know that you can determine exactly how old the roof is unless you know when it was done. You can determine how much longer it has to go by observing the wear and signs of leakage. Also the pitch of the roof will tell you a lot about the possible longevity of the roof. Usually, the steeper the longer it will last.