I believe it is common property, but whether it’s an insurance job or Strata cost is debatable as insurance could claim it’s just fair wear and tear if it’s been on since 1974. Flat roofing membranes don’t last as long as tiles or metal roofing. Last time I had a flat roof the membrane only lasted 12 years before leaking and requiring replacement.
I found the following excert from:
Principles and provisions of Memorandum No AG600000
2.3 Ceilings and Roof. Common Property.
a. Ceiling cornices.
b. False ceilings installed at time of registration of the strata plan.
c. Guttering.
d. Membranes.
e. Plastered ceilings.
f. Vermiculite ceilings.
2.4 Ceilings and Roof. Lot property.
a. False ceilings inside the lot added after registration of the strata plan.
b. Paintwork inside the lot.
It is produced by NSW Gov but it seems it needs to be approved by your OC to be valid. Don’t fully understand the implication.