30 to 50 years is not an unusual lifespan for green roofs in europe.
Green roof lifespan.
A properly installed and maintained green roof should extend the life of the roof 2 3 times its normal life.
Irrigation wastes water and energy both are what the green roof should be conserving.
By protecting the roof membrane however a green roof can extend the life of a roof by two or three times beyond its typical lifespan.
Green roofing can extend the lifespan of a roof by over 200 by covering the waterproofing membrane with growing medium and vegetation this shields the membrane from ultra violet radiation and physical damage.
As a result the roof structure can require less maintenance saving the owner money in replacement costs over the long term life of the roofing system.