Turning a flat roof into a planted one is one of the most frequent requests in urban refurbishment, but it requires checks that go beyond putting soil and plants on top.
Structural capacity: the first limit
The substrate, vegetation and retained water add considerable, sustained weight, far greater than a conventional flat roof. Before quoting any planted finish, you have to confirm (with a technician if necessary) that the existing structure can carry that additional load; if it can't, the green roof isn't viable without prior structural reinforcement.
Specific layers: waterproofing with root barrier
The waterproof membrane of a planted roof must include (or be combined with) a specific root barrier, since roots can puncture conventional waterproofing over time. Over it goes a drainage layer that drains excess water without waterlogging the substrate or overloading the roof.
Extensive or intensive: two different levels of demand
The extensive roof (low-maintenance vegetation, thin substrate) adds less weight and cost than the intensive (walkable garden with larger plants), which needs a greater substrate depth, programmed irrigation and a structure prepared to carry considerably more load.
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