Where it gets tricky for a McMansion is that green roofs tend to lend themselves to shallower gradients, not to 20deg-30deg pitches. We’ll assume for now that a McMansion roof structure [typically prefabricated timber trusses] has enough load capacity to bear a fully soaked green roof.
Here’s how it could potentially be done;
1. Remove the existing cladding – whether it be concrete tiles or metal decking. Metal decking could remain if the load isn’t too much. Replace with marine ply board;
2. Add the requisite layers of waterproofing, drainage cell, insulation and geotextile;
3. Add the perimeter angles to hold the soil/planting [sounds like it could be a tricky detail, but it is possible];
4. Add the soil profile and planting. For this one there are various methods available – I didn’t have any luck sourcing Australian examples/products so the US it is. There are proprietary soil stabilisation products available for steeper slopes with in-situ planting, or there is planting in plastic trays or even mats which come ready-established.
I agree with the final assessment of the post: I’ve not seen this proposal before. How much might it cost to retrofit the roof of an existing large home? It seems like the easiest way to make this happen would be to change buildings codes to require greener roofs and then the cost simply becomes part of the new home.
With more interest in greener dwellings (tiny houses, net zero energy homes, passive homes, etc.) plus the negative connotations of owning a McMansion or larger homes, I suspect more of these homes will be constructed with green features. However, I continue to wonder: will a large home with some green features, like a green roof, be considered green enough?