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Howard-
Cat poops tho :D
The substrate / density of most seedum puts them off, it seems - They just don't crap there. They sleep there instead.
The worst I've seen on either of my 2 growing roofs is some sort of owl pellet, and whole eggs / walnuts buried by the local idiot squirrels.
Seedum roofs are heavy though (up to 100kg / m2) - you'll need to check that the roof is able to support it without deflecting too much.
NotThamesWater
Rubber/EDPM with a green roof on top? Sedum for low profile, low maintenance, lightweight (less chance of increasing cost of structure). The plant stuff helps protect the roof finish.