We always advise clients with roofs with even a very shallow pitched roof that they should consider some sort of irrigation for their roof. Although sedum really doesn’t like being waterlogged, as a living plant, it does need water to thrive. We have noticed that as we have been having longer hotter dry spells that some of our roofs have got a bit sad.
This wonderful roof is having a full irrigation system installed which is obviously a very good solution. The pipes will soon stop being so obvious once the sedum starts to grow! A cheaper, but more labour intensive, option if your roof is accessible, would be to water it yourself in the evening in very dry spells and generally keep an eye on it.