The case imabouttoblowup mentioned falls into exactly the category that is carbon negative, though.

Let's say you have a lawn. The grass grows by combining water with CO2 in the atmosphere. It doesn't (mainly) build itself from carbon in the ground. That's why a big tree doesn't end up standing in a huge hole. If nobody mows the lawn then dead grass will end up rotting, but rotting it doesn't free up all its carbon. The result is humus which accumulates with each generation of grass. So over time, the lawn moves upwards, rising on a thicker and thicker layer of humus. A lot of that extra mass is carbon captured from the air.

If somebody starts a garden on their roof and leaves it to itself (and it doesn't die), then I would expect the same thing to happen there - a build-up of more and more organic material there. I've seen old grass-roofed cabins with thick wildernesses on top of them.

But anyway, carbon capture was not the purpose of this new law. From the article:

Green roofs have an isolating effect, helping reduce the amount of energy needed to heat a building in winter and cool it in summer. They also retain rainwater, thus helping reduce problems with runoff, while favouring biodiversity and giving birds a place to nest in the urban jungle, ecologists say.