A green roof is Low maintenance not No maintenance
The principles of a GreenRoofTops roof is that there should be just the right amount of substrate for the plants to grow in. This is very important as it must be enough to allow the sedum to florish, but not enough for weeds and other invaders to become established. The substrate should provide enough nutrition and water holding capacity for the sedum, but not for weeds. Don’t be tempted to water it every time it looks a bit dry but leave it to its own devices unless you have reached that third week without rain which is unusual in this country. If you cannot water it at all, then usually it will be just fine. Sedum is one of the most drought resistant plants and it will out-live pretty much anything else on the roof so don't panic!
We recommend to apply OSMOCOTE slow release fertilizer every 6 or 12 month in April or October time, depending on how your roof is looking.
Weeds: If for any reason you have got a weed problem and they have not died out during the summer then they are best removed by pulling. The best policy is to tackle it before it gets to be a problem.
If you do have bare patches then give the whole roof a good dose of fertilizer, if there are any good bits, then pull off a few cuttings and sprinkle them over the thin areas, Then put a bit of compost on top to slightly cover but not completely bury the cuttings. Do this in the autumn or spring and they will root very readily.