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 flourish, but not enough for weeds and other invaders to become established. However we live in a climate with weather conditions that are not completely predictable and therefore what might be perfect one year may not be the same another. The substrate should provide enough nutrition and water holding capacity for the sedum, but not for weeds. The principle being that whilst weeds may become established during wet periods, then during a decent dry spell they will die out each summer and therefore will not be long term residents on your roof. This does mean however that you cannot always be looking at a lush green carpet of a roof. If you provide the conditions to enable the sedum to grow like that by for instance having a deep compost layer, giving much fertilizer and water then yes, your sedum will grow very well, will lower very profusely, and look great for a while. However you will end up looking at a mass of dead flower stems which can smother the growing plant and at the same time may get infested with weeds. What you should aim to achieve is a fairly short plant with a sprinkling of flowers. Dont 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 or fourth 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 dont panic!
Sedum is a very responsive plant and is even more helpful to us in changing colour pretty readily. When very happy it is green and when stressed it will furn red. Some varieties are more reactive than others but they all do it to some extent. The stress levels of your sedum depend on the following: water, fertilizer and temperature. So during the winter when cold or the summer when dry it will naturally look redder. In the spring and autumn, then it should return to green. If it still looks red at these times of year then it needs fertilizer. For our blanket or module based roofs we recommend about 20g / m2 of a special slow release fertilizer ever 6 or 12 months depending on how your roof is looking.
Over a period of time as the natural base level of nutrients in the compost gets used up it is common for a the sedum on a roof to gradually get smaller and thinner, eventually bare patches will form. This may happen over a period of several years and you may not notice it happening. So I would recommend that you take a picture of your roof from time to time so you can see for instance what it looked like this time last year. If it is not so good then the chances are that it needs some fertilizer.
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 puling. You can use a spot treatment of Glyposhate with a weed dabber which are sold for patios but be aware that it will also kill the sedum. The best policy is to tackle it before it gets to be a problem as you are less likely to damage too much of the sedum when removing the weeds.
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 sprinkler them over the thin areas, then put a bit of compost on top to lightly cover but not completely bury the cuttings. Do this in the autumn or spring and they will root very readily. If in summer then watering several times a day until rooted is essential.