Hi, in the latest edition of the Gardener's World magazine there's an excellent section by Carol Klein on plants for different types of green roofs. Here's her list for roofs of under 50mm soil depth:
Acaena microphylla - dense carpets, drought tolerant, flowers June to September
Cotula hispida - forms mats of tiligree silver foliage with tiny button like daisies, flowers May to August
Sedums like rupestre, reflexum and acre
Sempervivums, and close relative Jovibarba, don't need soil! Legend has it that they stave off thunderbolts! Flower June to August
Sea Campion - Silene maritima - flourishes on thin dry soil and extremely tolerant of exposure. There are simple and double forms (simples are more likely to be pollinated I believe as doubles can cover the pollen which makes sense). Flowers April to September
Creeping Thyme - Thymus serpyllum - nectar rich. thrives in thin soil with sharp drainage and will form dense cover. Flowers May to August.
Plants for less than 100mm
Kidney Vetch, Anthyllis vulnararia
Thrift - Aremeria maritima
Poppies - Papaver miyabeanum, self-seeds
Auriculas - Primula auricula - loads of cultivars, alpine
Rhodohypoxis baurii - likes dry winters but as long as veru well drained, sit bulbs on sharp sand piles?
Encrusted saxifrage - Saxifraga cochlearis
Soil depth 150mm or so
Atrantia 'Roma' - shorter variety but sterile hybrid so won't spread itself.
Purple milk thistle - Galactites tomentosa - insects love it and birds love the seeds
Great Burnet - Sanguisorba officinalis - used to control soil erosion because of extensive root system!
Aquilegias - this is a surprise as I'd always thought they like moist, sheltered positions! They'll colonise happily apparently!
Stipa tenuissima, and cultivars - I know there's a lovely blue one, the name of which I forget now (I think it's a Stipa ??)
Viola cornuta - Carol finds it grows well on her roof, but she's in Devon I think! Perhaps this grows well in the shade of plants like Stipa.
Some of these are listed below I think. I'm about to build a roof garden over an extension so I'll be putting in some of these too.