There are so many perennials that you can try.  You can even try growing some annuals to give you more color.  Perennials you can try are sedum (stonecrop), creeping phlox, creeping thyme, candytuft, ice plant, dwarf daylilies, dwarf coneflowers, dianthus (pinks), catmint, bee balm (can get tall), speedwell.  

When you’re looking for plants, look for the lower growing varieties.  I would stay away from aggressive vines like Virginia creeper, crossvine, honeysuckle, trumpet vine, climbing hydrangea.  You could put in a clematis and let it ramble over the roof, but roots need to be in shade.  
You did not mention the shape of the roof or if it’s in sun or shade.  You probably already know that you will need to make sure that your shed can support the weight of all that will be on it.  A heavy layer of plastic will need to be put down to keep the constant moisture off of your roof shingles and help keep rain or any water from finding its way inside.  Then you need to install supports around the edge to help keep the soil from sliding off the roof.  

Have fun shopping at garden centers picking out your plants.  If you have any questions, please feel free to call the cooperative extension in your county and talk to an agent or a master gardener or Ask Extension.

Thank you for contacting Ask Extension and the Delaware Cooperative Extension.