Felting a timber shed roof?

A single row of blocks mortared inplace sounds good as a little stem wall. If the shed has 4" thick walls you could use 4" blocks and grout in foundation bolts in 2 holes on each side. The bottom plate should be a treated board. If you are using rolled roofing staple down a layer of 15 pound felt paper first and overlap the seams about 4". Put galvinized metal L shaped drip cap around the edges and tack it in place with small nails along the top edge of the metal. Then roll out the roofing material and cut it so it overhangs the eave and facia edges slightly, about 1/2". Use tar sealer in caulking tubes and put a couple beads all around under the edges. Then nail the eave and facia edges about every 3" or what ever looks good to you. Then cut the next course overlap the first about 4"( on this seam you may want to use a brush and coat it good ) and nail the seam about 1" up from the edge and every 2"or3"and so on.