In 2008 I did a simple 16 x 30 lean to roof on my 30 x 80 shop building. I lagged a 2 x 8 nailer board onto my exterior stud wall, then used 2 x 6 joist hangers to hold the rafters. I then used a glue-lam beam for rafter end support over 4 vertical 3 1/2" dia. steel columns set in 12" sq. concrete pads. Once the roof was up and shingled I had the slab underneath poured with a 1/2" slope for drainage which allows me to wash it down every so often. Used good 6 bag mix and #3 rebar @ 12" o/c and haven't had any problems or cracks.
