Someone at some point on here or Pnet suggested looking for mud splatter and putting the lowest rack above that. Sure enough, we have a canoe temporarily stored outside on upside-down plastic lawn chairs, maybe 8 inches clearance or so. There are mud-splatters on it from rain. I imagine that will be extra wear and tear in the long run, especially on woodwork. Sounds like you'll have a roof, but something to consider if the boats will be close to an overhang without gutters. I would also want enough space to allow ground moisture to dissipate. Again, depends on the size of the roof, how close the boats are to the edge, and what's under the boats (grass, mud, gravel, concrete pad).