[AFMUG] Nonpen on River Rock Roof

[Nate Burke]

Thoughts on pros/cons.  When putting a Non-Pen on a roof with river 
rock, (if that's what it's called, 1/2"-1" diameter stones, no tar) do 
you clear the rock and put the non-pen mats directly on the roof, or put 
the mats and non-pen on top of the rock?