text: how do i engineer a small structure to support a green roof.? i would like to construct an outbuilding with a green roof.  it would be dug into a slope and the roof would need to support foot traffic, dogs, as well as the occasional wheelbarrow full of heavier stuff.  i can find info on how to construct the green roof itself, but how do i calculate what is necessary to support it in this instance? you would build a green roof the same way you build a orange, or purple roof.  however it is slightly different from a blue, red, or yellow roof.    it depends on the square footage of the roof, the materials that you will use, and the restrictions for the open space under the roof to name a few.  if you have natural materials close to where the roof will be, then use them, like the trees, rocks, and vegetation.  but make sure that when you build it, over engineer it.  the weight from the roof, water, wind, and animals will add a force that needs to be addressed.