As artificial constructs that are designed to mimic nature, green roofs have a love/hate relationship with trees.
- On the one hand, green roofs are created not only to give buildings energy savings, extended roof membrane life, reduce water runoff and heat island effect, etc., but also to replace the piece of ecosystem eliminated by the footprint of the building.
- On the other hand, the plantings cannot threaten the integrity of the roof membrane and so deep-rooted plants can be an issue if not carefully considered in the planning stages.
The determining factors in appropriateness of plant material on green roofs are the weight-bearing capacity of the roof, the intent of the designer, and the financial boundaries of the project.
That said, in theory, one could plant any tree from a scrub oak to a redwood on a roof as long as you met safety concerns and had the budget to do so. However, the reality is that green roofs are extremely limited by weight-load capacity and root demands of plant material, not to mention budgets.
Mimicking nature is the basis for any sustainable practice and is the key guideline to green roofs that last. I suspect that you wish to design a green roof that not only is environmentally functional, but also yields usable square footage that has ornamental value, perhaps a brownstone roof garden. Large hardwood trees generally do not grow in the mountains, and an elevated garden of any kind will embrace the limitations of an alpine garden. Weight restrictions and cost of installation coupled with the altitude and exposure lend itself to this.
You will want trees that are ornamental or achieve a maximum height of approximately twenty feet or less unless you plan to root-prune and bonsai a larger specimen. Trees that have more of a lateral root tendency and less of a vertical taproot are preferable. I once went to an apartment where a roof-garden tree root had come through the ceiling above, not a desirable situation in any setting unless you want to mimic a hobbit house.
Also consider what the tree will be planted in. A garden I redesigned at The Solaire, a LEED Gold residential building in NYC, has amelanchiers planted in a 24"-deep green roof area that is approximately 15' x 15', and the roots occupy the space laterally. Chicago City Hall, on the other hand, has varying depths for its trees and shrubs, creating deeper below-surface planters for the trees. It is not unheard of to plant trees in planters placed in a shallower bed for the sake of economy.
Remember this when planting trees on roofs: the volume of soil in relationship to the growth needs of a tree will determine the lifespan of the tree, which is why green roofs tend towards trees that like altitude and are acclimated to living with less available organic material, as opposed to, let's say, the aforementioned redwood.
The minimum area per tree could be as small as one foot deep and three by three wide, but depending on the trees, the lifespan and tree development will be very limited. At minimum, I would give any tree at least ten cubic feet of soil, whether in a box or spread out over a green roof, if you want it to hang around for a few years.
For more information:
Read Mark Schrieber's Ask A Pro Q&A, "Are there any cost-cutting shortcuts for planting a green roof on my urban townhouse?"