The extensive green in contrast to the intensive green roof is lighter. It is much shallower and has a growing substrate that is not more than 200 mm in depth (Axter, 2008). This is perhaps the easiest to build given that the shallow substrate layout might not cost much and also the roof area clearance could be really low for this type of Green roof. The water requirements for this form of green roof are not as much compared to the intensive green roof system. So the construction of this form of a Green Roof in the YMCA building would mean that no extra water arrangement and planning have to be done. The minimal maintenance is also an advantage; hence, the YMCA management will no longer need to arrange for specially trained employees.
The cons to this form of an eco-roof are that the roofing will not support much variety of plants. Small low growth plants of the succulent variety are the most that would grow here. The green roof is not biodiverse and cannot be used to increase vegetation or even provide a form of place to live for birds, etc. The most beneficial outcome here is the water retention capacity that eventually leads to the internal cooling of the building (Axter, 2008).
On the other hand, the intensive green roof will present many advantages for a building owner who wants the building roof to be a biodiverse habitat (Appendix 1). Intensive green roofs are generally heavier, with a deeper layer of growing substrate, and support a wider variety of plant types. The heavy weight makes them have the requirement of more water, irrigation facilities and constant maintenance, which could be a problem in the long run. The high engineering landscape required might not be affordable in the case of the YMCA building. Moreover, the YMCA is not a roof garden for decking purposes either, and its main functional need is for the cooling of the building.