You’ll get a lot of simple single point answers on this, but there are a lot of reasons for and against.
Let’s start with a simple pro and con list:
Pros
- Pretty or at least attractive to most people
- Decreases thermal gains in the structure
- Increases the insulation on the largest surface area of most structures
- Increases the thermal mass of the building
- Decreases solar island effects
- Increases vegetation in the area
Cons
- More expensive than most roof systems
- More complex to setup and maintain
- Much heavier requiring additional structural support
- Requires constant maintenance
- Requires an irrigation system
- Can easily trap water against your roof/structure increasing the likelihood of leaks
- Growing organics can undermine and penetrate waterproofing structures causing leaks
- Can freeze or delay water runoff, increasing the weight of the roof
So basically, you do have an aesthetic, but you’ve more weight and water issues vs better thermal qualities. Most of these number will vary so much based upon the specific structure that an analysis would need to be done each time rather than a general value.
But, there is one issue that will kill it for most designers. Maintenance. Every homeowner swears to the heavens above that they will maintain the garden roof. They never do. Take a look at any sod or landscaped roof you can find. Maintenance issues. They blame the designer, the contractor, the landscaper, but they almost never do anything to maintain the roof other than watering it. Until that breaks too. It is a large commitment to have a sod or landscaped roof. Sure they were great in the medieval era where if it got a little damp they increase the fire inside to dry everything out. This doesn’t work with modern finishes and electrical systems. Imagine what would happen to your TV if it got “a little damp”.
So, there you go. A great idea from another era that only works today at great expense and/or effort.
MKDesign 