Green building now offers so many material and design alternatives sometimes it can be overwhelming. Today I would like to run through some of the competing benefits of installing a solar vs a living roof. This would seem to be fairly simple and straight forward but actually many factors are involved. Let's stick with the purely environmental benefits.
-The idea with solar power is that the power you produce on your roof doesn't have to be produced at a coal fired plant or by some other harmful means somewhere.
-The idea with a living roof is little more nuanced- in that you are recovering the space for flora (carbon reduction) which has been displaced by your residence, improving your insulation, cooling your roof, all while mitigating the urban heat island effect.
So what is actually more beneficial for the environment? Guess what- It is somewhat a matter of perception and is very involved if one is trying to quantify. It comes down to a complicated equation. (doesn't it always?)
First you have to figure out how much power your hypothetical solar roof will create over the course of it's lifetime. Then you figure out how many emissions would be produced if that power was created via conventional means. This is how many emissions aren't being created because you aren't consuming the power that would have necessitated their creation. However the production and disposal (or recycling) of solar panels (and the energy required to do so) creates it's own set of emissions which must be subtracted from that original total-emissions-avoided number. So what your left with is the total amount of carbon emissions which aren't being created due to the total lifespan of your solar panels. This emission reduction is the environmental benefit of your hypothetical solar roof.
On the green roofs side you must calculate the entirety of carbon emissions and pollution which are being breathed and processed by your rooftop plants in the course of your roofs 40 to 80 year lifespan and ad to that the carbon emissions avoided from the energy savings reaped from your insulatory and roof cooling benefits. The complexity of variables such as plant type and energy-use-avoided are staggering.
That is why I pose the conclusion that it simply isn't generally feasible to quantify/compare the 'exact benefit' of these two alternatives because there are so many ancillary factors involved. There are a few definite things to consider though when making your choice;
- A living roof is going to be more beneficial in an urban setting in the sense that it provides a place for plants where there is none. This is also where the "urban heat island" exists. (not to mention storm water mitigation, air quality improvement and aesthetic value)
- Foot per foot over equal time periods the emission reduction/avoidance is probably greater from a solar roof when you consider how that energy might otherwise be produced in the current social environment. (see next bullet) On the other hand theoretically this energy can also be produced elsewhere or by other renewable means whereas the flora that is able to grow on a living roof in a city would otherwise have nowhere to grow.
- So many little factors are involved that you really need to consider how optimal your location is for either or both. If your in a sunny rural area (with high energy prices)solar may be the way to go. If your in a predominantly cloudy urban setting then your a great candidate for certain living roofs. One thing to consider is that if your just going to be using much of that solar energy on extra power for air conditioning or heating, then a living roof might be just as good in terms of 'cutting out the middle man' and naturally keeping your house a little cooler, and/or better insulated.
I hope this was helpful and as I am not an environmental engineer or scientist I have much to learn about this overall issue as well. Please don't hesitate to comment.
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As they say here in the south: 
It's sunshine, it's free, it's simple enough—right? With all the things to consider, from interpreting the power bill and reviewing solar proposals to comparing paying cash with lower-cost offerings, the process of switching to solar can leave a homeowner buried in paperwork and as confused as ever.
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