The French just passed legislation requiring all commercial buildings to install solar panels or install plants on their roofs.

I am sure that the following questions were asked and smartly debated before passage:

• What externality is this solving that could not be done more cheaply with taxes or other output regulatory standards? For example, if you are concerned about water runoff, why mandate a particular way of controlling runoff rather than requiring buildings to have less runoff?
• Suppose it was decided that we understood what externality this mandate was dealing with (for example, carbon emissions), at what cost per unit of externality reduction did these solutions create? For example, how many tons of CO2 emissions per year are averted by installing solar panels on building roofs, of course considering that the panels themselves had to be manufactured in a carbon intensive industrial process? And then, how many dollars were spent per ton of CO2 emissions averted? And then, how does this compare to the damage caused by a ton of CO2?
• If we build a roof with plants on it, how much heavier will the roof be once it collects soil and rainwater? If the roofs are considerably heavier (remember that one gallon of water weighs 8 pounds, my current roof is about 1,000 square feet, so a 5-inch deep plant system on my current roof would be about 417 cubic feet of rooftop garden. Assuming that this soil is weightless (silly) but absorbs 50% of the water falling on it, it will add up to 1500 gallons equivalent of water to my roof, or about 12,000 pounds. I think this is a conservative estimate. Now, I am not a builder or an engineer, but I am pretty sure that designing a roof on even a small home like mine to withstand that weight is going to require more and stronger materials in the construction of the house, and almost surely will require gutter guards and a concrete supporting deck to install and support the new roof. Is it possible, given the requirement for the new materials, that installing green roofs is perhaps even harmful to the planet? It would be worthwhile to at least investigate a back of the envelope number, no?

I am also sure that when this law (and others like it) was passed, no one stood to benefit from its passage. There are no such things as corporations that build and install solar panels. There are no such things as corporations that build and install and design rooftop gardens. Those non-corporations probably have never contributed a dollar to a PAC, Super-PAC or have ever stepped foot to lobby the halls of Congress. Those sorts of “dirty” activities are reserved only for evil corporations that we don’t like.

Finally, I am sure that with the passage of this law the French government has established a system to review the effectiveness of those programs at delivering on the environmental goals of the program, we look forward to reading the annual reports on it.