Weekend Warrior #16 – Green Roof Vs Tree: Which is Better?

This one will start a fight for sure, and maybe that’s my aim. But this is a conversation that I never hear taking place during project work, and I think we need to start having it.

Before I wade in, let me be clear; I am a fan of green roofs. I think they have multiple benefits: they can be beautiful, create great amenity, comfort and urban habitat, they can cool the building etc. etc. But for Part #3 in this City Slicker series I’m going to use green roofs to illustrate my point.

If we’re looking for ways to maximise our environmental benefit per dollar spent, are there other ways to do that?

Here are some crude figures that I’ve harvested from the ‘net. I found many sites with variations on the data, so what I’m presenting are the apparent averages.

carbon 200

Hmmm… gets you thinking doesn’t it?

EarthDay1

Brown University, Providence R.I. Earth Day

Of course there are other examples of how we launch into more expensive ‘carbon solutions’ on buildings when what we could really be doing is increasing our carbon impact almost a hundred-fold for the same investment.

No doubt right now you’re thinking ‘but green roofs have other benefits…’, well of course they do, but so do trees. I’ve previously outlined the value of street trees but when we head to the countryside we unlock another raft of benefits, and I’ll outline these in the next post.

So this weekend’s challenge, should you choose to accept it, is to plant a tree. Just one. You won’t get a seedling for a mere $3.75 but it won’t be much more. Not such an easy challenge – trees get big, they need space and they need soil volume [around 42 cubic metres to live a healthy life].

It was National Tree Day last Sunday in Oz so here’s your chance to catch up in case you missed it ; )

[Assumptions used above];

  • 1m2 green roof absorbs 0.187kg co2/annum and prevents 0.702kg co2/annum through energy savings from less cooling;
  • 1m2 green roof installed =$100 for an extensive green roof. Figures range from $60 – $100 and sometimes up to $2,000/m2 for constrained existing sites requiring cranage;
  • tree cost = $3.75/tree, based on minimum 120 tree purchase, from Carbon Neutral
  • tree absorbs CO2 @48Lbs CO2/annum = 19kg CO2/annum. Depends also on climate, soil, tree species etc.
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