Extensive Green Roof Cost - How much should a living roof cost?

How much do we need?

Mercedes or Volkswagen?  Cadillac or Ford?

I'm not referring to quality because, in my opinion there is quality in each.

I'm referring to the 'custom extras' (including the value in the brand name).

Same way with a living roof.  One of my close roofing contractor friends a couple of years ago became really excited about getting into the green roof business because he saw a huge profit margin.  At the time a satisfactory green roof system, not including the underlayment or TPO layer was selling for around $20 to $25 per square foot.

Of course, the system he was considering was not basic, rather more mid-level in complexity and components.  There was a frame, a drainage mat, a water adsorption pad, another woven mat, the soil and potted plants.  But still, he felt he could make $12 or so per square foot even with the labot to install, insurance and his overhead.

That adds up to a profit margin of about $240,000.00 on a 20,000 SF roof.

Considering the other extremes;

• A complicated green roof system with multiple layers, or trays where the profit margin may be only $2 per foot for a total margin of $40,000.00 or
• A very simple system with a root barrier/liner, mat, soil and seeds where the profit margin could be $16 per square foot for a profit of $320,000.00 

Extremes usually don't happen, yet the point here is valid.  How much do we really need?

Green roofs have come along ways since the first sod roofs.  Some systems today are engineered to control almost every environmental variable.

But how much of all the technology and plastics and plastic trays and plastic liners and adsorbent mats and plastic weave and ground up rubber tire crumb anti-pollution media, and all else that goes into a green roof do we really need?

When do we start creating GMO plants specifically designed for green roofs and rooftop garden environments?  When do we start growing roboplants?

How about back to basics.

A simple extensive green roof should cost no more than the following:

• A strong root barrier/liner - 20, 40 or 60 mil depending upon application Cost: $ 0.60 per square foot
• Adhesive $0.30 per square foot
• A woven mat or honeycomb type fabric system, something to hold the soil in place (if you really want to be sustainable and cost-effective use hemp or burlap instead of a polyethylene weave) typical mat cost $ 0.60 per square foot
• Best Gro-Drainage Soil Mixture  $ 1.00 per square foot
• Seeds (C3,C4 and CAM native & food plants) cost $ 0.60 per square foot
• Labor to install $2.00 per square foot (probably high)
• Overhead, insurance, etc cost $ 1.00 per square foot 

Total Cost $6.10 per square foot for a quite nice system.  Note: This price does not reflect cost of just one manufacturer.  There are many great systems choices on the market so always shop around for the best price.

Sure, many sales reps will try and sell you the high end system.

Yet how much do we really need?

There are some beautiful vegetated roofs that have lasted for centuries across Europe without layers and layers of high tech plastics.

On a side note, the 60 square foot rabbit hutch roof I built this weekend at $25 per square foot would have cost $1,500.

My cost breakdown was:

• Root barrier/liner 60 sf @ $0.50 sf $30.00
• Grow mat 60 sf @ 0.50 sf $ 30.00
• DIY Soil Mix 60 sf @ 2" $5.00
• Seeds - Lots of Veggies! $ 5.00
• Labor - mine

Total cost $70 for a green roof (little over a buck per square foot) that will look every bit as good as the most expensive green roof in the world and last as long and provide food at the same time. For a photo refer to the blog here.

How much do we really need?

Green Roof Plants - "Assumption of Relative Consistency Models (Plant Behavior) Artistic Intuition and Plants Having a Mind of Their Own

Good morning!  We've been discussing the difference between C3, C4 and CAM plants over the last couple of days.

The discussion of the different types of plants focuses on varying patterns of photosynthesis metabolism processes.  All of this may sound complicated but it is really quite simple and the reason we are distinguishing between plants is so we can predict behavior and select the best plant for each individual and unique green roof!

Having the ability to predict how plants will survive, thrive and grow on a green roof is vitally important to the green roof profession.

Moreover, understanding how plants will behave allows for the project's economic costs and impacts to be analyzed.  Of course, there are many variables to consider when choosing what blend of C3, C4 and CAM plants to use, factors such as;

• Wind direction and speed
• Sunlight exposure
• Temperature ranges
• Average humidity per month
• Rainfall amounts per month
• Existing local seed source
• Nearby allopathic plants
• Smog and air pollution
• and other factors

The important component of modeling or predicting how the roof will behave with plants is the "assumption of relative consistency".

The "assumption of relative consistency" is a phrase that means - we know what to expect.

So for green roofs the assumption of relative consistency means we know there will be times when the roof will be very hot (mid day summer) and times when the roof may be very cold (early winter mornings).  There will be times when the green roof will have plenty of sunshine for photosynthesis (when the sun is high in the sky) and times when the days are short and the sun is low on the horizon (winter solstice).

The times when the days are short is when every bit of sunlight is important.  During the shorter days there is an assumption of relative consistency that deciduous trees will loose there leaves and sunlight can filter through the tree to the roof.

This assumption of relative consistency about winter sunlight where summer leaves had provided shade is a factor green roof design incorporates.

Yet plants, like children - and many adults - have a mind of their own also.

Deciduous sweet gum left, wont loose its leaves

The photo to the left is of a deciduous tree, Sweetgum , Liquidambar styraciflua.   This sweetgum is planted adjacent to a greenhouse and a green roof and provides summer shade but is supposed drop leaves during the winter to allow sunshine exposure.

There have been record cold temperatures and the solstice has come and gone so the environmental triggers for leaf drop have been present.

But the plant has a mind of its own.

And the leaves just will not fall off the tree - no matter how long I stare at this tree!

And the greenhouse and green roof are not getting the sun exposure I expected this year.

So the theory of assumption of relative consistency of plant behavior may not always hold true and definitely will have exceptions.

Certainly the exceptions to the rule do not justify discarding plant behavior through modeling.

But what is important is to realize that plants do have a mind of their own and green roof design, though highly technical is also a function of art.  The green roof artist has an understanding of how plants may behave on the green roof based not only on assumption of relative consistency data, but also on an inner intuition only a plant person can understand.

Happy Green Roofing!

Kevin