My Micro Green Roof - Planning

Last winter a huge wind storm, with gusts of up to 75 mph, passed through my town knocking down trees and power lines. I remember hearing the wind howl and watching the hanging lights inside gently swinging back and forth. One casualty was my garden shed's roof, which lost a few rows of shingles. By July I still hadn't repaired it, but was reluctantly preparing to buy some asphalt shingles from Home Depot and repair it.

Before buying the supplies, though, I spotted a book called Small Green Roofs. The authors' premise is that while big green roofs for houses or offices are expensive and complicated, any idiot with a hammer can build a small green roof. "Hey, I'm an idiot with a hammer," I thought to myself, so I kept reading. Although, the book wasn't a step by step manual, it still had all the information I needed. I guess the field is too new and once they can write step by step instructions, it won't be as interesting.  So I decided as long as I needed to fix my roof, I would try to make the project interesting.

Two initial concerns were with the wind and the fire protection of a green roof. As I mentioned above, the Caribou Valley in Nederland gets some strong winds. And unfortunately, wildfires are a yearly danger here, too.  I was pleasantly surprised to learn a green roof can be more resistant to wind than a shingled roof because the roots can grow it into one big mat that can't be lifted by the wind. Also, since a green roof tends to be mainly rocks, dirt and succulent plants, one source said the fire risk is equivalent to a tile roof.

Project Planning
My first step was to assess the shed's roof and do some background research for my area. I drew out the following map of the roof :

Next I calculated the maximum load for the roof. The authors say "You can have fun with all of the other elements of a green roof, but you must make sure the roof does not collapse." I calculated out about 107 sq feet of area for my shed. Next, using the handy load chart in Small Green Roofs I realized that if I put up five inches of rock and top soil on the roof, I could have a maximum load of around 4500 lbs ( ~2000 kg ) when it got wet!

So I started checking out my shed to see if it could handle the weight. Once again I was pleasantly surprised. The guy we bought the house from had built a solid shed. The rafters were spaced one foot apart. The walls were solid wood made of stacked two by fours.

Still, I talked to my Dad who knows a lot about carpentry, and he suggested I reinforce the main beam going across and some of the rafters. He suggested getting a long 2 x 4 and using liquid nails to glue it to the current beam, then attach them together with screws.

Local Research
The authors of the book are located in England, but I'm in Colorado. Our climate is semi-arid, alpine with 17 inches of precipitation per year, while they receive about 33 inches. So I searched on local green roofs and found some excellent resources:

• EPA Denver Green Roof
• Design Guidelines and Maintenance Manual for Green Roofs in the Semi-Arid and Arid West
• Denver Botanical Gardens Green Roof
• CSU Green Roof

I thought my local weather wouldn't allow me to have a green roof, but I was surprised how flexible they can be. Even though my climate is totally different from the UK, it still can support a green roof. In fact the typical green-roof plants are selected because they can grow in harsh conditions in rocky, nutrient-poor soils. Many of the plants that fit the bill are alpine plants and sedums. To be honest, reading through a list of appropriate plants for a green roof is like reading through a catalog of the plants growing in my yard! ( More on this in a later post ).

The Carpentry
There really isn't a lot to say about this. For the box to contain the garden I just decided on some 1 x 6's of weather-proof wood. I attached these to the roof with angle brackets. I also decided to follow my Dad's advice and bought enough 2 x 4's to reinforce the rafters and main supporting beam. Altogether, the lumber and hardware ended up costing me just under $50.

The Design of the Waterproof Layers
One goal on this project was to complete it without breaking the bank. Even a small roof like this could have cost $1000 or more. I was shooting for something in the $300 range. This was still a lot more that the $50 or so asphalt tiles would have cost me at Home Depot, but hopefully the final result is nicer :-)

So for the waterproof layer I decided on an inexpensive pond liner from Amazon.com. Checking amazon.com now, I see they have a 10' x 13' pond liner for just $60. Unfortunately for me the square footage was good, but the dimensions wouldn't work for me. So I ended up buying some pond liner tape for $20 so I could cut apart the pond liner and tape it together to the size I needed.

The Design of the Soil
My initial plan was to take a few wheelbarrows full of dirt and rocks and dump them on top of my shed. But after my initial calculations, I realized soil and gravel might be enough to collapse even my bomb-shelter quality roof when it got wet. So I looked around at the different alternatives and decided to stick with a standard for green roofs -- lightweight expanded shale. It's a lightweight rock made by heating shale to high temperatures. It's also pretty cheap. I could have gotten it in bulk from a local manufacturer for only about $50 / ton. Unfortunately hiring a construction truck to get it to my place would have cost me way more than the cost of the shale :-(

I also looked into zeolite which has a lot of great advantages for a green roof, but since I didn't own a truck and the guy I was talking to had it marked up to around $400/ton, I ruled that out as well.

So I paid a guy with a pickup truck to go to a building supply center in Longmont and pick up the ton and a half I needed. I decided on a mixture of 70% shale with 15% compost and 15% peat moss, ( according to the information I'd found from the sources listed above ). The cost of the shale and transport ended up being $200.

Plants
I thought this would the easiest part of the roof design process. I went to my local nursery and told them I was looking for recommendations for green roof plants. The response was along the lines of "Oh cool -- I have no idea what to recommend." They thought the green roof was a great idea, but apparently they didn't get a lot of business from green roofs and had no idea what to recommend. They suggested I visit the Denver Botanic Garden's green roof, which was a great recommendation because I could see the plants and create a list there and then go back and buy them. Ultimately I spent about $90 on plants for the roof.

Conclusion
Overall, I've been pretty happy with the design of the shed roof, although if I could go back there are some things that I'd like to change:

• Use natural substances - I used pressure-treated lumber and a PVC-based liner for the roof. Ideally I'd have liked something that could be recycled. When the shed is at the end of its useful life, both the lumber and the liner will have to be thrown away. All of the hardware can be 100% recycled or re-used. And the shale can be mixed into the yard soil to improve the drainage. It's just the liner and the boards that ultimately don't have anywhere to go.
• Use local plants - Since I live at around 8,500 feet a lot of the alpine plants that do well on a green roof also grow in my yard. In retrospect I should have saved myself $90 and just selected wildflowers and ground cover from my yard rather than buying plants at the nursery. One of the methods they mention in the book for populating the roof with plants is just to let natural plants take over. In retrospect I probably would have been better off doing this.