With less swearing than I originally thought would be needed for this redo, I got the chicken coop roof rebuilt and it’s working just fine. Water is filtering down through the soil and dripping out the pipe as intended.
The design is a pretty simple one:
* Box in the roof.
* Lay down a water barrier. In this case, 6 mil plastic that I bought in a roll. The trick (as I learned) is to wrap the edges of the plastic up and over the edges of box. Hold the plastic in place with strips of wood screwed down on the top and outside edges of the box. Don’t secure the plastic to the inside of the box; you’re going to create holes in the plastic that water will seep through. And don’t be fooled: water will seep through.
* Lay down chicken wire over the plastic. This will help keep the soil in place instead of sliding down hill. Again, don’t puncture the water barrier when you install the chicken wire. I stapled the wire to the wood strip on the upper edge of the roof; the opposite end of the wire is held down by the weight of the soil.
* Install a perforated pipe along the lower edge of the roof. I drilled holes every 4 inches on both sides of the pipe. I also capped the non-draining end of the pipe. To exit the box, the pipe had to go through the water barrier; I used a “permanently flexible”, waterproof caulk to seal the gap in the plastic around the pipe. Cover the pipe with a layer of clean gravel to prevent soil and other detritus from clogging the holes.
* Cover the whole thing with soil. I used potting soil that needed a home after taking down the container garden.
I found this basic design in a book called The Barefoot Architect, by Johan van Lengen. Anyone interested in low-tech, DIY, green building techniques should definitely check it out. Van Lengen has spent years working in Third World countries learning and developing all sorts of construction techniques with and for local communities. But don’t pick up the book expecting step-by-step instructions; it’s mostly simple line drawings with basic explanations.
Another thing you absolutely have to consider when building a green roof is the underlying structure that supports it all. Your standard roof is meant to have water run off; a green roof is meant to hold water. One cubic foot of water weighs roughly 60 pounds. Which means – depending on the scale of your project – you’re adding hundreds of extra pounds to the roof. Don’t assume that a standard structural design can accommodate this weight. Figure out exactly the sort of structure you’ll need to support the load your project will produce.
Since this is the first green roof I’ve built, I’m sure some additional tweaking will be needed. But for now I’m happy to have it complete so I can focus on other projects.
Like figuring out the leak in my own roof.
van Lengen, J. (2008). The Barefoot Architect: A Handbook for Green Building.