Here in Seattle we get rain during about 9 months out of the year, so our newly constructed cob oven needed a roof to protect it from the elements. A green roof seemed like the perfect fit – picking up the theme of earth-based ecological building from the cob material we used to build the oven. Besides giving us a chance to experiment with another alternative building technique, we really like the look of green roofs. There’s just something unexpected and delightful about seeing plants growing up in the air as an integral part of a building.
As green roofs have become more popular, an industry has sprung up to supply the specialized roofing materials, soil mixes, and plants that are required. It’s still a niche market though – you won’t find these things at your neighborhood hardware store. Given the small size of our project we decided to go for a low-tech approach, utilizing readily available materials. This wouldn’t be appropriate for a house roof, but should be just fine for a garden structure.
The roof deck consists of 1×6 cedar boards on 2×6 rafters.
On top of the boards goes a sandwich of materials designed to protect and waterproof the roof structure:
- paper – protects the plastic from below
- 6 mil plastic sheeting
- paper – protects the plastic from punctures from above
- pea gravel – to help drain excess water
- weed fabric – to prevent silting up of the drainage layer
Instead of a gutter we incorporated a drain channel into the lower edge of the roof.
A rain chain directs the water coming out of the drain channel.
The soil layer is quite thin, to keep the weight within reasonable limits. It needs to be light and free-draining, but also able to absorb some water to keep the plants happy. We mixed peat, top soil, and perlite.
On top goes a thick layer of wood chips. These will hopefully slow evaporation while not adding a whole lot of weight. Over time, they’ll also break down and provide nutrients for the plants.
Finally, time to plant. We are using a lot of sedums and grasses for drought tolerance. It’s a little known fact that Seattle is really dry during July and August – in fact there may not be a drop of rain during the entire 2 month period. Combine that with the shallow soil on the roof and it makes for very challenging growing conditions.
After a day of lifting buckets of gravel, soil, and wood chips, that pizza tasted really good.