Green (or living) roofs have much to offer! In urban areas inundated with impervious surfaces, they slow the run-off of stormwater and provide habitat and food for birds and pollinators. The aid in cooling of the structure below. They add beauty and whimsy, softening the harsh edges of buildings.
They also create some design challenges– The roof is significantly heavier than a traditional roof, requiring a stronger support below to accommodate the soil, rainwater, and snow. The growing conditions can be harsh for plants, with a restricted soil depth as little as 4 inches in smaller designs, and good drainage is a must, resulting in hot, dry growing conditions in summer.
I’m going to walk you through our design process and experience for our cob house roof. This is an experiment that I enjoy following and observing year to year, learning more about the resiliency of some of our dearest native plants.
Roof slope and Construction
Small green roofs do well with some slope for drainage. I chose a shallow slope to the south that corresponds with the slope of the land. Too steep of a slope would cause the soil substrate to shift due to gravity and rainfall, requiring some built in restraints to keep the soil substrate in place. On the low end of the roof, we would add a gutter to prevent water trailing back the rafters onto the cob house, which could cause a gradual erosion with time.
The wood framing of our cob house was designed to be strong enough to hold the green roof (and snow) without the additional support of the super strong cob. This required doubling up some beefy lumber for the headers (2x10s), and 2x6s for rafters. 3/4″ exterior plywood covers the top of the rafters, creating a solid base for our roof.
The Layers of a Green Roof
You know how sometimes the right people come at the right time? In 2017, I was approached by Jack Frane of Mundelein, looking for an Eagle Scout project. After listing some easier options, he asked what our plans were for the then unfinished roof of the cob house. His interests were piqued, and he had the added bonus of having an older brother in the roofing industry and experienced with green roof design. Together the family team of father and two sons proposed a design that topped any of my ideas and solved the challenges that I didn’t know how to solve. Blessed.
A pond liner material was glued onto the plywood, with enough extras lefts on the perimeter to wrap up the sides of the roof to create a water tight barrier. Leaks onto the plywood would greatly compromise the integrity of the structure over time. The extra material on the sides of the roof was folded over the edge and cedar trim was attached to cover this seam, both to secure the pond liner to the sides and for aesthetic reasons.
This left an exposed section of pond liner across the top of the sides. Exposure to sun would eventually degrade the pond liner, causing cracking. Jack’s brother’s employer, Jones and Cleary, manufactured and donated the custom copper trim that protects this section from the elements in a beautiful, elegant way.
On top of the pond liner is a drainage membrane. It’s a flexible material with raised bumps to allow water to drain down the slope of the roof and into the gutter.
Next, a filter fabric was installed. This fabric would help prevent roots and soil substrate from filling up the drainage membrane, reducing its effectiveness.
Now we were ready for the soil substrate! Adding topsoil isn’t recommended for green roofs, for drainage, water absorption, and erosion from wind. We used a mix designed specifically for our depth of planting (3-6″) made of light weight aggregates: Midwest Extensive Media Spec Sheet Huge thank you to Midwest Trading of Maple Park for donating this product to us!!
Choose your Plants Wisely
Frequently you see sedums recommended for shallower green roofs. With good reason–they thrive with little to no soil and water. As usual, I had a different plan. I like the more wild look of natives and wanted to see how they stood up the challenge. Native plants are known for having very deep, complex root systems, which doesn’t seem practical with only 4-6 inches of soil here. But what if their roots grew sidewise? Spreading out horizontally and capturing water from a broader area horizontally vs vertically?
I contacted the Chicago Botanic Gardens Green Roof manager and visited their trial garden on top of their Science Center. Here they have been evaluating both native and non-native plants planted in different depths of substrate and incorporating varying irrigation amounts over a number of years. Here are their findings: https://www.chicagobotanic.org/downloads/planteval_notes/no38_greenroofplants.pdf
Check out CBG Green Roof blog for more on their observations.
I was willing to do some drip irrigation, at least for the first couple of years to get the plants established, but I definitely wanted to steer towards plants that were more likely to survive without that kind of babying. I started with native plants that CBG awarded with 4-5 stars. Prairie dropseed, nodding onion, coreopsis and purple clover.. but as I became more familiar with the idea of plant communities, I started adding plants that I thought my thrive alongside the ones the CBG evaluated. Plant communities are groupings of plants that you often see growing together because they thrive in similar soil, water and sun combinations.
The current list of the plants on our green roof are:
Side oats grama Bouteloua curtipendula
Purple prairie clover Dalea purpurea
White prairie clover Dalea candida
Prairie Dropseed Sporobolus heterolepis
Nodding Wild Onion Allium cernuum
Prairie Smoke Geum triflorum
Partridge Pea Chamaecrista fasciculata
Hairy Penstemon Penstemon hirsutus
Silky Aster Symphyotrichum sericeum
June grass Koeleria macrantha
Lance leaf coreopsis Coreopsis lanceolata
Rattlesnake master Eryngium yuccifolium
Rough blazing star Liatris aspera
Little bluestem Schizachyrium scoparium
Wild Petunia Ruellia humilis
Due to our modern landscaping methods, we are used to seeing spaces between plants filled with wood mulch, or for people in the know, leaf mulch. This is not how nature grows. Most plant communities grow in multiple layers growing at different heights. Our eyes are generally drawn to the upper layers, with their lovely flowers and seedheads. However, hidden below is generally a matrix of low growing plants called a Groundcover. This layer provides food and shelter to small creatures and insects, shades the soil from drying out from the sun, dissipates the impact from heavy rainfall, and can creates positive competition to discourage weed growth. I’ve noticed for years that fully vegetated beds creates less weeding for me and so this idea of a living mulch is practical. Its success may be determined by proper plant choice.
The living mulch idea can be applied to a green roof, shading the soil to slow the drying out process by promoting a thick cover. I chose to interplant sedums for this purpose. While they aren’t necessarily native, I think they have the potential to live harmoniously with the exist native plants.
We added some drip irrigation lines as a precautionary measure to help our newly planted specimens. During the first year, I watered weekly during times of low rainfall. During the second year, I watered whenever I felt the bottom layer of the soil was dry. I’ll continue to push the limits of what our plants can tolerate, dryness-wise.