Designing Living Roofs – Part 1
I’ve been exploring various aspects of living or green roofs since I first started my own architectural practice about ten years ago. In fact my very first project was to fix up a shingled Victorian in San Francisco. It had a tiny garage in front of the house dating back to 1912 and a nice garden that wrapped around the garage. Wanting to expand the garage to fit two cars and occupy the entire footprint where the garden stood, I decided to make the roof of the new garage a giant planter.
There have been many reasons which led to planting rooftop gardens over the years. In this first project, I valued having a nice green space in the tiny urban lot. I also felt strongly pushed to design in a way that begins to counter the urban heat island effect resulting from the over-paving of our denser developed areas. San Francisco also suffers from a rare policy that requires all roof water to be diverted into our sewers and then treated in sewage treatment plants – an extremely energy-intensive process. When we plant gardens on roofs, much of the rainwater is taken into the garden where it feeds the plants, evaporates into the air or, if designed properly, recharges the city’s ground water.
Placing what is essentially a large planter on top of a building is inherently challenging. Water is heavy and we must design a structure that can support the weight of a fully saturated planting medium and which will protect from any lateral wind or seismic loads that might occur during a deluge. Protecting against plant roots as they try to work a way through waterproofing is also a concern as is guarding against the sharp tools gardeners use which are not friendly to most roofing materials.
The first fix up project was a good first living roof project because I was only working over a garage and a little moisture seeping in above cars is much less problematic than similar leaks above living spaces. I essentially ended up building a concrete bunker in the side of a steep hill. This allowed me to have generous soil depth to accommodate large plants and their correspondingly large root structures. Typically, we are pushing to minimize soil depth in order to reduce the structural requirements. Since the garage was holding up the existing hillside and the house above, we ended up with a sloped garden whose depth went from 18 inches in the front to about twelve feet deep at the rear. Subsequently, we could plant shrubs and even substantial trees. For waterproofing, we used a torch-down membrane and covered it up with a drainage mat and a root barrier. We filled the planter with a conventional planting soil that was sprayed on top of the garage with a gunite hose. The plants consisted of trees, bushes and a fast-growing iceplant that served to stabilize the sloping soil.
In subsequent projects, I have had different reasons for designing and implementing living roofs, and have faced different challenges. I will follow-up with accounts of other green roof projects shortly. Stay tuned!