A while back I was able to take part in a green roof install and it occurred to me that it would be a great way to share a bit about green roofing systems and how they are built. Most of the green and garden roof projects that I have worked on in China are for show. They are public spaces where you can sit and have tea with clients or entertain investors with a company BBQ. However, this particular project was not overly decorative or landscaped and was not meant to be a public space. The client simply wanted to have better insolation and hoped to improve their building using green solutions.
We were dealing with a flat roof, about 300 square meters, atop a building in the Jiangsu Software Park in Nanjing (see first photo). There were air-conditioning units, heating pipes and vents to be aware of but over all it was a fairly straightforward flat roof. The final green roof was not meant to be open to the public and would only have limited walking access. However, it would be acting as a protective layer of waterproofing and insulation. The goal was to lower the heating and cooling costs by 30%. The whole install took less than a week and followed the step by step method that is laid out below.
Cleaning the roof: As preparation for the instillation it was necessary pressure wash the roof. This removes grit and other loose objects that may cause problems during the rest of the instillation process. It also ensures that we can get a good look at the roof itself, including any cracks or separations between roof and utility units (pipes, a/c units and the like) which, if found, would need to be fixed, sealed and water proofed.
Waterproofing: After the roof is clean and dry, we seal it with a whole new layer of waterproofing material. Traditional flat roofs are covered with a rubber material that is rolled out and are sealed to the roof. Then the seams are heated with a torch to ensure the are sealed together. In our roofs we are using a two-part sealant that is sprayed on. The two different substances mix as they leave the spray gun and adhere to the roof building up a thick protective layer that is both waterproof and able to grip to nearly any surface. If we found any leaks during the cleaning, they would be first hand painted over with an extremely smooth coat of this water proofing material to ensure they are filled and sealed. After that, the main protective layer would be sprayed on.
Drainage and root barrier: Once the roof is waterproofed it is important to ensure that drainage still occurs properly. An inch or so of retained rain water will add a tremendous amount of weight to the roof structure (which could be dangerous). So we will do a drainage check that allows us to both check that the roof is sealed properly and that the water is draining the way it should.
Above the waterproofing we add a root barrier to keep plant roots from eating away at the waterproofing and the roof itself. Plant roots are amazingly resilient and most rooftops are not nearly as structurally sound as we would like them to be.
Grow medium: next comes a layer of fabric to contain the soil that will sustain out plant life. This is simply a fiber material that will allow water to pass through it yet won’t let our soil erode away. Above this comes the actual grow medium. Again, for our projects we are using something a little less standard.
We have developed a lightweight soil that still retains water. By grinding up Styrofoam and mixing it with organic fertilizer and silica, we can remove a lot of weigth while still providing a lot of surface area for water partials to grab on to. Also, it is important to remember that the waterproofing material that was sprayed on the roof will also protect against some heat lose but the added solid above that is a far better insulator.
Organics: The top layer of our green roof is a woven mat of straw. It would also be perfectly easy to use burlap or some other coarse material. There are several reasons for this layer. First, it provides a small protective layer for the plants which will be planted in the soils below it. Secondly, it helps defuse rain droplets and start the wicking effect. We want water to spread out across the roof as much as possible so every little bit helps. It is also important to note that our lightweight soil is so light that it tends to blow away in high winds. As simple as a straw mat is, it’s perfect for breaking apart the lifting force of wind and keeping our soil on the roof until the vegetation takes root and ties everything together.
For this roof we used a robust little plant called sedum. It retains water well, grows and spreads easily, yet does not grow tall or shed leaves of any kind, making it a natural choice for a low maintenance green roof. You can actually chop up sedum, mix it with organic compost or other soils and simply spread it around; it will pretty much always take root. Sedum also comes in several varieties that can give your roof a bit more visual variety.
This install was a learning curve for our company’s design staff. We actually had the whole design team come out and help plant to the roof. It’s important to get away form the desk every so often but it’s even more important to get to know the other facets of the company works. Hands on experience like this ensures that everyone knows exactly how a roof is put together and gives them valuable knowledge about issues they should be aware of when designing.
A few months later the Sedum has really started to fill out the roof and given it some excellent green coverage.