I recently completed a large fynbos landscaping and rehabilitation project at the new Oudebos Cabins tourism development in Cape Nature’s Kogelberg Nature Reserve. Part of the project involved developing 12 roof gardens on the new cabins and meeting room. While I have installed roof gardens in the past on traditional level concrete structures, the Oudebos Cabins provided some unique challenges as the buildings are made entirely of wood, and half of each unit has a steep pitched roof.
Together with project architect Justin Cooke and Marijke Honig of Think Ecologic cc we designed wooden frames that fitted on top of the roofs to hold the soil. We were limited to 150mm depth for these frames owing to the limited carrying strength of the underlying wooden building. The roofs and frames were first waterproofed using berbigum waterproofing.
Once the wood had been waterproofed we installed Dorkin, which is like large sheets of glorified cookie cups that aid drainage and also retain some water for uptake by fine roots. The Dorkin was then covered with biddum geofabric. Because we had very little soil to work with we added a layer of Coir woven geotextile and approximately 15% of the soil mix was made up of Coir pith /peat blocks added to the soil mix (both sourced from the Coir Institute). Local soil from the site was mixed with the coir, which also inlcuded 40% fine compost from Agri-organics.
The mixing of the soil mix and lifting onto the roofs was done by machine, which greatly sped up the process. I had learnt the hard way on a previous roof garden project where we lifted the soil using ropes and buckets!
We kept some of the coir pith blocks out of the soil mix and used them to create a 30mm peat layer on top of the coir woven geotextile before raking the soil mix up to height of the the 150mm wooden frames.
The picture to the right shows the soil mix about to be added on top of the coir geotextile and coir pith layer.
Once the mix had been levelled we covered the soil with Biojute which is a coarse, biodegradable fabric woven into a open mesh from rugged heavy jute yarn. This was sourced from Maccaferri and holds the soil in place while the plants establisih. It will eventually break down by which time the plants roots will hold the soil in place.
A drip irrigation system was then laid with lines 300mm apart and drip points also 300mm apart. The final step was then the planting. I worked on 9 plants per square meter. We selected a range of succulents and grasses. Most of the fillling plants were made up of a local Erepsia (E. anceps sourced from the site) and Aristida junciformis which grows naturally in the Kogelberg, and has proven itself to me previously as an excellent roof garden plant.
While we have had some unplanned for challenges including the emergence of weeds and grasses (because of using soil from the site), and water supply issues resulting in mortality of some grasses and succulents, the gardens are establsihning well and we are expecting full coverage by next summer.