Main image by Louis van Zyl
Justin Sam, Managing Director of Rooftop Landscapes, has been in the rooftop and vertical landscaping industries for the past eleven years. As one of our country’s foremost players in this field, he shares some of his knowledge and expertise on key considerations, pitfalls and the importance of upfront consultation before installing a rooftop garden.
Q: What are some of the biggest issues in rooftop gardens?
Justin: “Specifiers sometimes do not plan the project with the correct structural reinforcement in the roofs. In some cases, wastewater is not planned for and the result is drainage and irrigation water coming out on the client’s driveway or paving due to a lack of planning. In some cases, planters are too shallow – depth of the parapets around the roof does not allow for enough soil depth – this should be planned upfront.
Q: What plants work in a rooftop garden?
Justin: “Most full sun loving plants work well in a rooftop garden. But some will never grow as tall as those in ground-level gardens, as they’re effectively bonsaied in their planter. For trees you need a minimum of 500mm of soil depth for the roots to develop and grow, while most grasses and shrubs only need a depth of less than 150mm.”
Q: Is there a difference between residential and commercial properties?
Justin: “Both need to follow the basic planning and considered design elements and maintenance. Commercial projects are often larger, with both trafficable and non-trafficable sections.”
Q: Does maintenance require special attention?
Justin: “Yes absolutely, maintenance is part of all our projects. Once a month we go out to do maintenance of plants and irrigation, and remove weeds. It should be budgeted for and requires a lot of hard work up on a roof. We have been doing this for years and clients have realised how important it is as part of this type of garden.”
Q: How should people go about plant selection?
Justin: “Some plants and trees have aggressive roots, which can be a danger to your waterproofing. It is best to avoid these completely. Consider the final heights of plant and trees and how that will look from the ground-level and roof-level vantage points.”
Q: Any specific system that you prefer?
Justin: “We have two systems. We have a build in-situ solution where we use geo-textile layers to layer up the roof and put the soil on top. The second is a modular system, using planter trays manufactured by a company called Greensquared, which works very well.”
Q: Which system is better?
Justin: “Honestly, it all the depends on the type of project – in some case built-in-situ solutions will be better and in other cases a modular system works the best. Therefore we offer a free consultation to all our clients, as being able to get the right advice and selecting the best suited system is vital to the success of a project.”
Q: What should architects be asking from experts such as yourself?
Justin: “Ensure that you contractor has significant experience with rooftop gardens and waterproofing. One client had to do a roof three times before he got hold of us to help him redo the garden from scratch. This demonstrates the importance of working with a reputable contractor.”
Q: So waterproofing experience is vital?
Justin: “High-spec waterproofing is a must for certain projects and only reputable contractors will know they must specify this as a requirement. Structural implications and soil types are other vital considerations – for example, soil may have too much clay content and may retain too much water. This is why we have our soil mixes tested in an independent laboratory to ensure that it functions as required.”
Q: How important is compost?
Justin: “This is another key consideration, knowing how much and the type of compost to use for each type of rooftop garden. As plants are feeding from it, the compost degrades – too much compost means that the garden depth will sink.”
Q: Has a decking area on rooftop gardens become popular?
Justin: “Absolutely, as not only does this offer a tranquil space for the homeowner, but it also means additional usable space is added to the building.”
Q: Which one of your projects stands out for you?
Justin: “A 120m² garden with succulents and grasses – where the house was designed around the rooftop garden from the onset, and it was such a beautifully successful project that included a jacuzzi and the necessary privacy-screening plants.”
In our next edition Justin will talk to us about his passion for hydroponics and vertical gardens.
Seven key considerations for a successful rooftop garden
- Know-how: Consider how the functionality of the garden pairs with the specific building situation, orientation and function of the roof garden. Consider whether the roof garden will be trafficable, or simply to provide an organic decorative element to the design. Consider how the decorative features of the roof are to be viewed from ground level as well as roof level.
- Landscaping: Having limited space calls for careful design and expertise, as all plant types have ideal growing conditions. Think endemic and waterwise planting.
- Types of rooftop gardens:
a. Intensive rooftop gardens: Replicate ground-level gardens. They have a growing depth of 200mm or more and are normally installed over concrete roof decks.
b. Extensive rooftop gardens: They are covered with low-growing plants, with a growing medium depth of less than 200mm.
c. Biodiverse rooftop gardens: They recreate pre-existing ground ecosystems and are designed with specific biodiversity objectives in mind, such as attracting birdlife or insects.
- Practical factors: The building and roof must be engineered to withstand the additional load of watered soil and plants, considering both construction and building foundations.
- Irrigation: Drip irrigation works well, as rooftops tend to be breezy. A tap on the roof is useful for cleaning.
- Soil and feed: Plants need regular feeding, as roots have limited space to grow and develop.
- Prevailing weather: Roof gardens are more exposed to the elements, thus consideration of the average temperature, wind direction and velocity in both the summer and the winter and glare are essential considerations.
Key elements that all rooftop gardens should include:
• A good drainage system.
• A high-quality waterproofing and root protection system.
• A lightweight medium and subsoil drainage system.
• Bespoke pots and made to measure planters.
Our sincere thanks and appreciation to Justin Sam from Rooftop Landscapes for sharing his expertise with us.
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