The technical side of green renovation: flat roofs

Architects, gardeners, landscapers and roofers from all over the world know how to find Sempergreen for greening buildings. Fortunately, they involve us earlier and earlier in the renovation or design process. After all, there are several factors to consider when installing a green roof or living wall. The earlier this is done in the design phase, the easier it is to install the greenery later in the project. 

Max de Vos, working in Research & Development at Sempergreen, is a specialist in the technical aspects of green renovation. In a series of articles, Max shares his knowledge. In this article, Max talks more about renovating flat roofs. 

Early collaboration leads to best results! 

Green renovation on flat roofs offers huge benefits but also challenges that are crucial for architects, landscapers and roofers to consider at an early stage. Avoiding surprises later in the process is best achieved by seeking collaboration early on. Our experienced specialists can help find solutions to challenges you may encounter on your project. This article highlights the technical aspects of green renovation on flat roofs and shows why it can be valuable to work closely with our experts from the start.  

What questions are important in the design phase? 

How strong is the flat roof to be renovated? Will the entire roof be renovated or built on an existing structure? Of course, it is very important to know how much weight the flat roof can handle. But the very first question is obviously ‘what will be the functions of the flat roof to be renovated?’. Is the roof going to be used intensively? Will it be a so-called living roof or will it be used for water storage, for example? Will the roof be visible and from where? What is the role of sustainability and, last but not least, what is the financial scope for the flat roof to be renovated?   

All these questions lead to the best possible solution. For this reason, our Sempergreen specialists like to get involved early in the design phase. Together, we will create a beautiful green roof that will last for years and years.

Bearing capacity: how much weight can my flat roof support? 

Sempergreen has many different green roof solutions in its portfolio and these also have different weights. When it comes to a water-storing green roof system, a fully water-saturated roof will have a large weight, which obviously needs to be taken into account in the design phase. If there is a need to collect water for sustainability purposes, we can provide several solutions to meet this demand. Want to know more about roof water storage? We will soon discuss this subject of water retention in a new article. So keep an eye on the news on our website!

Credits: Bert Rietberg

Eaves and vegetation-free zone: give your flat green roof a nice finish 

Besides the load-bearing capacity, the height of the roof edge also plays an important role. The more intensive the vegetation on the green roof, the higher the roof edge will have to be. A vegetation-free zone should be created around a flat green roof so that water can be drained easily. Moreover, this edge of gravel has an important fire-resistant function. It ensures, in case of fire, that the fire does not spread. Of course, this layer should not extend above the roof edge because of the risk of gravel falling due to wind or shifting. When renovating a flat roof, this can therefore very well be taken into account, provided the Sempergreen specialists can provide timely advice. This advice is of course free of obligation, so if you have any questions, feel free to contact us using this form.

Irrigation: easy construction and management on flat roofs 

Of course, you want your flat green roof to be beautiful at all times. By nature, Sedum plants can survive very well in prolonged dry conditions. Sometimes architects, landscapers, gardeners or roofers choose to install an irrigation system anyway. For instance, as a result of the flat roof size. It can also be relevant for housing associations where the question ‘who is responsible and where does the water come from?’ often arises. A key advantage of flat roofs over pitched roofs is the ease of construction and management of irrigation. Whereas irrigation on sloping roofs can be a bit more complex, flat roofs are more efficiently and easily managed independently. For large projects, it is therefore important to consider in advance whether an irrigation system should be directly included in the design and construction.

Maintenance: twice a year in spring and autumn

Maintenance is also simple with a Sempergreen flat green roof. Most flat green roofs can be walked on and are therefore easy to maintain. We recommend maintenance twice a year in both spring and autumn. This maintenance consists of removing undesired vegetation that birds or wind have brought onto the roof, fertilising the vegetation so that the plants are strong and healthy and that, for example, grasses have less of a chance, and touching up any spots where less plant growth has been achieved. Want to know more about flat green roof maintenance? On this page, we elaborate on the maintenance of various types of green roofs.

Vegetation choice: consider the load-bearing capacity of the roof 

The load-bearing capacity of flat roofs is, as mentioned, a crucial factor. The choice of vegetation has an effect on the weight of the green roof. The rule of thumb is: the greater the diversity and growth height of the plants, the higher the weight per m2. Besides weight, local legislation can also be important in making vegetation choices. More and more municipalities have a policy of preferring native planting. Sempergreen has a number of types of vegetation blankets in its range that comply with this. And do you love flowers and bees? Or would you like to create a very special cheerful, flowering roof for your client?  Then read about Sempergreen's biodiversity products. 

Does your flat roof lack sufficient bearing capacity for a biodiverse planting? But you want to increase the ecological value of your green roof? Then add a Sempergreen Biodiversity Package to your Sedum roof!  

Hail and snow, thunderstorms, wind, rain and heat

No stress! All Sempergreen products that turn a flat roof green are capable of withstanding almost all types of weather. Because we grow all over the world, this applies to all types of weather we encounter. Due to climate change, extreme rain showers are becoming more frequent. Heavy rainfall that used to occur only once every 50 or 100 years is now becoming more frequent. These large volumes of water create peak loads on sewers and drainage systems, and this is only going to increase in the future. To prevent flooding from excess rainwater, green roofs are a perfect solution. You will soon read more about the solutions we offer for collecting and channelling large amounts of water. 

As far as wind is concerned, Sempergreen's products can also withstand a lot. When a green roof covers a very large area (e.g. in large shopping centres, sports halls or conference centres), it is advisable to consult our specialists on this subject in good time. There are good solutions for eliminating any risk of wind-blown-up and the earlier this is taken into account in the process, the easier the solutions will be to implement. 

Finally, a green roof also has a very beneficial effect on lowering the ambient temperature. Large cities and densely built-up areas suffer from the so-called urban heat-island effect, ‘islands’ where the temperature is significantly higher because of the many buildings and the great lack of cooling nature. The more green roofs we can create together, the more impact we have on urban temperatures. All the small, green islands together are a strong fighter in the climate crisis.

So what are the technical advantages of a flat green roof?

As you have read above, green roofs have a beneficial impact on the urban heat-island effect. But green roofs also have a good impact on the heat regulation of buildings. The extra layer of greenery can provide cooling in summer. In addition, green roofs increase the lifespan of the roof by up to 60 years which naturally ensures that a renovation with greenery is also technically much more sustainable than one without. Want to know how we can best advise you? Then get in touch here without any obligation. 

Green, greener, greenest. 

Integrating solar energy with green roofs is an emerging trend that we at Sempergreen lovingly embrace. After all, we want to green our beautiful planet and that means more than just making roofs and facades literally green. We believe sustainable energy is also very important. That is why we have had a wonderful partnership with Over Easy Solar since 2024. Over Easy Solar's vertical PV system (the name says it all) is very easy to integrate with our flat green roofs. But what exactly should you take into account? Read more about solar in combination with a green roof. And do not hesitate to contact us if you want to know more about this unique, particularly sustainable roof solution.     

Conclusion 

Green renovation of flat roofs requires a detailed approach and early cooperation between architects, landscapers, roofers, gardeners and our Sempergreen specialists. By paying attention to bearing capacity, technical details, water management, irrigation, maintenance, vegetation choice, solar energy and financial aspects, surprises can be avoided and sustainable, (multi-)functional green roofs can be realised. Renovation with a flat green roof is an investment in the future that will pay off in many ways. Find out who all actually benefits from a green roof or green facade here.

Stay tuned for the next articles via our newsletter, where we take a closer look at drainage, circular substrates and other technical details essential for successful green roofs.

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Read previous articles in this series here: 

 Green renovation of sloping roofs