Green roofs create comfort and improve air quality

Steep green roof - green roof for gable roof

A steep green roof is a personal eye-catcher in detached houses, terraced houses, carports or sauna huts.

A steep green roof adds greenery to the yard. It absorbs rainwater, allowing it to run off and evaporate from the roof in a slow and controlled manner. This in turn prevents flooding of the yard, for example in asphalted yards. The soil layer of the green roof insulates heat, thus equalising the temperature of the space below in summer and winter.

The structure of a steep green roof

The waterproofing of a steeply pitched green roof is usually done with a single rubber bitumen core. On top of this, a root protection layer is installed, which can be root-protected rubber bitumen or other strong root-protection membrane. A drainage sheet is installed on top of the root protection layer.

BMI Icopal's Fonda Green drainage sheet acts as both a drainage layer and a root protection layer, so no separate root protection layer is needed. On steep roofs, it may be necessary to support the soil layers, for example with battening, so that the substrate does not run off.
 

Gently sloping green roof

A gently sloping green roof can be installed on the roof of a market, a residential building or even a carport, for example. The picture shows the sloping green roof of the Heinola Bird Garden - it fits naturally into the environment.

The green roof equalises the temperature of the air below and at the same time protects the waterproofing layer of the roof itself. In housing associations, for example, a car roof with a liverwort roof is also beautiful to look at from the top floors. In the rain, it absorbs water and thus prevents flooding of the yard during heavy rains.

On a sloping green roof, waterproofing is generally provided by three layers of rubber-bonded waterproofing layers, the top layer being root-protected. A drainage and water retention layer is installed on top of the waterproofing layer.

The water-retaining layer can be, for example, a thin layer of rock wool. The waterproof layer is to be topped with a substrate and selected vegetation, such as liverwort or meadow grass. The design of a sloping green roof can be, for example, a sloping pent roof or a so-called flat roof that tilts inwards.

Inverted green roof: courtyard deck and roof garden

The thick layer of soil in an inverted green roof, such as a courtyard deck or a roof-top urban garden, allows for a wide variety of vegetation. It provides good sound insulation, effectively absorbs rainwater and balances the temperature of the space below.

Of course, more demanding plants need more care - watering, fertilising and, if necessary, cutting back lawns, trees and shrubs. A rooftop garden in the city is usually a joint project between the residents of a housing association or a landscaper of a public space.

Garden cover, green roof inverted structure

The structure of the waterproofing of a pitched green roof is very similar to that of other sloping green roofs.

In the inverted construction solution, waterproofing layers are topped with water-resistant thermal insulation and a drainage layer between the waterproofing and the thermal insulation.

You can build a green roof on practically any kind of roof. The most important thing is to build your roof carefully, professionally and with quality materials. On flat and sloping roofs and steep pitched roofs, different materials and construction solutions are used for waterproofing. 

A drainage layer is placed on top of the actual waterproofing layer of the green roof. It acts as a water reservoir for plants and ensures that excess water can safely escape from the waterproofing layer. 

Loose green roofs should always be installed by a roofing professional with a firefighter's certificate and knowledge of waterproofing. You can build a steep green roof yourself if you wish, as the materials used do not require the installer to have a firefighter's licence. However, experience with roofs and construction is an advantage. The installation instructions must be followed carefully to ensure that the green roof works. On steep roofs, it may be necessary to support the soil layers, for example with battening, so that the substrate does not run off. 

In green roofs and roof gardens built on courtyard decks, the movement and thicker growing medium puts a strain on the structures. Their waterproofing layer is protected underneath the thermal insulation. This is called a reverse green roof solution. 

Courtyard decks and roof gardens require load-bearing capacity from the structures below. For these green roofs, it is particularly important that the waterproofing is done correctly. For inward-pitched green roofs such as garden decks and roof gardens, several layers of bitumen are used as waterproofing. These green roofs are best left to a professional to design and build.

Benefits of a green roof 

• Improves amenity, is beautiful to look at and creates opportunities for urban farming

• Produces oxygen and cleans the air, trapping dust, emissions and carbon dioxide

• Binds rainwater to help manage stormwater and prevent flooding

• Equalises the temperature of the space below and the ambient temperature in urban heat islands

• Extends the technical life of the roof by protecting the waterproofing of the roof

• Can maintain biodiversity by providing a habitat for rare field plants and the insects that benefit from them

Green roofs are easy to maintain, but they also need regular maintenance. Selected plant species can be allowed to take over naturally from the roof. Regardless of the type of roof, excess tree trunks and, in cultivated areas, weeds should be removed. Of course, you can let various weeds grow freely on the roof of a wild sauna hut if you wish. 

Caring for green roof plants

Green roofs - including liverwort roofs - should be watered during the dry season. Proper fertilisation of plants contributes to their well-being. Liming the liverwort cover every four years or so is good for liverwort. Otherwise, liverwort can manage with a little fertilisation. However, spring fertilisation can encourage liverwort to flower. In autumn, it is a good idea to remove excess leaf litter and conifer needles from the roof to prevent them from acidifying the soil too much. 

Also check your green roof twice a year

Green roof technology should be checked twice a year. Check the roof gutters on sloping roofs to ensure that water can move freely through them. In some models of roof wells, it is also possible to check the condition of the waterproofing and seams at the same time. The gutters of a pitched roof or other steep roof are inspected and cleaned of debris. 

With a thin layer of thatch, the condition of the waterproofing can be easily checked by lifting a piece of thatch from the roof. After the check, the piece of cabbage is put back in place.

Those considering a green roof may fear that the structures will not be able to support the weight of the roof, the roof will leak or the plants will not grow properly. However, problems with green roofs are rare when the weight of the roof, waterproofing and growing medium are taken into account during the design and construction phase. 

Structures are designed to withstand the weight of the green roof even when wet

Roof gardens and courtyard decks are generally built on new sites where the structural engineer has pre-designed the structures to withstand the weight of the green roof, both dry and wet. The load-bearing capacity of the structures must be ensured in the renovation project. For example, a liverwort roof is thin and does not require exceptional durability from the underlying structure. This also makes it a safe option for roof renovation. 

Quality installation is key

Correct installation is the key to waterproofing green roofs. Particular attention is paid to the careful execution of the various joints and seams. A green roof is actually a very durable solution as a waterproof roof: it protects the waterproofing layer from UV radiation and mechanical stress, unlike a conventional roof. 

A constant moisture level in the growing medium during planting and sowing is ensured

To ensure that the plants start growing, the substrate must be of the right quality and evenly moist at the sowing and planting stage. The growing medium should be kept moist until the plants have taken root. Properly rooted plants are more drought-tolerant in the future. 

Sometimes not all the plants chosen for a roof will thrive on that particular roof, for example because of varying light conditions. Then you can try planting new species - or let the ones already on the roof take over freely.

Old idea, new uses 

Wildly flowering or subtly green roofs have been built throughout the ages. Finland has decades of experience with modern green roofs. Very few problems have been found with the waterproofing of roofs. A green roof is a very durable, safe and attractive roofing solution when designed and installed correctly and with quality products. 

BMI Finland has been supplying and installing waterproofing for green roofs for decades. We have expertise in the best products, different construction solutions and waterproofing installation.

The price of a green roof depends on the size of the roof, the steepness, the structural solution chosen and the vegetation. You can build a green roof for a domestic rubbish shed or a bike shed in a housing association on a small budget. In new urban developments, the price is also influenced by the future use of the roof: whether you want a liverwort roof, a grass roof or a diverse garden. 

When drawing up a budget, it is essential to take into account the benefits of green roofs as a source of clean air, a source of human comfort and a source of environmental diversity. 

If you would like more information or ideas for your own green roof plans, please feel free to contact us!