Green roofs and walls: types, benefits and how they are built

Roofs are the largest unused surface in our cities. Instead of bitumen and metal sheeting they could hold a meadow, a garden or a place to relax – and that is not only attractive but also useful for the building, the people in it and the whole neighbourhood.

Green roofs and walls are no longer exotic. The technologies are well established, and in Bulgarian cities, with their hot summers and heavy spring downpours, the benefits are particularly tangible. Here is what you need to know before deciding whether to green your roof or façade – from the types of system to the layers, upkeep and the most interesting examples around the world.

What is a green roof?

A green roof is a roof on whose waterproofing a root barrier, drainage, a filter and a growing medium with plants are laid. It can be flat or gently pitched and is designed as a complete system.

There are two main types:

• Extensive green roof – a thin layer of growing medium, typically from a few centimetres up to around 15 cm, with hardy plants such as sedums, grasses and herbs. It is light, needs almost no watering or upkeep and is often not intended to be walked on.
• Intensive green roof – a deeper growing medium that supports lawns, shrubs and even small trees. This is a true roof garden that you can walk on, but it is much heavier and needs watering and regular care.
• Semi-intensive – an intermediate solution with more varied planting and moderate upkeep.

What is a green wall?

A green wall is a vertical surface covered with plants. It can be built in two ways:

• Green façade – climbing plants that grow from the ground or from planters and climb the wall or a tensioned mesh. Simple, inexpensive and long-lasting, but slow to grow.
• Living wall (vertical garden) – a modular or textile system fixed to the wall, with plants set directly into it and automatic irrigation and feeding. The effect is immediate and much richer, but the system needs constant maintenance.

What are the benefits of green roofs and walls?

The main benefits are rainwater retention, less overheating and a longer roof life.

• Rainwater – the growing medium holds back part of the rainfall and slows run-off, easing the load on the sewers during heavy rain.
• A cooler roof in summer – plants and growing medium shield the roof from direct sun, and evaporation cools it. Especially useful for the top floor.
• Protection for the waterproofing – it is hidden from ultraviolet rays and sharp temperature swings, which extends its life.
• A cooler city – more greenery reduces the urban heat island effect.
• Biodiversity – a habitat for bees, butterflies and birds.
• Sound insulation – the extra mass reduces outside noise.
• Quality of life – views of greenery and extra space to relax.

Why does this matter in Bulgaria?

Hot summer days and increasingly frequent heavy rainfall make both main benefits especially relevant. The top floor beneath a flat roof in a prefabricated panel or cast-in-place concrete block often overheats in summer; a green roof, together with good thermal insulation, makes these rooms more comfortable. In the densely built districts of Sofia, Plovdiv or Varna, every green roof is also a small extra green space where there is no longer any room at ground level.

What are the drawbacks and risks?

The main risks are weight, leaks and upkeep – and all three are managed through good design.

• Weight – water-saturated growing medium in particular loads the structure. For existing buildings, a check by a structural engineer is essential.
• Waterproofing – a leak under a green roof is harder to find and repair. That is why root-resistant membranes and carefully executed details are used.
• Upkeep – even an extensive roof needs inspecting and its drainage outlets clearing at least once or twice a year.
• A higher initial cost compared with a conventional roof.

How is a green roof built, step by step?

1. Structural check. An engineer determines how much additional load the roof can carry.
2. Waterproofing and root protection. Reliable waterproofing and a root barrier are laid.
3. Thermal insulation. On an inverted roof it sits above the waterproofing and must be suitable for a damp environment.
4. Drainage layer. Carries away excess water and retains some of it for the plants.
5. Filter layer. A geotextile that stops fine particles from clogging the drainage.
6. Growing medium. A special lightweight mix, not ordinary garden soil, which is heavy and compacts.
7. Planting. Plants are chosen according to sun, wind, the depth of growing medium and the local climate – in Bulgaria they must withstand summer drought and winter cold.
8. Irrigation and upkeep. Drip irrigation for intensive roofs and regular inspection of the drainage outlets.

What are the famous examples of green roofs and walls?

The most memorable projects show how varied the result can be.

• ACROS Fukuoka, Japan – a building by architect Emilio Ambasz whose park-facing façade steps down as a series of planted terraces.
• Bosco Verticale, Milan – Stefano Boeri's two residential towers with trees and shrubs on their balconies.
• Musée du Quai Branly, Paris – Jean Nouvel's museum with the famous living wall by botanist Patrick Blanc.
• CaixaForum, Madrid – a cultural centre by Herzog & de Meuron with a vertical garden by Patrick Blanc on the neighbouring party wall.
• One Central Park, Sydney – a residential complex by Jean Nouvel with vertical gardens on its façades, again in collaboration with Patrick Blanc.
• PARKROYAL on Pickering, Singapore – a hotel by the practice WOHA with sky gardens on several levels.
• Vancouver Convention Centre, Canada – one of the largest living roofs in North America, planted with native species.
• The High Line, New York – a former elevated railway turned into a linear park above the streets of Manhattan.

How we approach it at DARCH Projects

A green roof or façade should be built into the architectural design from the start – together with the structure, drainage and access for maintenance. When renovating existing buildings, we assess the options as part of the energy-efficiency retrofit design and show the result in a visualisation.

Frequently asked questions

What is the difference between an extensive and an intensive green roof?

An extensive green roof has a thin growing medium and hardy plants such as sedums and grasses. It is light, needs almost no watering and is usually not meant to be walked on. An intensive roof has a deep growing medium, supports lawn, shrubs and small trees and is used as a garden, but it is much heavier and needs regular maintenance.

Can a green roof be added to an existing building?

Often it can, but first a structural engineer must check how much additional load the roof slab can carry. Extensive roofs are the lightest and the easiest to add to an existing building. The condition of the waterproofing also matters – it is usually replaced with a root-resistant membrane before the drainage, growing medium and plants are laid.

Does a green roof need maintenance?

Yes, although it varies by type. An extensive roof needs inspecting once or twice a year – clearing the drainage outlets, removing weeds and filling any gaps. An intensive roof is maintained like an ordinary garden: watering, mowing, pruning and feeding. Without upkeep, even a well-built roof gradually loses its appearance and its function.

What is the difference between a living wall and a green façade?

A green façade is formed by climbing plants that grow from the ground or planters and climb the wall or a mesh – simple and long-lasting, but slow. A living wall is a system of modules or pockets in which plants are set directly into the vertical surface, with automatic irrigation. It gives an immediate effect but needs more maintenance.