Green buildings

Green roofs

Image credit: Audrey Timm

Only choose hardy plants which are able to survive in rooftop conditions

Plants on extensive roof gardens should be able to survive and grow in high temperatures, full sun, high wind speeds and periods of drought. These should be able to survive in a thin substrate layer, a small waterbuffer and fluctuating temperatures.

The best plants for extensive roofs are:

  • Perennial, low, seed-forming, drought tolerant plants
  • Succulent plants with grey hairs or a thick blue layer on the leaves (sedum and sempervivum are available as plants or as ready-made living mats)
  • Plants with strong woody twigs or thin leather like leaves
  • Annuals that bloom and seed freely for continued presence.

A larger variety of plants can grow on an intensive roof garden

An intensive roof garden provides plants with more soil, water and fertiliser availability. Selection factors for plants include the height of the roof, wind speeds, irrigation availability, sun position, relation to other buildings, climate zone

The best plants for intensive roofs are:

  • Plants that don’t get too big (maximum weight capacity), grow in full to half sun, can tolerate wind (do not use plants with lots of seeds that blow away, high plants with breakable or falling stems or plants with strong spreading roots)
  • Perennials should be suitable for full sun and fully cover the ground
  • Eco-roofs should consist of native or near-native plants
  • Shrubs should be winter hardy, not be sensitive to wind, grow in full sun and grow in normal to dry soil, solitary shrubs can be pruned like trees. Low, drought tolerant shrubs are good choices
  • Trees should only grow up to 6m high, not be sensitive to wind, have a crown that is not too big and open
  • Trees with shallow roots must be anchored

Create green roofs on new and existing buildings and fulfil the need for green where space is limited in the urban environment.

 

Criteria for building a green roof

Criteria
Depth of soil7-10 cmfor sedum, moss
25 cmfor shrubs
80 cm for trees (0.75m per m canopy)
Weight30-130 kgfor 2-15 cm soil
130 -300 kgfor 15-20 cm soil
250-1000 kgfor 15-80 cm soil
MaintenanceA conventional roof requires inspection 1 x per 5 years
An extensive green roof requires inspection 1x per year and weeding (no irrigation required)
An intensive green roof requires inspection 8 x per year (weeding, pruning and fertilizing) and always requires irrigation in dry periods.

Benefits of green roofs

  • Life of the roof is up to 20 years longer
  • Insulates the building against cold and warm
  • Gives a “green” image
  • Absorbs noise and vibrations
  • Makes solar panels more efficient
  • Eases the peak flow in stormwater sewers; lessens the urban heat island effect; produces oxygen; absorbs carbon dioxide
  • Captures particulate matter, absorbs gaseous pollutants
  • Improves the living and working environment

…The price of a garden is the same on the ground as on a roof: the land is already paid for! The extra costs of the roof construction/preparation, special soil mixture preparation and transportation of the materials to the roof make up the difference… Niek Roozen, landscape architect

 

New and existing developments

In new & existing development:

Check local building codes and permits and have a structural engineer check the plans to be sure the roof is strong enough to hold the weight. It is also wise to compartmentalize the roof so it is easier to find the sources of possible leaks and prevent damage to neighbouring roofs.

In existing development:

The existing roof must be in good condition, strong enough, insulated and waterproof. Check if the insulating layer is under the waterproofing layer or over it and design accordingly. A layer of root barrier is also needed.

 

List of trees successfully used on roof gardens in The Netherlands

Nothofagus antarctica                                 

Pyrus salicifolia ‘Pendula’

Amelanchier lamarckii                                  

Pinus nigra ‘Nigra’

Taxus baccata                                                  

Betula utilis ‘Doorenbos’

Cornus mas