Architecture Student Chronicles

A green roof is partially or completely covered with vegetation and a growing medium, planted over a waterproofing membrane.

Green roofs usually do not involve construction of complex drainage and irrigation systems. The use of “green” refers to the growing trend of environmentally friendly and does not refer to roofs which are merely colored green, as with green roof tiles or roof shingles.

Traditional sod roofs

There are different ways in which green roofs are constructed. Container gardens on roofs is one of the type of green roof, where plants are maintained in pots, are not generally considered to be true green roofs, although this is an area of debate. Rooftop ponds are another form of green roofs which are used to treat greywater.

It has various advantages and serves several purposes such as absorbing rainwater, providing insulation, creating a habitat for wildlife, and helping to lower urban air temperatures and combat the heat island effect.

There are two types of green roofs namely;

Intensive roofs

Intensive roofs are thicker and support a wide variety of plants and are heavier requiring more maintenance.

Extensive roofs

Extensive roofs are covered in a light layer of vegetation and are lighter than an intensive green roof.

Chicago City Hall

The term green roof may also be used to indicate roofs that use some form of “green” technology, such as a cool roof, a roof with solar thermal collectors or photovoltaic modules.

The hierarchy of plant life offers an extensive palette for a landscape designer to draw from. Many types of vegetation can be used in a very effective way to modify the micro-climate of the site. Vegetation can be classified as grass, low shrubs, high shrubs, deciduous trees, evergreen trees etc.

We are going to discuss how each type of vegetation can be useful in various ways…

Grass

• Stabilize soil
• Retain rainfall
• Build soil
• Harbour insects and rodents

Low Shrubs

• Cover ground
• Retain moisture
• Mulch soil
• Shelter small birds and animals

High Shrubs

• Visual screen
• Channel winds
• Mulch soil
• Shades ground
• Provides flowers and berries

Deciduous trees

• Mulch soil
• Seasonal shade
• Channel winds
• Shelter structures
• Bare fruits
• Visual screens

Evergreen Trees

• Cool breeze
• Block winter storms
• Visual screen
• Retain soil
• Add acid to soil
• Shade ground

Green Roofing System

Extensive green roofs are often not accessible and are characterised by:

• Low weight
• Low capital cost
• Low plant diversity
• Minimal maintenance requirements

The growing medium typically made up of a mineral based mixture of sand, gravel, crushed brick, peat, organic matter and some soil.

It varies in depth between 5-15cm (2’-6”) with a weight increase of between 72.6-169.4 kg/m² when fully saturated.

18th century Green Roofs

Advantages of Green Roofing

• Lightweight roofs generally does not require reinforcement.

• It is suitable for large areas.

• It is suitable for roofs with (0 to 30^o slope)

• It involves low maintenance costs and has long life.

• There is no need of irrigation and any specialized drainage system.

• Less technical expertise needed.

• Often suitable for retrofit projects.

• It looks more natural and is relatively inexpensive.

Disadvantages of Green Roofing System

• Less energy efficiency and storm water retention benefits.

• Roof insulation can be provided using a roof system of precast hollow terracotta curved panels with nominal GI reinforcements.

• A nominal layer of concrete of only 2” thickness at the crown of panel was poured into place.

• The hollow terracotta layer works as heat resisting layer.

For Flat Roofs

• Effective roof insulation can be provided using vermiculite concrete.

• It reduces the roof construction by 60%.

• Roof insulation can be provided by terracotta tiles flooring above the waterproofing material and lime concrete is laid.

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