Topic 4 Installation and development of green roofs

Several considerations must be reflected upon in the development of a green roof…

  • What are the structural requirements for a green roof?
  • What kind of process do we follow for its installation?

Structural requirements depend on the type of roof constructed as well as environmental conditions…

  • Intensive roofs require greater structural stability and more elaborate safety measures.
  • Extensive roofs require less structural stability due to limited human access requirements.

This topic is divided as follows:

  • 4.4.1: Important considerations before the instalment of green roofs
  • 4.4.2: Green roof instalment: A practical guide

4.4.1. Important considerations before the instalment of green roofs

Dead load (D)

The weight of the building medium, stationary and any permanent materials of construction.

Live load (L)

The load to be produced by the intended use of a green roof, such as the soil medium and vegetation, as well as snow, ice and rain.

Wind load (W)

Temperature load (T)

For greater live load (e.g. intensive roofs, climate variable) there has to be greater dead load, hence greater capital cost.

It is best to get a structural engineer to determine what your existing roof needs to support the weight of a green roof.

4.4.2. Green roof instalment: a practical guide

  • Green roofs generally consist of multiple layers which provide a growing surface and drainage of excess water.

    More specifically, these layers can include:

Vegetation – or plant – layer

Soil medium

Soil filter fabric

Drainage mat

Waterproof geomembrane

It is imperative that water not absorbed by the roof is drained properly so the installation of the geomembrane is of utmost importance

  • The first step in the installation is a monolithic type waterproof membrane (rubber or plastic) on top of the roof decking.
  • Once the membrane is placed, it is best that is tested with an Electric Field Vector Mapping (EFVM) to detect leaks.
  • Test must be performed immediately after membrane has been installed by moistening the latter and running a current through two probes.

Leak test is performed as follows:

  • Examination wire is set up in rectangular loop around test area and is connected to pulse generator.
  • Every 3 seconds, a pulse is delivered for duration of one second.
  • An electric potential difference is created between wet layer of medium and grounded roof deck. In case of leaks, current will flow from medium through hole to the roof deck.
  • Using receiver, the direction of current can be determined and hole can be pinpointed.

Top the first layer of the geomembrane with one or more thin sheets of foam insulation suited for contact with damp soil.

    • This is only required in cases where it is needed to increase the R-value for the roof.

Following, set a drainage mat with capillary spaces on top of the insulation. Place the mat so the felt side faces up in order to keep the soil from clogging the mat.

    • The mat diverts any excess rain water not absorbed by soil medium or vegetation.

Above the drainage mat, place a soil filter fabric that serves as a root barrier.

  • These are often geotextiles that retain the soil and keep vegetation roots from penetrating drainage layer.

You can frame the sides for the roof with mesh gutter guards, wood or other edging that will permit drainage to hold soil in place.

Add the soil medium

  • The optimum soil conditions consist of ½ solid particulate matter, ¼ water and ¼ oxygen.
  • Correct aggregate can increase porosity and soil’s capacity to retain water and nutrients, as well as improve aeriation and drainage for plant growth.
  • Addition of compost provides high quantity of organic material for enhanced growth.

Add the vegetation layer

  • For extensive roofs, choose succulent or cacti-like plants which need minimal hydration (maximum root depth of 10 cm).
  • Intensive roofs have endless vegetation choices.
  • In the absence of precipitation (in dry climates), the installation of external water source or irrigation system is strongly recommended.
  • This ensures that the growth medium receives adequate nutrients and moisture under climate conditions.