Planning of Green Roofs and Green Walls

Andras Bela Olah PhDolah.andras.bela@gmail.com

Lecture 2: Structural design

Historical example 1.The Hanging Gardens of Babylon:

It was a real green roof, moreover it was a real flat roof (!)

„…Furthermore, the walls, which had been constructed at great expense, were twenty-two feet thick, while the passage-way between each two walls was ten feet wide. The roof above these beams had first a layer of reeds laid in great quantities of bitumen, over this two courses of baked brick bonded by cement, and as a third layer of covering of lead, to the end that the moisture from the soil might not penetrate beneath. On all this again earth had been piled to a depth sufficient for the roots of the largest trees;… ”

/Diodorus Siculus; Ctesias Cnidus/

Thus, the appearence of bitumen in nature (on the surface), the invention of arch, and the absolute monarchy and the great number of slaves/war prisoners together made possible the creation.

The possible builder was Sin-Ahhe-Eriba (Ninive 7th Century BC) or Nabu-Kudurri-Usur (Babylon 6th Century BC) or both.

Hanging Gardens of Babylon

Historical example 2.The scandinavian sod roof:

It is a real green roof, but it is always pitched due to the lack of waterproof insulation (there were not any natural bitumen source in Scandinavia).

The roof structure is an absolutely traditional wooden log structure (the whole building is a log house). The role of tiling is played by multiple layers of birch bark. It is excellent for tiling.

Northmen were not interested in environmental benefits ( ), but they were highly interested in stabilizing (weighing) the birch bark tiling and in heat insulating. Furthermore, they needed affordable solution.

That is why they used sod layer (double layer), by the lower layer the grassy side was downwards(!), and only by the upper layer was the grassy side upwards. This defended the birch bark from hummus acids and helped the rooting after establishing.

The sod covering was made together with birch bark covering (team work!). Extensive green roof, due to scandinavian climate.

The Scandinavian Sod Roof

Present day green roof technology I.

The real green roofing started only in the 1960ties, when the usage of waterproofing membranes became common and widely used. This means that real green roofs appeared simultaneously with real flat roofs.

This green roofing movement started in Germany (West-Germany) and even nowadays the german green roofing industry is considered to be the best worldwide.

The idea based on the necessity of urban environment improvement. In the 1960ties the rate of green zones in downtowns were already very small. The ideology was supported also by the modernist architects (Le Corbusier).

The affordability of the usage of waterproof membrans opened the way towards creating again green roofs.

Present day green roof technology II.

There must be mentioned a few societies on this field:

FLL: Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau =The German Landscape Research, Development and Construction Society.They provide a worldwide accepted guideline collection in this topic:

The German FLL-Guideline for the Planning, Execution and Upkeep of Green Roof SitesThis guideline collection is permanently renewed, the last occasion was in 2008. This is necessary due to the technical development on this field.

Present day green roof technology (problems)

There appeared problems but then appear unexpected advantages.

Problems: -the soil can not be put directly onto the waterproof membrane, a drainage layer is necessary.-The drainage layer must be kept clean, which requires a filter membrane-The soil itself can not be ordinary soil, there are special requirements, it must not be hardpan and it should be as light as possible.-The roots are extremly strong and almost irresistible, thus sometimes there is necessary a root barrier layer.-There can occur problems, with the edges (a parapet wall is necessary and there must be also a drainage between it and the growing medium)-The delf must be designed carefully (it is possible that the delf is frozen, while the water in the drainage layer is still liquid. The slack water must be avoided.

Present day green roof technology (scheme of the structural layers)

The structural layers in reality

Filter membrane

Filter membrane, usually geotextile. Permits water and retains growing medium. Agressive roots can penetrate it.

Drainage layer

Dorken membrane or Delta membrane. Originally invented for avoiding the damages caused by soil movement, which comes from frost. It can retain some water, which can be used up by the plants. It is usually 2-6 cm thick.

Root barrier

It is usually a copper layer (copper is poisonous for roots). It is very important that it has to be permeable for water.

Waterproofing membrane

These are usually plastic layers. It can be glued (by waterproof glue) or welded.

Different structural designs

In case of intensive green roofs the structural design is always like this.

Due to the thick growing medium the lower layers are always unfrozen (in Central-European countries).

The thick „soil” layer together with the plants act as excellent heat insulator in every season.

In case of extensive green roofs the situation is a little bit more complex. Considering a Sedum roof with only about 5 cm thick growing medium, the heat insulation ability can be queried (despite the fact that the drainage layer is also a good heat insulator and the water contain highly increases the heat capacitance).

Thus, there used to applied an additional heat insulation layer.

In case of the so called straight layered green roof the heat insulator layer is beneath the waterproof membrane.

In case of inverse layered green roof the heat insulator layer is above the waterproof membrane.

There exist also dual layered green roofs with double heat insulation layers.

Straight layered extensive green roof

Inverse layered extensive green roof

Green roof layers and parapet connection

Layers of a pitched green roof

The structure is similar that of the flat green roofs, but here must be inserted retaining structures, which do not let the growing medium creep.

Thank you for your attention!