green roof structure basics

1. Planning of Green Roofs and Green Walls
Andras Bela Olah PhD
olah.andras.bela@gmail.com

2. Lecture 2: Structural design

3. 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.

4. Hanging Gardens of Babylon

5. 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.

6. The Scandinavian Sod Roof

7. 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.

8. 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 Sites
This guideline collection is permanently renewed, the last occasion was
in 2008. This is necessary due to the technical development on this
field.

9. 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.

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

11. The structural layers in reality

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

13. 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.

14. 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.

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

16. 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.

17. Straight layered extensive green roof

18. Inverse layered extensive green roof

19. Green roof layers and parapet connection

20. 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.

21. Thank you for your attention!