PRINCIPLES OF DESIGNING GREEN ROOFS

INSULATION LAYER RESISTANT TO ROOT OVERGROWTH

The basic requirement for roofing is resistance to plant root overgrowth. To build green roof systems, single- and double-layer materials are used. As a rule, single-layer materials made of plastics (foils, membranes) are resistant to root overgrowth. Double-layer materials should have a minimum acceptable thickness for the undercoat layer of 4.00 mm, and for the surface layer – 5.00 mm. Both layers should be made of bitumens modified with SBS polymer. Reinforcement (the carrier insert) should be made of a polyester nonwovenfabric with a minimum weight of 150 g/m2 in the case of the undercoat layer and 180 g/m2 in the case of the surface layer. Application of the third layer of the base roofing membrane with minimum parameters, such as in the case of the top roofing membrane, constitutes additional protection. The surface layer should be resistant to root overgrowth. Such resistance is ensured by adding a special chemical agent or an insert made of copper foil. In the case of normal polymer-bitumen roofing membrane which is not resistant to root overgrowth, it is possible to apply additional protection against root overgrowth by adding a 0.4 mm thick foil with min. 0.15 mm folds. Roofing should be installed in compliance with the manufacturers’ instructions and applicable standards.

Roof penetrating elements: to reduce the number of connections on the roof surface, roof penetrating elements should be concentrated in one place. Green roofs should be accessible for maintenance purposes. It is recommended to design a water connection on the roof surface or near it.

projektowanie zielonych dachów

PREVENTIVE FIRE PROTECTION

Construction and insurance supervision requires that roofs made as “fire-resistant roofs” are protected against fire penetrating from above and from thermal radiation. Based on the testing methods according to DIN 4102 part 7, the surface layers of green roofs, which fulfill these standards in the case of the specific structure, are tested. Vegetation mats by Xero Flor are fire-resistant and they fulfill DIN standards.

WHEN A GREEN ROOF IS A FIRE-RESISTANT ROOF?

Roofs covered by intense greenery on which greenery is watered and cared for, are usually made on a thick layer of substrate. Such roofs without the need to fulfill further conditions are resistant to fire and thermal radiation, and hence considered to be fire-resistant.

Roofs covered by extensive greenery, i.e. greenery which does not have to be watered and which is rarely subject to additional care, are considered to be fire-resistant when they fulfill the following conditions:

  • the substrate layer is at least 3 cm thick,
  • the substrate contains at most 20% of the weight of the organic part,
  • in the case of larger surfaces covered by greenery, there is partitioning (e.g. a gravel lane) every 40 m,
  • in the case of openings on the roof surface (penetrations, fanlights), gravel bands (approx. 50 cm wide) are arranged.
zielone dachy przegrody ogniowe

PROTECTION AGAINST WARMTH AND MOISTURE

Calculating thermal insulation, only layers of sealing or roofing from the inside can be taken into consideration in compliance with DIN 4108 in a typical case. Layers of a green roof should not be taken into account calculating this parameter.

WIND UPLIFT FORCES

Surface layers of roof greenery must stay in place even with strong gusts of wind. Required additional ballasts are compliant with DIN 1055, part 4. Here, a slightly simplified method of solving the above problem was presented. In the central part of the roof, an additional ballastwith a thin substrate layer is sufficient, where it is not possible to detach layers covered by greenery as a result of wind uplift forces. However, the situation is different along the edges and corners – places especially exposed to wind uplift forces. In such areas it is usually necessary – depending on the roof form and height – to use additional protective ballasts in the form of gravel or concrete slabs.

0.5 (50)0.8 (80)1.1 (110)

Wind uplift forces in the case of a roof surface with an incline of up to 8° depending on the horizontal roof projection a x b and building height hThe height of the building from the ground
0 – 8 m8 – 20 m20 – 100 m
Accumulation pressure q in kN/m² (kg/m²)
Roof rim and aerodynamic pressure coefficient (pressure) CpPressure force (wind uplift or required additional ballast in kN/m² (kg/m²)
Relations between dimensions
b/a=<1.5; h/a=<0.4
Corner rim Cp=-2.0
Edge rim Cp=-1.0
Central part Cp=-0.6
1.00 (100)
0.50 (50)
0.30 (30)
1.60 (160)
0.80 (80)
0.48 (48)
2.20 (220)
1.10 (11)
0.66 (66)
Relations between dimensions
b/a=<1.5; h/a=<0.4
Corner rim Cp=-1.5
Edge rim Cp=-1.5
Central part Cp=-0.8
1.00 (100)
0.50 (50)
0.30 (30)
2.24 (224)
1.20 (120)
0.48 (48)
3.08 (308)
1.65 (165)
0.88 (88)
Relations between dimensions
b/a=<1.5; h/a=<0.4
Corner rim Cp=-2.5
Edge rim Cp=-1.0
Central part Cp=-0.6
1.00 (100)
0.50 (50)
0.30 (30)
2.00 (200)
0.80 (80)
0.48 (48)
2.75 (275)
1.10 (110)
0.66 (66)
Relations between dimensions
b/a=<1.5; h/a=<0.4
Corner rim Cp=-0.3
Edge rim Cp=-1.7
Central part Cp=-0.8
1.00 (100)
0.50 (50)
0.30 (30)
2.00 (200)
1.36 (136)
0.64 (64)
3.30 (330)
1.87 (187)
0.88 (88)

PLANT LAYER

EXTENSIVE GREENERY

Extensive greenery includes plants with the lowest vegetation requirements which are self-sufficient and develop on their own. They reproduce vegetatively and generatively, and actually they do not require cumbersome care (apart from recommended annual inspectionsin compliance with the manufacturer’s guidelines) and additional watering. Plants adjusted to extreme weather conditions and with significant regeneration capacities are used. In green roof systems it is recommended to use local plants with minimum vegetation requirements, such as mosses, perennials, sedum, herbs, grasses, etc. Methods are different: starting from planting vegetationin appropriatequantities per 1 m² (from 10 to 25 pcs/m²) to distribution of ready green vegetation mats.

zazielenienie ekstensywne
zieleń ekstensywna

SUBSTRATE VEGETATION LAYER

Extensive mineral substrates aim to allow plants to develop properly and fulfill the role of a ballast layer in specific cases.

Components of substrates should be products of mineral origin with a small addition (max. 10%) of organic components. They should be characterized by a stable skeletal structure which will be resistant to frost and UV radiation. The air content in substrates – with the maximum water saturation – should not exceed 10%. The salt content in substrates should practically not exceed the value of 1.0 g/l of the substrate. The substrate should have proper capacity to accumulate water at a level of at least 15% of its volume. Technical requirements concerning the thickness of the substrate layer on the roof are limited to the maximum weight a given roof can carry. The vegetation layer (substrate) thickness depending on the designed form of greenery is illustrated by the table below.

grubość warstwy wegetacyjnej

VEGETATION LAYER MADE OF HYGROSCOPIC ROCK WOOL

Hygroscopic rock wool is a special kind of mineral wool. It is supposed to ensure the best vegetation conditions for plants; it is characterized by significant water absorption and it is resistant to decay processes. It is used in construction of extensive green roofs as a substrate replacement due to its very light weight. It is used as a vegetation and storage layer on which green vegetation mats are placed. Using the arrangement of layers listed above, a green roof may weight approx. 45 kg/m².

FILTRATION PROTECTIVE LAYER

The filtration layer should be characterized by at least 10 times higher permeability than the vegetation layer thanks to which it can protect the drainage system and drains against siltation. The filtration layer should be made ofa 100% polypropylene structural nonwoven fabric with a weight of 200-250 g/m², free from chemical binding substances, characterized by long-term durability (also in terms of drainage stability and filtration) as well as resistance to water, chemicals and decay. The nonwoven fabric has to have good water permeability in the horizontal and vertical direction as well as the ability to diffuse water vapor. The filtration layer in the nonwoven form is distributed directly on the drainage, and under the vegetation layer, leaving a fold ofat least 10 cm between particular bands.

DRAINAGE LAYER

In compliance with DIN 4095 on the slabunder the substrate, the drainage layer is placed which will discharge excess water, being an additional ballast and negatively affecting development of plants. It is supposed to be resistant to changing weather conditions (moisture, temperatures below zero) and be characterized by significant mechanical resistance tostretching, elongation, friction and compression. The drainage layer must be constructed in such a way that it functions for a long time. Drainage should discharge water in the amount of 0.03 l/s m² = 300l/s ha. Natural drainage used sometimesmade of rinsed gravel should have a granulation from 16 to 32 mm and a minimum thickness of 5 cm. Drainage made of artificial materials, such as loop mats, bucket mats or geotextiles is light and low, but still very durable and most of all compliant with the standard; geotextiles used in them filter water perfectly preventing siltation of drains. It is assumed that starting with anincline above 2º the drainage system is not required (using the Xero Flor system).

GRAVEL LAYER (BAND)

To ensure good communication and access to technical devices installed on the roof around the roof edges (attics and adjacent facades of higher parts of buildings) and technical devices (ventilation, culverts, skylights, smoke dampers, control chambers), a band made of rinsed gravel with a fraction of 16-32 mm and a thickness of approx. 10 cm should be made. The gravel band width should be at least 50 cm.

CARE OF GREEN ROOFS

The Xero Flor extensive system is basically self-sufficient and cares for itself; natural processes in the substrate provide nutrients for plant vegetation. The state of stress in plants caused e.g. by the lack of water, is balanced using specially selected mechanisms. Death of old plants on one hand, and sowing and development of young plants on the other hand take place in a closed system with continuous biological renewal and regeneration of plants. Reasonable circulation which can be carefully supported is not subject to any intensive care treatments. Hence, no herbicides need to be used. Damage to vegetation is also caused by introducing foreign plants into the system. Minimum care treatments are appropriate fertilizers, regular inspections and cleaning of roof drains in spring.

WATERING

During the growth phase, i.e. immediately after planting extensive greenery on the roof, the entire roof should be watered in such a manner that mats and the substrate absorb as much water as possible. Then, in the next 2-4 weeks immediately after installation, the entire roof should be watered. After the growth phase extensive roofs usually do not require any watering, however during long-term draught it is recommended to water the entire roof surface to achieve complete water saturation.

pielęgnacja zielonego dachu

FERTILIZING

Planting extensive greenery, fertilizing can be limited to the use of fertilizers in spring and autumn. Organic fertilizers with prolonged effects should be used. The amount of fertilizers used should range from 5 to 30 g/m² depending on the type of fertilizers.

INSPECTION OF WATERING SYSTEMS

Hollows on the roof and places directly next to roof drains are critical points with increased accumulation of water. Due to increased susceptibility to the development of unwanted vegetation such places must be particularly inspected, and weeds and plants growing around these places must be removed.

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