How to make Intensive Green roofs?
Introduction
Intensive greening of a roof can be considered comparable to more traditional soft-landscaping schemes in terms of potential use and diversity of design. Planting can include lawn, shrubs, bushes and even the occasional tree. Once planted, the vegetation imposes a fairly high demand on the composition layers and needs regular irrigation (or a water retention system) as well as a regular supply of plant nutrients. This type of green roof can only thrive with regular maintenance. An intensive green roof can easily be combined with hard landscaping for pedestrians and vehicular traffic.
Due to the vegetation’s demands for water and nutrients, along with the planting itself, the typical minimum weight of this type of green roof would be around 282 kg/m2 with a minimal layer depth of 217 mm.
When designing intensive green roofs, these high loadings must be taken into consideration.
Features of an intensive green roof:
- wide choice of suitable vegetation
- total freedom of design
- easily combined with pedestrian and vehicle accessible areas
- construction height – starting from 217 mm
- dead load – starting from approx. 282 kg/m2
Design considerations
1.1 Standards
CE-marking hEN 13252
Geotextiles and geotextile-related products for use in drainage systems must bear a CE marking pursuant to the European Standard hEN 13252 ‘Geotextiles and geotextile-related products – Characteristics required for use in drainage systems’. Drainage systems are defined as systems that collect and transport precipitation, ground water and/or other liquids or gases via a geotextile or a geotextile-related product.
This European Standard covers more than just geotextiles such as filter membranes and filter fabrics. Geotextile-related products such as geocomposites (drainage channels and drainage panels) and geospacers (nubbed film and nubbed panels [so-called eggbox-shaped dimples]) are also covered by this standard (hEN ISO 10318 ‘Geosynthetics – Part 1: Terms and definitions’).
The manufacturer is responsible for issuing a Declaration of Performance (DoP) relating to the essential characteristics of the geotextiles and drainage channels that it has brought onto the market pursuant to the European Standard hEN 13252). Part of the Declaration of Performance is an internal production control system that is assessed annually by a certification institute (Notified Body). The product therefore satisfies the national provisions set by the European Member States with regard to the essential characteristics of construction products. All geotextiles and geotextile-related products brought onto the market by manufacturers have to bear the CE-marking.
FLL-Guideline for the Planning, Construction and Maintenance of Green Roofing, published 2008
There are no European standards which specify the design of green roofs. The FLL-(a research institute in Germany) has drawn up a guide for the design, installation and maintenance of green roofs.
This guide sets out the basic principles and requirements that, in general, apply to the design, installation and maintenance of green roofs. This guide is based upon scientific research and practical experience in building green roofs in Germany over the past 20 years.
In many countries throughout Europe this document has been accepted as the key guide for building green roofs. This guide is also based upon this FLL-Green Roof Guideline (2008).
1.2 Design loads
The structural deck should be able to withstand and to absorb the static and dynamic loads imposed during construction and final use.
The load on a roof is determined by the:
- dead permanent load imposed by the weight of the construction
- permanent load imposed by the composition of the green roof
- variable load due to e.g. maintenance work or traffic load
The assumed permanent load of an intensive green roof at maximum water capacity:
* Weight approx 1,5 tonne/m3 at maximum water capacity
** Optional for usage in a non-root resistant waterproofing membrane
There are no published standards governing the construction of elemental pavements over a roof deck in the UK or RoI. Through research in close cooperation with the Technical University in Munich, Germany, the following load classes can be distinguished based upon the intended use of the roof deck:
The construction of elemental pavements over roof decks for the various load classes is described in the section “Trafficable roofs”.
1.3 Roof constructions
The structural deck needs to be able to carry the extra load of the extensive green roof composition. The waterproofing membrane should be root resistant and as with the thermal insulation, be able to carry the permanent load of the extensive green roof composition.
The following roof constructions are recognized:
Cold roof construction
This is a roof construction with an independent ceiling enclosing an air space between the structural deck and the ceiling. When insulation is used it should be placed below the structural deck with a ventilated airspace in between. The load bearing capacity of the structural deck is generally minimal and must correspond to the calculated weight of the extensive green roof. The cooling effect of an extensive green roof can affect the physical properties of the structure. Freezing temperatures on the underside of the structural deck may result in frost damage to the vegetation.
Warm roof construction
This is a roof construction without a ventilated airspace beneath the structural deck. When insulation is used it should be placed on top of the structural deck. It is recommended that a vapour control layer be placed on top of the structural deck underneath the thermal insulation. In general, all types of green roof systems and all forms of vegetation are suitable for use with this type of roof construction.
Inverted roof construction
Insulation is placed on top of the waterproofing membrane. Should an inverted roof be selected for greening, moisture diffusion measures should be considered. When an extensive green roof is installed, a damp-permeable drainage layer must be placed over the thermal insulation in order to protect the insulation from accumulating moisture (internal condensation) over time.
In general, all types of green roof systems and all forms of vegetation are suitable for use with this type of roof provided there is sufficient dead load to prevent uplift of the thermal insulation due to water and wind.
Roof construction without thermal insulation
On top of the structural deck the waterproofing membrane is installed without any thermal insulation. A characteristic of this roof construction is that the space beneath the roof is not heated. Basically all types of green roof systems and all forms ofvegetation are suitable. Freezing temperatures on the underside of the structural deck may result in frost damage to the vegetation.
1.4 Thermal insulation
Thermal insulation needs to be CE-marked based upon the hEN 13162 – 13171 ”Thermal insulation products for buildings. Factory made … Specification”.
There are two different methods for installing thermal insulation to a roof deck:
- IRC = insulation placed above the waterproofing membrane – inverted roof construction
- WRC = insulation is placed beneath the waterproofing membrane – warm roof construction
A cold roof has been omitted as this type of roof construction is rarely used nowadays.
The waterproofing membrane and the applied thermal insulation should be able to withstand short and long term loadings. Should any deformations of the thermal insulation be expected, it should be taken into account when detailing the waterproofing membrane (roof outlet, roof edge, roof protrusion, etc.). For load class 1, the roofs built on an insulated roof, the thermal insulation should meet minimum load class “dh”. For load classes 2 and 3 the thermal insulation should meet load class “ds” respectively load class “dx”. The suitability of thermal insulation is to be demonstrated by the
manufacturer.
Recommendation
If a paving needs to be installed on top of an insulated roof, it is recommended that an inverted roof construction with XPS insulation or a warm roof construction with cellular glass be chosen. With an inverted roof the waterproofing membrane should be fully bonded with the structural deck, in order that any leak in the waterproofing membrane can be located easily. The XPS insulation panels offer extra protection of the waterproofing membrane during installation of the paving composition.
It is important that a damp-permeable drainage layer is placed on top of the XPS insulation. This allows the panels to dry. Water absorption due to internal condensation will be minimised. It is not necessary to install a separate vapour control layer as the waterproofing itself acts as one. The drainage layer should not damage the top of the insulation panels. Full bonding of the waterproofing membrane is also possible with a warm roof construction if cellular glass is used as thermal insulation. The cellular glass panels are fully bonded with the structural deck and all joints are filled with bitumen. The waterproofing system is thereby fully bonded with the cellular glass panels.
Suitability of the various types of thermal insulation:
* Compressive strength at 10 % deformation in accordance with hEN 826 “Thermal insulating products for building applications. Determination of compression behaviour”
1.5 Waterproofing systems
Continuous waterproofing systems
Roof constructions are, in general, protected against the penetration of water by a waterproofing system (bitumen, synthetic or liquid-applied).
When designing and choosing a waterproofing system, the intended use, applicable standards, regulations and standards of good practice have to be observed. Roof decks should be constructed with adequate falls.
The waterproofing system should be designed to suit the anticipated use. To maintain the integrity of the waterproofing membrane, and to ensure proper construction of the paving, it is essential that the membrane system is being laid as flat as possible.
On trafficked roof decks, horizontal loads caused by vehicle overrun can excessively compress the waterproofing membrane. Such loading should be avoided and therefore separation and slip layers should be built in to the structure.
The waterproofing membrane beneath any vegetation (intensive or extensive planting schemes) should be root resistant or protected against root penetration by a separate root barrier. Root resistance can be proven if the material has passed the FLL-root resistance test or is covered by the British Board of Agrément (BBA) Certification for green roof applications.
The membranes can be applied in one or two layers and attached to the structural deck according to the following methods:
- loose laid and ballasted
- mechanically fixed
- fully bonded.
The composition of a fully bonded waterproofing system can be as follows:
Bitumen – modified bitumen waterproofing membranes
(APP – SBS)
- at least two layers
- first layer: a polyester based roofing felt fully bonded to the structural deck (pour and roll)
- top layer: a root resistant APP or SBS waterproofing membrane fully bonded (torched).
Synthetic waterproofing membranes
- at least two layers
- first layer: a polyester based roofing felt fully bonded to the structural deck (pour and roll method)
- top layer: EPDM, ECB, POCB or TPO waterproofing membrane fully bonded to the first layer.
Liquid-applied roof waterproofing
- Liquid-applied roof waterproofing is regarded as a single layer system.
- It should adhere to the entire surface and be applied in at least two discrete layers.
- A suitable geotextile should be placed in between the layers as a reinforcement.
- The manufacturer should have European Technical Approval in accordance with ETAG 005 “Liquid Applied Roof Waterproofing Kits”.
Mastic asphalt
- The concrete sub-structure needs to be primed before nstallation.
- As a sub-layer – a root resistant APP – SBS torch-on embrane.
- The asphalt layer with a minimum thickness of 25 mm should be installed on top of the sub-base.
Water-resistant concrete
- Requirements for water-resistant concrete are specified in hEN 206-1 “Concrete. Specification, performance, production and conformity ”and hEN 8500 “Concrete – Complementary British Standard to hEN 206–1 Parts 1 and 2”.
- Cracks in any direction should be limited to ≤ 0.2 mm.
Recommendation
It is recommended that a waterproofing membrane is fully bonded with the structural deck. In many installations leakages occur due to incorrect detailing, poor choice of materials or errors/damage incurred during installation. When a loose laid waterproofing system is damaged, the point of leakage is difficult to locate as the water can move freely over the structural deck. Fully bonded waterproofing systems give much more security if they are installed on a closed structural deck. This means that with insulated roof constructions the choice is limited to a warm roof with cellular glass or PUR and an inverted roof with XPS insulation.
If it has been decided to install a warm roof construction in which the waterproofing membrane is not fully bonded with the structural deck, it is recommended that separate compartments within the vapour control layer be created. In case of any damage to the waterproofing membrane, any leak can be located more easily.
1.6 Details
Basically, the same waterproofing detail principles apply to green roofs as to flat roofs. The waterproofing membrane should be brought up above the surface level by at least 150 mm at roof upstand details e.g. parapets, abutments, and roof protrusions.
Roof edge
At roof edge details a clear strip of clean gravel (min. 16/32 mm) or concrete slabs should be installed for maintenance and inspection purposes. The minimum recommended width is 300 mm. To prevent the growing medium being washed into the clear strip, an Edge Retaining Profile should be installed.
If the waterproofing membrane projects over the roof edge into the ground, it is recommended that the waterproofing membrane extends a minimum 500 mm beyond the edge and at least 200 mm over a joint.
Facade
At facades, the waterproofing membrane should be brought up above the highest roof edge by a minimum of a 150 mm above service level. This is not always possible at door thresholds, therefore at those door thresholds where a channel drain is installed, the waterproofing membrane can be brought up above the surface level by 50 mm.
Along facades, a clear strip of clean gravel (min. 16/32 mm) or concrete slabs should be installed for maintenance and inspection purposes, and to act as a splashguard. The clear strip between the facade and the vegetated area helps to prevent any water run-off adversely affecting the development of the plants. To prevent the growing medium being washed into the clear strip, an Edge Retaining Profile should be installed.
The minimum recommended width of this clear strip is 300 mm.
Roof protrusion with or without roof upstand
The roof upstand of e.g. roof lights, ventilation shafts and other roof protrusions must be at least 50 mm higher than the roof edge or the emergency overflow. The waterproofing membrane must be brought up to a minimum of 150 mm above service level.
Around roof protrusions with or without a roof upstand, a clear strip of clean gravel or concrete slabs should be installed for maintenance and inspection purposes. To prevent the growing medium being washed into the clear strip, ab Edge Retaining Profile should be installed. The minimum recommended width of this clear strip is 300 mm.
Roof outlet
Where a roof outlet is positioned within a vegetated area, an inspection chamber complete with access cover is placed over the outlet to protect it from plant growth and impurities. The inspection chamber should not affect or impede drainage efficiency and must be accessible at all times.
A clear strip of clean gravel (min. 16 – 32 mm) should be placed around the inspection chamber. The recommended width of this clear strip should be a minimum of 300 mm. To prevent the growing medium being washed into the clear strip, an Edge Retaining Profile should be installed.
Roof outlets lying outside the vegetation area are normally positioned within a gravelled area, with a gravel guard covering the outlet. When a roof outlet is located within a hard-landscaped area, an inspection chamber fitted with a suitable grating should be placed over the outlet.
To be continued.
Design of falls – roof slope
Designing for stormwater
Fall protection
Fire prevention
Wind loads
Protection from emissions
Design features
Protection from calcium deposits
Maintenance
The composition of an extensive green roof
Composition
Root barrier layer
Separation and slip layer
Drainage layer
Filter layer
Drainage Systems
Water retention layer
Growing medium layer
Vegetation layer
Intensive green roofs combined with hard landscaping
Intensive Green Roof System
Roof with sufficient fall 1 in 80 to 5 °
Roof with insufficient fall < 1 in 80