The word ’Green’ frequently gets attached to other terms, usually to imply some sort of environmental-friendliness credentials. But in the case of green roofing, it is not only asserting environmentally positive credentials but describes a roof which is literally green – albeit often with sprinklings of many other colours of nature.
What is a Green Roof?
Let’s start with a green roof definition; A living roof or green roof is a roof onto which vegetation is intentionally grown or habitats for wildlife are established.
Green roofs come in three principal formats, commonly known as ‘extensive’, ‘intensive’ and ‘bio-diverse’ (confusingly often referred to as ‘brown roofs’!)
Types of Green Roofs
Extensive roofs are the simplest and most economical variation. They are generally formed from a layer of sedum matting (little plants pre-grown on a matt and delivered to site in a roll, a bit like rolls of turf) laid over a thin layer of a growing medium. The growing medium isn’t soil, but a designed mix of crushed brick, lytag and organic matter. For an extensive green roof, this will be in the region of 60-90 mm deep and will weigh, wet, about 90-100kg/m2. Below this, there should also be a drainage layer of some description to remove excess rainwater from the roof and prevent the vegetation from drowning.
Intensive green roofs are the area where an architect or landscape designer’s imagination can really be creative. Intensive roofs can be built to almost any depth to accommodate ranges of planting from small wildflowers through shrubs and bushes, up to small trees. They may also incorporate water features, hard landscaping, and a variety of recreational amenities.
Bio-diverse roofs – often referred to as brown roofs – tend to be a more ecological feature as they will be designed to form habitats for birds and insects. The growing medium will be left uneven and may contain large stone, brick rubble and logs or timbers to allow the creation of wildlife habitats, and the substrate will be seeded to allow it to grow naturally on the roof. In some cases, the roof covering will not even be seeded but left bare for the roof environment to develop naturally.
So having identified the main types of a green roof, what are the main design considerations to be taken into account?
Benefits of a Green Roof?
The uses and benefits of a green roof, usually a flat one but pitched green roof designs work too, covered with natural vegetation of some description are well documented – recreational amenities, visual aesthetics, carbon capture, air-cleansing, local cooling, rainwater attenuation, and ecological effects being the main ones. There may also be green roof incentives such as enhancing a building scheme such that it assists with obtaining Local Planning consent. However, there is often confusion about how the different types of green roof and how they should be constructed and maintained. So in this month’s blog post let’s take a look at the basic types of green roof and their component parts, in other words – the green roof design.
Green Roof Design
The design will usually begin with the required appearance or amenities. If the planting is to be substantial requiring deep layers of growing medium or includes substantial items such as water features or hard landscaping, then the roof structure itself needs to be capable of supporting the imposed weight. The structural design of the roof and its support will therefore have to be considered accordingly.
The growing medium for intensive roofs needs to be carefully designed to suit the type of planting being proposed, in terms of depth and grading, and the drainage layer will need to have sufficient flow capacity to drain larger quantities of water from the correspondingly greater volume of growing medium substrate.
Filtration and protection membranes will also need to be incorporated to prevent fine particles from migrating into the drainage layer, and provide protection against the actions of future maintenance and tools.
The Waterproofing System
However, the most critical component of the green roof covering, as far as the building itself is concerned, will be the waterproofing system. Once constructed and covered over, the roof waterproofing will be practically inaccessible for maintenance so needs to be designed, installed and protected effectively to provide a reliable trouble-free environment both above and below the roof.
Designers are always looking for the ‘best’ waterproofing system for flat roofs, but in reality, there is no best system. Most of the modern flat roof covering types when correctly installed to a suitable specification will provide the required security. There are a few provisos to this though in relation to green roofs in particular.
A waterproofing system for a green roof needs to have accreditation for that purpose. A testing process where the waterproofing is placed in a planter containing soil and some fast-growing plant roots for a period of time proves the system resilient to root penetration. In the UK a BBA certificate indicating suitability for green roofs is usually accepted as the required accreditation.
Green Roof Installation
Having selected a waterproofing product with the right qualification it is important to make sure it will be installed by skilled operatives. The installing company should be approved by the roofing system manufacturer, and the installing operatives should also have been trained and approved by the manufacturer’s technical team. Ideally, the manufacturer will have provided a specification indicating the component products required and the method of waterproofing specific details. Deviation from this specification is likely to invalidate any guarantee offer from the manufacturer.
Another common requirement for green roofs, promoted by the NHBC and others, is that the complete system from the deck up, of waterproofing, protection layers, insulations and green roof components should all be supplied and installed by a single contractor. In this way, there are no split liabilities regarding product supply and installation, and no friction between different trades working on different parts of the installation. There will be a single point of contact should there be any issues with any part of the green roof covering. Procurement in this way will also reduce the green roof cost due to not having a mixture of different suppliers and installers.
So, rather than asking what is the best roofing system, a designer could look for a supplier who can provide the complete package of green roof design advice, accredited products, manufacturer-approved waterproofing installation and green roof build-up, including hard-landscaping if required.
AAC waterproofing is one such supplier. We have approved installers for a variety of flat roof waterproofing systems including single-ply membranes, EPDM pre-fabricated sheeting and liquid applied waterproofing systems (we only offer cold-applied systems to avoid the pit-falls of hot-works).
We can also advise on and supply green roof components which we will install in accordance with the architect’s vision.
In summary, when it comes to green roofing, we are the complete package.