Following advances in roof-deck technology over the last decade, and the need to provide an answer to the ever-present problem of energy efficiency in buildings, we have seen the resurgence of the roof garden, not only in private residences but also in the public realm
Increasingly, as the value of land for development spirals upward, landscape exists as a thin membrane draped across the architectural fabric of our cities. Spaces that we frequently perceive as resting effortlessly on mother earth or nestling lazily between buildings are often the roofscapes of car parks, leisure centres, offices or stations.
Whereas once the emphasis of roof gardens was to create a simple green carpet, today's roof gardens employ high technology to create spaces that work every bit as hard as the architecture around them to bring added value to urban schemes.
Historically, roof gardens have been flights of fancy, and an opportunity to enact fantasy amid the familiarity of the built environment.
The roof garden built at Derry and Toms in 1931 included a series of interrelated gardens with exotic themes such as the Moorish and Elizabethan gardens. In the late 1950s, landscape architect Dan Kiley designed a roof garden over a department store in Oakland, California, which included olive trees set into flowing mounds. In 1976, Sylvia Crowe designed a roof promenade for recreational use by the employees of Scottish Widows.
Roof gardens are expensive commodities, and are hard to justify when they provide a purely private or aesthetic function.
The re-emergence of roof-deck technology came about not as a response to a visual need or an aesthetic consideration, but as an answer to providing energy efficiency in buildings and as a way of reducing storm water runoff in urban environments. In Berlin, for example, stipulations that new buildings could only discharge 20 per cent of storm water into sewer systems meant that the remaining water had to be dealt with on site either by evapo-transpiration through extensive planting, by infiltration systems or through use in irrigation or WC flushing systems.
Although such regulations led to a surge in roof-deck technology, the gardens created were a myopic solution, which resulted in flat carpets of succulent vegetation. These plants, although able to cope with the extremes of drought, temperature and desiccation experienced on exposed sites, offered little contribution to the landscape value of roof-deck spaces.
But advances in waterproofing and drainage technology, combined with the development of specialist growing media, meant that a normal soil profile could be compressed into a much shallower depth, and stay within the loading tolerances of modern building technology.
During the past decade we have begun to see roof gardens emerge as part of the public realm. They aspire to be places in their own right, rather than just the icing on the concrete cake. It is more appropriate to refer to these new spaces as perched landscapes, because they are more likely to be found connecting the fabric of urban platforms than sitting isolated on top of buildings.
The pressure on the designer has always been to create landscapes which look and feel natural on the surface so that the public is unaware that the trees that they see are suspended many metres above the ground level. The key to achieving this has been to recreate natural growing conditions for all types of plants in a completely artificial environment, and to hide all of the drainage and irrigation gubbins below the surface.
Construction techniques will vary according to the constraints of the site and the intensity of its usage. Important decisions have to be made early on in the design process about the end use of the roof garden, and its overall functionality. In the design of the public realm for Paddington Central, the developer and the architect had considered from inception that the spaces should be fully integrated and mesh with the existing urban fabric.
Although the brief for this site led to an intense development over an extensive area, many roofscapes are intended as purely visual pieces, to be viewed from spaces surrounding or above them. Good examples of this include the roof garden for the German Institute for Standardisation (DIN) in Berlin, or the roof gardens of Topher Delaney.
Whether planting is to be integrated in the slab construction to create the effect of being on ground level, or built into raised planters, the key elements of any roof garden profile will be the same, consisting of:
linsulation and/or void-filling layer;
ldrainage layer; and lgrowing media.
Waterproofing membrane It is vital that the waterproofing membrane is procured from a reputable firm that can supply, install and guarantee the works for a minimum of 10 years. Companies such as Erisco Bauder supply complete systems including insulation layers and an extensive planting mat, which simplifies the process of procurement. However, where more elaborate construction is required, it is often better to separate the waterproofing works from the landscape construction due to the specialist nature of each operation.
Most commonly the waterproofing membrane is a bitumen mastic layer, laid over the concrete slab with or without a screed layer to falls. Systems which do not use a screed to direct water to drainage outlets rely more heavily on a drainage layer to act as a buffer between the slab and the growing medium, and place a greater strain on the integrity of the waterproofing membrane. In areas to be planted, the waterproofing membrane will include a root-protection barrier, which helps to prevent root penetration of the waterproof membrane.
The most common point for the failure of waterproofing membranes occurs where there are expansion joints in slabs, or roofdeck penetrations. Therefore, the design of services and levels changes should be considered carefully at an early stage.
Insulation or void-filling layer Most often the spaces beneath the slab will offer some kind of accommodation and will require insulation. Even car-parking areas will often require insulation to the roof-deck slab to prevent condensation and dripping on to vehicles below. In areas where differential soil depths are required, or where there is a transition from a planted to a paved surface finish, it is common for inert void-filling material to be used to make up levels and reduce the loadings associated with saturated growing media. Load-bearing void fillers, such as lightweight concrete aggregate, or high-density polystyrene materials, such as Filcor by Cordek, are often used where the finished surface will take pedestrian or vehicular traffic. Filcor can be specified with different densities to take various types of traffic, and the material can be cut to size or even sculpted to create flowing contours or profiles beneath the topsoil layers.
Where there is no requirement for profiling, or the soiling zone is well contained, loose void-filling material such as Hydroleca or Optiroc, a lightweight expanded clay aggregate, can be used. It is important that such materials are not used in areas that will receive vehicular traffic as they will tend to break down and subside, and it is important to wrap loose void-filling materials in a geotextile filter membrane to prevent loose fines entering the drainage system. In planted areas, leca provides a cheaper alternative to polystyrene or block void fillers, with the added advantage that it provides an expanded drainage buffer in the sub-soil profile.
Drainage layer In many ways the drainage layer is the most critical element of any roof-garden system.
The most common cause of plant failures on roof gardens is waterlogging due to failure of the drainage system. Therefore, it is not surprising that a multitude of drainage systems have evolved. Some specialist systems combine water-retention reservoirs, a drainagelayer filtration membrane and a rootprotection membrane in one easily rolledout mat. This may improve the speed of laying, and offer the benefit of easy handling on roof decks with difficult access, but care must be taken to avoid over-specifying where elements such as the root barrier have already been procured in the waterproofing package. Where soil depth is limited, drainage mats can reduce the space required for the drainage layer to as little as 20cm.
Products such as Floradrain and Enkamat provide a drainage layer which is sandwiched between two geotextile filter membranes.
This solution works quite well where the slab is quite even or is laid to drainage falls. In areas where the slab level is inconsistent or the depth allowance is more generous, a drainage layer of lightweight expanded-clay aggregate (Hydroleca or Optiroc) provides a drainage sump, which ensures that the growing medium does not become saturated. It is important to ensure that such drainage layers are wrapped in a geotextile filter membrane of not less than 100g/m 2, such as the Terram 1000 Pr used at Paddington Central, to ensure that fines are not washed through into the drainage outlets.
It is also important that individual planting cells have at least one inspection tube that allows access to the drainage outlet. This allows manual inspection to establish whether the drainage layer is functioning correctly, and may provide early warning signs of a blockage in the drainage outlet. The inspection tube also provides the possibility to flush through or suck out any blockages at the point of discharge from the planting pit.
Growing media The specification of soils and growing media is in itself a subject worthy of much attention, and is currently under review by the Landscape Institute. In order to sustain plant life, a good topsoil must be free from pests, disease, noxious or invasive weeds and harmful quantities of toxic substances. It must have a tilth or structure, which allows sufficient drainage and air-filled porosity for the gaseous exchanges that occur around plant roots, and sufficient organic content for the retention of important trace elements and nutrients.
However, saturated topsoil is very heavy and, even at depths of 400mm, can impart a loading factor of 8-9kN/m 2. On roof decks where loadings are critical, this can be nearly halved by the importation of lightweight topsoil. Normal topsoil will contain up to 20 per cent dry weight of stones ranging from 2-5mm in diameter. Lightweight topsoil is manufactured by substituting the stone content, which is necessary for good drainage and air-filled porosity, with inert and lightweight particles of the same size such as perlite or vermiculite. This does not affect the performance of the soil, but significantly reduces the saturated weight of the soil.
Such a specification is currently being used to allow tree planting on a new roof garden for the old Harrods Depository in Knightsbridge, west London. The use of expanded clay aggregate as a void filler enables trees and shrubs to be planted side by side with a minimum of imported topsoil, sufficiently reducing the load factor to enable intensive planting in an area which would otherwise only be capable of supporting lawn or succulents. A firm that specialises in the supply of lightweight soils is Boughton Loams. Other specialist soils, such as tree sand, allow the growing media to be compacted without compromising the drainage or water-holding capacity of the soil. These manufactured soils enable trees to survive in situations where traffic and paving cause compaction of the root zone.
Various companies now manufacture tree sand in the UK under different trade names such as Urban Tree Soil, Metro-Sand, and Heicom Tree Sand. Tree sand is a proprietary product and as such should come with a performance guarantee. Metro-Sand provides a guideline on the volume of tree soil required for each tree, which is:
(D esir ed C r o wn D iamet er) 2= Volume f 4 Metro-Sand (m 3) Soil depths required by plants vary according to climate, the type of plant and the extent to which they will receive artificial irrigation.
That said, the growth of a plant can be severely stunted or adversely affected unless certain minimum depths are maintained. The diagram provides a recommendation for minimum soil depths (and tree pit widths) for various plant types, assuming that an automatic irrigation system is present.
Derek Lovejoy Partnership has been involved in the design and implementation of roof gardens for more than 40 years. During that time there has been a steady movement of roof gardens from private or corporate ownership into the public realm. Increasingly they have an important role to play in the creation of memorable urban spaces that engage the community and bind our multi-layered cities together. As development of roofgarden technology leaps forward, becomes cheaper, and fuses with outstanding product design, there will be a great difficulty in telling those landscapes which are on the ground from those which have been built on the deck; and the sky will no longer be the limit.
Peter Wilder is an associate with Derek Lovejoy Partnership
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