Roof, terrace & podium gardens

How does Roof, terrace & podium gardens work, step by step?

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   Step 1: Fix the structural allowance and the loading strategy first

   The landscape design starts from what the structure will carry. The engineer states the allowance for the deck, and the design team works within it using the saturated weight of every layer, the mature weight of the planting, and the imposed load of people and maintenance plant. Where deeper soil zones, large trees, water features or heavy planters are wanted, they are positioned over the structural elements that can take the load rather than placed for effect and checked afterwards. On a refurbishment, the existing structure is surveyed and assessed before anything is promised. This exchange between the landscape architect and the structural engineer is the single most important conversation on the project, and it happens at concept stage, not at construction.

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   Step 2: Get the waterproofing right and prove it

   The waterproofing system is the element that protects everything below it, and once the garden is built the membrane is inaccessible. The system, the falls of the deck, the upstands at every parapet, wall and threshold, and the detailing at every outlet and penetration are designed as a whole. Installation is inspected and the completed membrane is integrity tested before anything is laid over it - flood testing or electronic leak detection are both common, and the test is witnessed and recorded. Any subsequent trade that works on the deck works to a protocol that keeps loads, wheels and dropped tools off the membrane.

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   Step 3: Install the root barrier and the protection layer

   Roots seek out water and will exploit any joint or weakness in a membrane, so a root-resistant barrier is provided either as an inherent property of the waterproofing system or as a separate sheet laid over it, with laps and upstands detailed and sealed to the same standard as the waterproofing itself. A protection layer then goes over the barrier to take the mechanical punishment of construction traffic, drainage boards, plant and the growing medium - typically a heavy protection sheet or board. Skimping on the protection layer is a false economy: most membrane damage on planted roofs is caused during the construction of the garden above it, not by the plants.

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   Step 4: Build the drainage and reservoir layer

   Water has to be able to leave the build-up freely and reach the roof outlets, or the growing medium becomes waterlogged, heavier than designed and hostile to roots. A drainage layer - commonly moulded drainage and reservoir boards, or a granular layer where the weight allowance permits - is laid over the protection layer and connected to outlets that remain accessible for inspection through purpose-made chambers. Many board systems also hold a shallow reservoir of water in their profile, which the medium above draws on between irrigation cycles. Outlets, overflows and the route water takes across the roof are set out with the designer and are not obstructed by planters, paving or edging.

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   Step 5: Lay the filter layer and place the growing medium

   A geotextile filter layer separates the growing medium from the drainage layer and stops fine particles migrating down and blinding it. The growing medium is then placed over it. It is a specified engineered mix rather than topsoil - typically lightweight mineral aggregate blended with a controlled proportion of organic matter, designed to hold enough water and nutrient for the planting while staying inside the weight allowance and draining freely. Depth follows the planting: shallow across extensive areas, deeper in the zones intended for shrubs and trees, with the transitions designed rather than improvised. Medium is placed without tracking heavy plant across the build-up, which usually means conveyor, blower or crane and barrow.

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   Step 6: Plan the access route for materials and, later, for maintenance

   Nothing walks onto a roof garden. Growing medium, trees, planters, paving and furniture all arrive by crane, hoist, conveyor or blower, and the route, the lift positions, the crane time and the working hours are planned as a construction operation in their own right - frequently the operation that determines when the garden can be built at all. The same question is then asked about the next twenty years: how does a maintenance team get tools, replacement plants, medium and green waste on and off the roof, and where is the water supply and the power for them. Access, safe edge protection or anchor points, and a route that does not run through a private apartment are designed in, not left to whoever inherits the building.

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   Step 7: Design for wind and choose planting that will take it

   Exposure at roof level is the reason many rooftop planting schemes fail. Wind speeds are higher than at ground level and are funnelled and accelerated by the building form, drying foliage and medium and rocking anything tall. The response is a combination of shelter and selection: windbreaks, screens and parapet treatments to reduce exposure where they can, and species chosen for wind and drought tolerance, with tough low planting used in the most exposed positions. Trees and large shrubs are anchored, commonly using below-medium root anchoring or ballasted systems rather than above-ground stakes, because the medium alone will not hold them. The wind study for the specific building governs, and it is often produced for the facade design and then read across.

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   Step 8: Irrigate, commission and hand over the maintenance regime

   A roof garden has no groundwater and a shallow, free-draining medium, so an automatic irrigation system is provided on all but the most minimal extensive schemes. It is designed with the planting, zoned by planting type and by exposure, run in drip or sub-surface form to limit evaporation and wind drift, fed from a supply and a controller that form part of the building services, and detailed so that pipework does not compromise the waterproofing. Commissioning covers every zone, the outlets and overflows are proved clear, and the as-built build-up is recorded. Handover to the building manager includes the drainage layout, the irrigation zone plan and controller settings, the inspection frequency for outlets and anchors, and the maintenance access arrangements.