Roof garden and podium explainer and FAQ

A landscaped podium is a communal outdoor space, serving adjacent buildings, often appearing to be at ground-level, but constructed above a building e.g. carpark, a residential or commercial properties, rather than earth.

• • They are sometimes referred to as a raised courtyard or a courtyard podium.
  • Podiums may include paving, decking, seating, flower beds or raised planters and plants.
  • Podiums are usually intensively planted with a mix of plants that may include trees, shrubs, herbaceous and flowering plants. Sometimes they include lawns, wild-flower meadows and water features.
  • A well-known example of a podium is the award-winning Barbican Beech Gardens, in London.

•  Compare with: rooftop garden.

A rooftop garden or roof garden is landscaped, recreational green space on the top floor of a residential or commercial building.

• Rooftop gardens may include paving, decking, seating, flower beds or raised planters and plants.
• Intensively planted roof gardens may include trees, shrubs, herbaceous plants and flowers and lawns or wild-flower meadows.
• Extensively planted roof gardens include low maintenance plants such as sedum.

Compare with: landscaped podium.

Landscaped podiums and rooftop gardens – usually include the following components:

• Load-bearing roof structure
• Waterproofing
• Insulation
• The drainage and water retention layer (podiums and rooftop gardens often include blue roof or sustainable drainage system (SuDS))
• Paving – often pedestal paving or raised paving – and or decking
• Planters or flower beds, including:
  • Waterproofing/root barrier
  • The substrate or growing medium
  • Vegetation.

Paving slabs are lain on top of raised pedestals or supports, rather than a base of sand and cement.

Pritchard & Pritchard recommends pedestal paving for podiums, rooftop gardens, terraces and balconies, because of the following benefits:

• It reduces the total weight of the paving
• Improved drainage
• Better compatibility with blue roof systems
• Benefits to heat dissipation due to airflow
• Speed of install
• Easier to get a perfect level (even if the roof has uneven surfaces)
• Increases future flexibility (as the slabs can be easily lifted, replaced or removed)

A blue roof incorporates a sustainable drainage system (SuDS), which retains water following a downpour, to prevent the damaging run off and manages how the water is slowly dispersed.

Blue roofs are perfectly compatible with hard landscaping, especially if pedestal paving is used.

BREEAM (Building Research Establishment Environmental Assessment Method) state that living roofs, including podiums and rooftop gardens, can deliver the following benefits:

• Encourage people to go outside and engage in outdoor activities
• Reduce in the urban heat island effect, as plants absorb heat
• Improve air quality, through plant respiration
• Improve the city’s roofscape and reduce the visual impact of development
• Reduce surface water runoff and can act as a natural buffer helping to prevent flash flooding. See: Blue Roof
• Reduce noise travelling in or out through a building’s roof
• Reduce a building’s energy consumption by lowering the cost of heating in winter and cooling in summer
• Reduce the urban heat island effect by absorbing heat into the plants
• Provide habitats for wildlife and improve biodiversity.

SEE: How do living roofs reduce energy costs of a building?

According to BREEAM (Building Research Establishment Environmental Assessment Method):

• Living roofs “can help to reduce demands for artificial heating and cooling by contributing to insulation performance and reducing demands through solar gain and wind generated heat loss. This both helps to reduce energy demand and its associated costs as well as conveying resilience to future climate change in a visible manner, thus assisting in demonstrating corporate social responsibility”.

Studies:

• Research has found that living roofs can reduce heat flow through building roofs in summer can be reduced by approximately 80% via green roofs.
• Research by Nottingham Trent University found that a living roof kept a building 4.5°C warmer in winter and 15°C cooler in summer than a conventional roof.

The German FLL Guidelines state that roofs with a high degree of structural diversity, e.g. intensive, extensive green with raised areas and woody vegetation have the ‘highest faunistic diversity’.

Various studies have found that living roofs, particularly those with deeper substrate and a mix of substrates, have had a beneficial impact:

• More diverse vegetation provides foraging resources for urban bees.
• The creation of mounds provides refuge for invertebrates from extreme temperatures.
• Careful planning of substrate mix improves invertebrate species richness e.g., spiders, beetles, sometimes rare species.
• The provision of habitat resources, like perches, food and water, can accommodate the breeding needs of ground-nesting birds.

The construction and planning guidelines of many countries and cities explicitly require provision for green roofs, walls, roof terraces and sustainable drainage systems (SuDS). Some places have gone further, introducing incentives and enshrining urban greening into law.

The direction of travel in the UK is clear from the guidance for developers and planners in London.

Sustainable Design and Construction SPG provisions (2014) for London statesin APPENDIX 4, 6.4.3:

“New development should incorporate Sustainable Urban Drainage Systems and green roofs where practical with the aim of achieving a Greenfield run-off rate, increasing bio-diversity and improving water quality. Surface water run-off is to be managed as close to source as possible.”

Living Roofs and Walls Technical Report: Supporting London Plan Policy (2008) recommended the following planning policies:

Major developments where feasible should incorporate living roofs with the following objectives:

• Accessible roof space
• Adapting to and mitigating climate change
• Sustainable urban drainage
• Enhancing biodiversity
• Improved appearance.

The report also made the following recommendations:

• A minimum of 70 per cent of the roof space should be vegetated to provide maximum benefit for SuDS, building energy performance and biodiversity.
• At least 25 per cent of the total roof space in any one development should be accessible to residents and/or workers.

Like any ground-level garden, if left alone, nature will soon take over. Different type of podiums and roof gardens require different maintenance regimes. Pritchard & Pritchard recommends the following maintenance plans:

• Roofs and podiums with low maintenance extensive or sedum planting require four visits to inspect and tend per year.
• Roof-top gardens and podiums with intensive or mixed planting require monthly visits to inspect and tend.
• Roof-top gardens and podiums with lawns require fortnightly visits to inspect and tend.

Maintenance regimes for living roofs require the following tasks (according to German FLL Guidelines):

• Inspection of the paving to ensure it remain safe and free from hazards and detritus and litter
• Inspection of drainage system and inspection shafts
• Inspection and functional testing of irrigation systems
• Removing unwanted foreign vegetation
• Fertilizing and feeding
• Pruning and cutting back plants
• Repeat planting or seeding
• Pest control
• Refilling of substrate in cases of erosion
• Mowing, scarifying; aerating; sanding of lawns
• Mowing wild-flower meadows

See: Roof-top garden maintenance

A properly installed, irrigated and maintained living roof should last for over 40 years. There are living roofs in Germany built using same light-weight type of system used today (pioneered in Germany) that date back to the 1970s, for example Pliensau cemetery, near Stuttgart.

Arguably, the world’s oldest roof garden is on top of 400+ year-old medieval fortified tower house Torre Guinigi in Lucca, Italy.

Estate agents Oliver Burns estimate that roof terraces increase a property’s value by 10-30%, while Marsh & Parsons estimates buyers are prepared to pay 12% more for a property with outside space.

Every installation is bespoke. The cost calculation takes into account the following factors:

• Expected use of roof and footfall
• Roof size
• Elevation, access and requirement of crane
• Location
• The load baring capacity of the roof – is it sufficient to support planned use of roof; intensive planting etc.
• Proportion of roof covered by paving or deck versus planters or lawn
• Planting mix
• Maintenance package

See: Roof-top garden maintenance.