The town had never seen one, the architect had never built one, the budget was as tight as a tourniquet, there was almost no time to plan it, and everything depended on the specification. We tell the story of how a primary school in Kent got itself a curved living roof
When Architects Design Partnership won the competition for an infant school in Sevenoaks, Kent, there were a few raised eyebrows among the locals. New buildings in this commuter town veer towards the vernacular of brick, weatherboarding and wall-hung tiles. The winning design was for a curved, ground-hugging building with a sweeping, grass-covered eco-roof – unlike anything else that had ever been built in the town. It was also the first roof of its type that ADP had tackled – and with the new school year looming, the project team had next to no time to tackle the specification issues.

Roger Fitzgerald, a partner in ADP, says that there was a good reason why a traditional building would not have been appropriate for Riverhead School. "It's in a landscape setting, not a brick-and-tile setting," he says. "The green roof of the building goes hand-in-hand with the sweep of the parkland."

Kent council was impressed with Fitzgerald's reasoning. The site was on green-belt land, and it was agreed that a gently arched roof would be less obtrusive than a traditionally pitched one. Also, a pitched roof wasn't suitable as the team had decided to restrict the building to a single storey. "We wanted to keep the scale of the building down to child level," says Fitzgerald. He adds that the deep-plan design of the building would have meant that a pitched roof would, in any case, have been too high.

After winning the council and planners over with its low-slung, environment-friendly concept, ADP then had to work out the specification and detailed design of the unusually shaped roof. "The curve of the roof meant there were many issues concerning the junctions and detailing," says project architect Alison Golding. Not to mention the challenge of specifying a living roof. On top of this, the architect had to work closely with main contractor Buxton to keep to a tight budget.

The curved shape of the building is defined by 52 m long hollow-steel arches; these form the superstructure of the school. They are 7 m apart and planted at each end in large blocks of concrete. Macalloy bars tie the ends of each arch together underneath the school.

The deck of the roof is formed from 15 m long sheets of Plannja metal fixed to the steel arches, and on top of these sits the build-up for the green roof (see factfile 'Dramatic build-up').

The flat roofs of the main hall and entrance area punctuate the curved plane of the green roof (their rectangular and triangular designs were inspired by a school project on geometric shapes that Fitzgerald's son had been asked to do).

Vegetation's what you need
ADP originally wanted a turf roof, because it would have blended into the surrounding parkland. But after researching the problem, the architect discovered that grass had drawbacks. "The problem," says Fitzgerald regretfully, "is that it requires 30 cm of soil. This would have implications for the structural design of the building.

It would need larger, heavier structural steel arches, deeper foundations and heavier profiled metal, which all adds to the cost of the building."

To avoid this, ADP specified a sedum roof, which requires a 10 cm bed of soil. Sedum, a kind of rock plant, is also easier to maintain, being wind-, frost- and draught-resistant. Even better, it doesn't have to be watered, which means that maintenance costs are minimal.

After evaluating the proposals of a number of suppliers of eco-roofs, ADP chose a system from Erisco-Bauder, which has provided 1000 such roofs in the UK over the past 20 years. "Erisco-Bauder has a good reputation and lots of experience," says Fitzgerald. "We chose it partly because it grows its sedum in East Anglia. We wanted to get it from where it's grown to the site as quickly as possible to keep it in good condition."

Richard Kulakowski, regional manager at Erisco-Bauder, thinks that ADP was drawn to its system because the company provided every element of the roof build-up, and offered on-site backup and a 20-year guarantee. "We will maintain the roof until it is fully established," he explains. "A roof will usually acclimatise over 12 to 15 months.
After that, we will supply the client with a maintenance schedule." Kulakowski reckons the roof only needs inspecting twice a year: at the onset of spring and winter, when flashing, outlets and gutters need to be checked. In winter, leaves also have to be removed to prevent the sedum suffocating under mulch.

During its research, ADP realised there were other benefits that helped offset the extra cost of its sedum system. Eco-roofs improve acoustic roof performance and the mass of the soil provides additional thermal insulation. The soil also helps to protect the waterproofing system by reducing thermal and structural movement and preventing UV light from prematurely ageing the roof membrane, making the roof less likely to fail. The roof also helps the environment by providing a habitat for insects and birds.

Although the arc of the roof caused design headaches, it actually made specification simpler. Sedum roofs require at least a 1º slope to allow drainage to take place. If water accumulates on a sedum roof there is a danger that grass seeds would establish themselves and dominate areas of sedum – and when the water evaporates, the grass would die, leaving unsightly patches. What's more, the symmetry of the arc caused the weight of the sedum blanket to counterbalance itself on each side, which meant that few retention grips were required.

Where the curve of the roof did cause specification problems was in the interfaces with rooflights, the steel arches, partitions and I-beams. Even before the designs were detailed, ADP had trouble finding a manufacturer that could supply the circular rooflights required. "You think it would be easy to procure round rooflights with today's modern buildings, but we had difficulty," says Fitzgerald. "In the end we found Rooflight Systems through a journal, which had pictures of some unusually shaped rooflights."

TA Colbourne was the approved subcontractor for the Plannja P200 metal roof deck. Although contracts manager Tim Bench said that once the design details were drawn up, the roof was "relatively straightforward", roofing specialist Mat Tribbeck recalls there were headaches on site. He says that around the arches and rooflights, the metal deck had to be inched into place.

Minds in the gutter
"If you're building a one-off design you need to work as a team, as you don't know what you're going to come across," says Philip Mumford, construction director at main contractor Buxton, which chaired all the meetings between ADP and the contractors. "The entire process required a lot of co-ordination."

One of the biggest design challenges for the project team, according to Mumford, was the guttering. This involved supporting a metal trough full of stones across a 7 m span without it sagging in the middle (see factfile and graphic 'Dramatic build-up').

ADP's Golding says the teams of contractors and suppliers were excited about coming up with new details. "Nobody knew how we were going to it. It was a really enjoyable project as we had to actually think about how the materials related to the objects around them."

Wherever possible, the project team looked to save costs to make sure the school came within its £2.1m budget. An off-the-shelf brise-soleil shading the classrooms was going to be specified, but after looking at the costs, Clare Appleby, the project architect, designed her own using metal poles. Once Appleby had worked out the angle of the sun, structural engineer Anthony Ward Partnership drew up the specification. The clusters of chains to allow water to drain from the roof gutters were also designed by Appleby, after she saw how costly proprietary systems were. She had to work out how many chains to use and how to bundle them to ensure that the drips fell down the chains.

To ensure the specification of the building met Kent council's brief for an environment-friendly school, ADP used BRE to carry out a green audit with the help of an Arup engineer. "The audit helps to keep you on your toes," says Fitzgerald. "Knowing it's coming forces you to justify your design decisions as you go along." As a result of the audit the brise-soleil was installed and recycled materials were used on site, including local timber and glass.

For Fitzgerald, one of the most pleasing aspects of the curved roof is that even a five-year-old who's knee-high to a grasshopper can see the sedum from the ground – the common danger of specifying flat eco-roofs is that they are only ever seen by helicopter pilots and maintenance engineers. Thanks to ADP's curvy top, Riverhead School will be able to spread the green message for generations to come.

Grass isn’t always greener …

The sedum roof at Riverhead School is an example of an extensive eco-roof. These are constructed using vegetation such as sedum, succulents, indigenous herbs and turf. The grass roof rejected by the architect is an intensive system, which can include many types of plants and shrubs and are usually planted for recreational as well as aesthetic reasons. As well as requiring more soil, intensive roofs require more water. To retain water, Erisco-Bauder uses a layer of pocketed polystyrene or polyethylene that can retain 22 litres of water per m2. This allows soil depth to be greatly reduced. The exact soil depth and amount of water needed is determined by the type and mix of plants. The roof at Riverhead consists of eight types of sedum. As ADP found out during its investigations into green roofs, using grass means the building would have to be specified with a stronger structure. One way that specifiers could reduce the load would be to specify less soil but increase the amount of watering by installing specialist irrigation systems. These range in expense from “leaky pipe” systems, which are perforated hoses, to more expensive pop-up sprinklers. These are activated by timers or sensors in the soil that switch the system on when its moisture content falls below a certain level.