How do you install a green roof on this?

Stepping out cautiously onto the sloping roof of the new University Campus Suffolk (UCS) building in Ipswich you become conscious of a few things. First, it’s very steep. It’s also very high and really quite windy. And the current surface is slippery insulation foam.

In a few weeks the 1,400m2 roof will be covered in sedum to create a sustainable feature that has helped the building achieve a BREEAM ‘excellent’ rating. But standing at the apex of this slope an important question presents itself. Green roofs are heavy and made of loose soil and organic material, so how do you install one at a 20-degree pitch and keep it there once it’s laid?

Located on a compact site between the Ipswich Waterfront and a main road, UCS is Suffolk’s first ever university building, funded through a collaboration between the University of East Anglia and the University of Essex. With a £20m build cost it will provide a self-contained hub for the Ipswich campus, to include teaching and administration facilities, lecture theatres, plus facilities for local residents including exhibition spaces and a cafe. It opens this September.

The building’s distinctive sloping roof was designed as a direct response to the location, as Chris Davis of architect RMJM explains:

‘We tried to maximise use of the site. Height wasn’t a problem on the waterfront facade where there are already several tall neighbouring buildings, so we went up to six storeys there. But we had to taper it to two storeys on the rear street-facing facade where there are two-storey listed buildings. The result is a highly visible sloping roof – some people call it the “cheese wedge” – and we decided to make it green to boost our environmental credentials.’

Bauder was the obvious choice to supply the roof. With a warehouse literally around the corner and a sedum-growing farm just 25 miles away near Cambridge, it was very convenient. RMJM also wanted a complete roofing solution, which Bauder could provide.

‘We discussed a number of green roofing options with the client,’ says Bauder technical manager Rob Woolston. ‘They eventually went for the sedum blanket which, unlike substrate, can be restrained and held in position. It’s a solution we recommend for roofs with pitches of up to 30 degrees. Substrate requires drainage boards, filter sheets and other elements, virtually all of which have to be secured individually.’

A thick, fleshy, cactus-like plant, sedum is light and can withstand extreme hot and cold temperatures. It’s grown in huge lengths of blanket, which are stitched into a polythene fleece underlayer before being lowered into place. At UCS the sedum will attract wildlife and help reduce rainwater through absorption.

The roofing installation is being supervised by Adrian Humphries, director of Accurate Roofing. He says safety was a ‘big priority’.

‘We carried out risk assessments and decided to recommend using roof ladders, mainly because we were expecting frost. The frost didn’t come, however, so we haven’t needed the ladders, but we have had level platforms put in to store materials safely.’

Roof construction will be completed in sections over a 20-week period. A vapour barrier is first laid on the concrete deck, with 200mm of polyisocyanurate flatboard insulation glued on top (see diagram, right). To prevent the insulation from slipping, several continuous timber supports have been fixed to the roofing deck at 5m intervals. A waterproofing underlayer and cap sheet will then be mechanically fixed to the timber at the 5m centres, with a 100mm overlap ensuring that the fixings are adequately covered.

With a U value of just 0.13, the roof at UCS easily exceeds building regulations for insulation. ‘It’s what any responsible architect should be doing now, as any extra money spent on insulation can quickly be recouped during the building’s operation,’ says Davis.

During the final week of work the huge rolls of the sedum blanket will be lowered by crane onto the waterproofing and positioned by three operatives. Even a lightweight sedum blanket can become unstable when saturated at pitches above 10 degrees, however, so at UCS it will be held in place by a series of hooked carpet-gripper style retention strips placed at 2m intervals and bonded to the surface of the waterproofing membrane. The number of retention strips required depends on the length of the sedum blanket, which will be cut to fit the shape of the roof.

When CM visited the site in March, Accurate Roofing was six weeks into the job, but behind, with just the vapour barrier and some insulation down. ‘High winds prevented the main contractor from completing the concrete roof deck on time, which in turn held us back. The scaffolding around the roof perimeter is also incomplete, so we can’t access certain areas,’ says an irked Humphries. ‘The problem is that we come under the most pressure to get things done because we’re always the last trade on site, but if other aspects are incomplete that affects us.’ He’s confident the work will be completed on time, however, and plans to use more workers to get the work done if necessary.

Work has also been affected by the discovery, during the detailed design stage, that run off from copper cladding on two service elements that protrude through the roof could poison the sedum. ‘It has meant designing a separate drainage system for the elements, which previously drained straight onto the sedum,’ says Davis. ‘Thankfully, once it’s laid the roof is designed to be virtually maintenance free. Because of the steep pitch we designed it for as little access as possible but there’s a man-safe system in place if anyone needs to get up there. A “leaky pipe” irrigation system will keep it moist in the summer, and the same system can also apply granular slow release fertiliser once or twice a year.’

At an extra cost of £20-£40 a square metre sedum isn’t the cheapest of roofing options, but Bauder says the protection it provides can easily double the lifespan of the roofing membrane. And let’s face it, you’re effectively getting several roofs in one, as the plants change from bright red, to pink and then green as the seasons progress.

It will be an attractive sight once completed. But definitely one best viewed from the ground… cm