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Specifier's choice:Parliament Hill School, Hampstead

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Improved access, high visual impact and environmental friendliness were the key requirments demanded of Haverstock Architects in designing the Courtyard building.

Sutherland Lyall talks to project architect Tom Gibb about the products specified and why

Parliament Hill School is right on the southeast corner of Hampstead Heath in north London. It comprises a typical collection of brick secondary school buildings from the late 19th century, linked at the north end across the back of a parking courtyard to a 1950s flat-roofed modernist block.

In the first instance, the local authority wanted a new design and technology (DT) block. Haverstock Architects' solution was a single-storey grass-roofed building with a sinuous plan, which linked the southern ends of the old building with the middle of the '50s block, closing the southern end of the courtyard. Staff cars have been banished to another part of the site. There is actually not a great deal of courtyard left because the architect is planning to install a pronounced grassed berm along the north side of the building, almost filling the remaining triangular space.

The new Courtyard building, as it is known, contains three media studios, a CNC machine section and storage - plus a nonDT section for student services, which is integrated into the school's entry and security system. In addition, there is a covered but otherwise open walkway curving alongside the south perimeter of the building.

The media studios are actually workshops because they house, among other bench tools, heavy milling kit including metal lathes, as well as a CNC machine and CAD computers. The north side has brickwork up to sill level, and then strip windows above that. Under the open walkway canopy, the south sun-facing elevation has big windows extending down to the ground. The special feature in the grass roof comprises six tall, slightly canted roof lights, clad in cedar boarding. Haverstock has also designed a second block for the school to the south, to be known as the Corner building (pictured right), but it is in its early stages.

Curriculum consultant Haverstock and three other practices were invited by Camden Education Department to put a fee bid together. Project architect Tom Gibb says: 'We took on a lot of work to enquire about the needs of the school and used a curriculum consultant. Often the largest problem schools face is identifying what they really need.

'It was interesting at the beginning when we did some design quality indicators with the staff. One of the things which came out of that was that they wanted the building to have a high visual impact, and it is on the back of that we have included the green issues. We were also aiming to improve accessibility. Having enclosed the courtyard, you can get around the school without being exposed to the elements.' The contract is a JCT98 local authority version with quantities. Gibb says that for the second building, there is to be a twostage tender because the site is riddled with services, so there is a lot of enabling work involved. Of the appointment of Basildon-based Gee Construction as the main contractor, Gibb says: 'We went through a long process of selecting from a large list, taking references and running a series of interviews. It took quite a lot of time. The professional team was appointed direct by Camden, although we appointed the acoustic and landscape consultants, respectively Paul Gillieron Acoustics and John Tierney Associates. We've got a good track record with these people.'

Haverstock uses NBS in writing specifications. But, Gibb says: 'What we try and do is write performance specifications, especially in situations where we have to go to a subcontractor for advice. In NBS there is that standard 'or equal' clause, but we try to include as much detail as possible and we really do suggest that the main contractor looks elsewhere. We have a good relationship with the contractor and if they can save us money they do.' He elaborates: 'Originally we specified high-performance windows with flush-bonded glazing. Then the contractor suggested that the Comar P5/7 section from Abbey Fabrications was cheaper and had very slender sections. Abbey have more than 900 profiles and they are beginning to use them to develop products. In the next contract, I'm looking for a silicone bond between glazing panes. It is a product they are hoping to bring out soon.

'The contractor also offered an alternative internal floor structure. We had included a cast-in-situ floor with Bison planks as an option. But the contractor offered a Milbank block-and-beam system, which was good for lead-in times, so it has worked to our advantage. Once we were out of the ground it was very quick. The whole building appeared in a week. Or at least the bones of it.'

Industrial look The industrial aesthetic that the design team has followed ties-in with the quasi-industrial activities which take place in the building.

Window surrounds, for example, are galvanised steel and the window strip along the north elevation is made from two layers of Reglit. Gibb explains that he has plain C-sections back-to-back: 'They have a reasonable U-value and come in quite a few colours. We have used an amethyst blue for the outside, which is slightly obscured - you can see shapes through it. Inside we used the clear-wired glazed channel. Opening windows by Abbey Fabrications are being made at the moment.' For the floors, Gibb says: 'We have gone for a 100mm-thick screed and are using an industrial shake-on product, Sikafloor [Sikafloor-2 Syntop Metallic Dry from Sika], which takes the moisture from the concrete.

When you power float it, it forms a very hard finish. We have gone for two shades of grey, a darker grey for the CAD-CAM area and lighter for the media labs. The reason for the monochrome scheme is because we wanted to emphasise just one element - for example, a brilliant yellow wall. And we wanted to emphasise the light chimneys as well.' Internally, the walls are mostly Smooth Face Thermalite from Marley, and services are simply bolted on to the walls. This is not simply going for the cheapest option, but part of a deliberate visual strategy. The Hanson Omnia precast roof panels were chosen because they 'produce really good soffits, ' says Gibb.

Managing energy Explaining the environmental strategy, Gibb says: 'We used a heavy mass construction with exposed concrete soffits and night purging. This maintains internal temperatures and provides thermal comfort for users, without resorting to high energyconsuming air conditioning or comfort cooling systems. The courtyard building uses mechanical ventilation with heat-recovery in normal operation and user-controlled windows for background comfort.'

Environmentally, the green roof helps with this as well, although there is a trade off between roof loadings and thermal mass benefits. But as Gibb points out, it gives the school some of its green space back. It also provides a high level of thermal insulation, which far exceeds the current building regulations.

'We went to all kinds of grass roof manufacturers and they were very helpful which eased things during design development. They could answer questions all the way through, ' says Gibb. 'But I came to the conclusion that the Alumasc system was deeper, so in times of drought would hold more water. The alternatives tend to be a kind of carpet. And Alumasc does a complete roofing system.

'With Alumasc you can go for a mix of plants. We are going to be planting over two years before it reaches maturity. We have used a full mat system, a mesh of woven cable which relies on the weight of growing medium for anchorage.' Early designs had a walkway on the roof. Gibbs says: 'There is a slope on the north side and we have a design for a bridge on to the roof and a paved walkway. It is on a back burner now but the roof has been designed to take those live loads into account.' At the moment there is an Alumasc safety system with built-in anchorages, which obviates the need for a handrail.

The profiled galvanised steel canopy on the courtyard building provides an essential covered walkway for teachers and students, while also offering solar shading to the southern elevation. At first the design team had gone for a completely green roof, but it was more economical to plump for a lightweight structure - supported on slightly canted props.

The roof structure is a concrete slab and uses 50mm Hanson Omnia cast planks, which have reinforcement loops sticking out of the top. Gibb says: 'Where the roof structure isn't doing much work we have used expanded-polythene formers and the screed is cast over them - a bit like a concrete honeycomb. Waterproofing is part of the Alumasc system. It's interesting because it never really dries and can take a certain amount of movement. It's similar to wax to the touch.

And there is a root barrier and insulation on top. We were able to cast a properly flat slab because the system can cope with that happily - it allows quite a lot of water to sit in it and keeps the plants moist. Eventually excess water can run into the gulleys.' The roof is edged by a 300mm band of washed gravel to prevent plants spreading.

Is a green roof more expensive? Gibb answers: 'Not if you compare it with, say, copper. But yes, it is more expensive. Here though, because it is overlooked, an asphalt roof would be inappropriate. And also the planners would take a view because of the water- runoff scenario with asphalt or steel decking. It would be like a car park, with all the surface water going straight into the sewer system.

Fittings and fixtures The design team's policy was to not use plastics on sustainability grounds but also, Gibb says, 'because they are not very hard wearing in this kind of school environment - the end pieces go missing, especially skirting trunking. We have used galvanised steel three-part trunking and MK sockets and switches with a metal face.' For architectural ironmongery, Gibb says: 'We went to a specialist, James Thorley at ItsFitz, who was able to come to us with a schedule of his proposals which we could change and mix and get the best quality for the best price.' Lighting in the workshops uses Media Lab lights. The reason couldn't be better:

'We have them in the office, ' Gibb says. 'They are high quality, not overly expressive, very rectilinear and simple. They are from Fagerhult and we have used the diffused and direct fittings, mounted at the optimum 600mm from the ceiling slab soffit.

Bricks are in keeping with the surroundings. Gibb says: 'The old school has three different stocks and we have tried to match the tone of these with an Ashdown Cottage Mix from Ibstock Tayor Maxwell. Block paving, a random mix of Woburn from Charcon Hard Landscaping, is meant, Gibb says, 'to emphasise the arrival of people on the site and take them through the route along the south side of the building'. Street furniture from Broxap & Corby, such as galvanised steel collapsible bollards and litter bins, fits in with the industrial aesthetic.

Gresham Wood has made bespoke furniture, such as the fixed furniture around the media labs, and the CAD-CAM room has a layout desk, shelving and storage. Most of this is made using birch-faced plywood. The Leaderflush doors are solid, with one linear glass element.

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