Adam Khan’s wetlands centre floats between land and water
The design of the Brockholes Wetland Nature Reserve buildings in Lancashire draw inspiration from the villages of Iraq’s Marsh Arabs
Architect Adam Khan Architects
Structural engineer Price & Myers
Location Preston, Lancashire
“A collection of roofs bobbing like hats above the reeds.” Such is the mysterious sight that should greet nature lovers approaching the £5 million visitor centre of Brockholes Wetland Nature Reserve near Preston in north-west England, according to its architect, Adam Khan. The five buildings that make the visitor centre are indeed mainly composed of roofs. What’s more, they occupy what Khan calls a “magical location… amongst the reeds, perched ambiguously between land and water”.
What makes the perching location even more ambiguous and magical is that the cluster of buildings will quite literally float in freshwater that has naturally filled a former gravel pit. Rather than bobbing hats, the true and remarkably apt inspiration for Khan’s design was the vernacular villages of the Marsh Arabs, which float among the natural reed beds of southern Iraq.
The five buildings are all single-storey, lightweight pavilions that sit on a floating pontoon. The pontoon will be made of concrete filled with lightweight rigid foam that should rise and fall with the water level. With no need for excavation or foundations, the pontoon will have the softest of impacts on the nature reserve.
The scheme by the fledgling, south London practice of Adam Khan Architects won an international competition in 2007 and is currently out to tender based on full detailed design and specification. The visitor centre will be developed by the Lancashire Wildlife Trust, which runs the 127ha nature reserve, with funding from the North West Regional Development Agency and the Forestry Commission. In the competition brief, the trust called for “an exemplar of sustainable design”, that would be “carbon neutral in construction and operation (taken over the first 25 years), drawing on local resources where possible”.
Adam Khan Architects has responded with a design that attains the new record-breaking “outstanding” level of Breeam rating. In the name of sustainability, the buildings will be largely prefabricated off site, which as well as ensuring higher workmanship, should reduce wastage and disturbance to wildlife. As for drawing on local resources, however, it has been less successful. No local natural materials have been used, though the internal roof lining at least is a fully recycled material.
As with the floating village as a whole, the five buildings that compose it were partly inspired by those of the Marsh Arabs. Though not made of local reeds, they are all lofty single-storey pavilions with steeply pitched roofs rising to a central ridge. Khan’s other source of inspiration was the traditional English barn of a similar shape.
The hollow pyramidal shape of each pavilion acts like a thermal chimney, making mechanical ventilation or air-conditioning unnecessary. Fresh air is drawn in though windows and vents at the bottom and let out through opening rooflights at the top, and this passive stack effect is reinforced by sunlight warming the air through the rooflights.
All five buildings are rectangular in plan, and instead of flat gable ends, all have hipped roofs that provide extra bracing and have a constant steep pitch of 60 degrees.
The eaves of the perimeter walls have been kept at a constant 2.3m height to add a convivial, domestic scale and draw a sharp horizontal line between windows and roof.
The grandest volume is the dining hall, which seats 150 people and is separate from the adjoining self-service servery and entrance foyer. Full-height windows around three sides admit ample daylight and offer unrestricted views of the surrounding wetlands.
The other four pavilions house an exhibition hall, education spaces, conference centre and shops, which all together are intended to cover the entire running costs of the nature reserve.
When it came to roof coverings, Khan set out once again to emulate the Marsh Arabs by using the same natural material that could be literally picked up on the site, even in north-west England. Reed thatch. “But that was a fire hazard, so we abandoned the idea,” he says.
Instead, the architect has gone for the next-best natural roofing material — oak shakes. Although it isn’t grown commercially near Preston, oak in its natural, untreated state is durable and nearly maintenance-free. Unlike timber shingles, which are sawn to an artificially smooth surface, shakes are split to leave a rough, natural, rustic surface. And over the years, oak mellows to what Khan calls “a lively golden-silvery grey”.
The shakes are 8mm thick and they will be nailed to battens in triple overlapping layers like slates with copper or stainless steel nails.
Directly below the natural oak shakes, the roof switches into something much more artificial and factory manufactured — structurally insulated panels. These comprise rigid plastic foam sandwiched between panels of orientated strand board. The intention was to boost the thermal insulation of the roof to attain the “outstanding” Breeam rating. The SIP panels are specified here as a double layer 260mm thick to achieve a U value for the full build-up of the roof of just 0.11 W/ÞC/sq m. They will be fully prefabricated off site to cover the roof pitch from eaves to ridge in one piece.
To achieve the “outstanding” Breeam rating, the roof must also be highly air-tight with an air leakage rating of just 3cu m/hr at an air pressure of 50 pascals. “The SIP panels themselves have an air-tightness rating of 2.8cu m/hr, so it’s just the junctions that are critical,” says Khan. “They require careful detailing of the vapour barrier at eaves and ridge.”
The roof soffit will be finished in sprayed acoustic foam made of newspaper pulp.
“It has a bobbly finish and it kills sound dead, like a covering of fresh snow,” says Khan. The acoustic foam is also a waste material that has been fully recycled, and that makes it the most sustainable material in the whole visitor centre devoted to sustainability.
A zig-zag lattice of exposed portal frames in glue-laminated timber supports the roof of the large dining hall. “You can see the skeleton as in old barns,” comments Khan.
The portal frames are exposed for the practical reason that, if they were incorporated into the depth of the roof, they might cause cold bridging that would compromise its very high thermal insulation. The splayed, concertina configuration, with frames connected to their neighbours at top and bottom, brings another practical bonus: by reinforcing each other the frames can effectively double the distance they span across the hall.
The rafters are splayed with a constant 11 degree angle between pairs of rafters all the way round the roof, and this matches the regular geometry of the roof with its constant 60 degree pitches. Well, that’s how it looks on a quick viewing. But at the hipped corners where five rafters meet, complications arise and the rafters cope with this with splays that vary up to 17.5 degree. As Nick Reichinger of structural engineer Price & Myers admits, “We had to devise some incredibly smart geometry to get them to work.”
The glulam members also demand mild-steel reinforcements in key locations, and these are all craftily concealed. The vertical columns and ridge collar beams are both made up of two halves sandwiching a steel flitch plate running their whole length. In addition, slender steel ring beams are incorporated into the roof fabric at eaves and ridge levels.
The portal frames will be erected on site in groups of four. The split columns and ridge collar beams will both be assembled in two halves along with their intervening flitch plates.
Architect Adam Khan Architects, Client Lancashire Wildlife Trust, Structural engineer Price & Myers, Environmental engineer Max Fordham, Quantity surveyor Jackson Coles, Fire consultant WSP Fire, Project manager Lend Lease
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