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A school designed by Keith Harnden Architects on the edge of Salisbury's Cathedral Close makes use of natural materials and allows pupils to engage with their surrounding environment

Leadenhall Primary School is on the edge of Salisbury Cathedral's Close, in a former wooded garden to the 12th-century house reputed to be the original home of the cathedral's master mason. Keith Harnden Architects has designed 14 new classrooms and a school hall on this sensitive site in a way that has won, astonishingly, universal plaudits from the local civic society. It is astonishing because this is no pastiche, rather a series of pavilions with copper roofing and timber walling that fall away westward through the old trees down to the banks of the River Avon.

One range of five classrooms runs in a straight line along the north boundary of the site. A second, staggered, group of three classrooms connects at the west with a similarly staggered group of five to form a kind of V formation, protecting the big hall for drama and gym behind. An equally valid view is that the hall is an anchor around which the classrooms cluster. Whatever the case, Keith Harnden has facetted and stepped them so that they all get views of the river and the water meadows on the other side. Each of the classrooms has one splayed corner to which is attached a small deck and a short stairway (set at different angles) down either to grass or gravel. This is on the garden side of the classrooms: the access entrance is on the other side where there is a network of raised open walkways with open pergolas overhead. As Harnden says: 'When the children go out of their classroom it is into the garden rather than into a school corridor.' It needs to be said that the school is refreshingly comfortable about kids toughing it a bit; typically, when other schools closed during last winter's heavy snow, it remained open.

Better believe it This is architecture founded on a belief in the necessity for sustainability. And it would be very difficult to think of this scheme in terms other than timber. Harnden says: 'I love using traditional materials - it is their formation and the way the materials can be composed that interests me.' So too the way they will weather - the copper to its characteristic dull green and the Douglas fir to its silvery grey.

The timber is mostly Douglas fir with planed kiln-dried Canadian Douglas fir windows and mullions made off-site by local company Winchester Joinery.Walls are vertical Douglas fir planks with 75mm cover strips at 150mm centres. The ribbed decking is 150 x 19mm planks of Balau hardwood with 10mm spacing between them. Harnden says: 'It's the wood they make jetties out of, and it's a nice colour. But it is contained in a Douglas fir framework of handrails and balusters, horizontal framing and posts.

'Douglas fir has a durability about it that is better than cedar - and cedar leachant eats copper. But when you put Douglas fir and copper together they seem to sing.'

The contractor came across a source of Douglas fir that had been grown in the nearby New Forest. Harnden says he was enthusiastic that the timber could be sourced locally, and because the price was quite a lot less than Canadian timber. But, he says, 'I was disappointed at first, it has more knots in it and is a little more ragged in places than if it had been carefully selected for export. Still, this Douglas fir has the same lovely mellow colour: it's the character of the grain and its warmth that I love, and the English version is now mellowing nicely.

Screwing down The inner structural steel frame (which has a zinc phosphate primer with site-applied bitumen paint over site welds) sits on helical piles - possibly the earliest recent use of the system in the UK since it was invented here a century ago for piling piers. It is now produced by the New Mexico firm of Ed Crocker, and Harnden was able to persuade local piling contractor Bullivant to import and install Crocker's 75mm diameter hollow galvanised steel tubes with a helical outer face. They are simply screwed into the ground in sections to the selected depth, using a modified auger drill. Bullivant installed 82 of them in eight days and in relative silence. They are not cheap, but as Harnden says: 'It was infinitely quicker and they were installed precisely.And the process was non-intrusive to tree roots.'

But the biggest thing in this ancient garden, where Constable sat and painted Salisbury Cathedral, is that some time in the future the whole scheme can revert back to nature.

Harnden says: 'We found, when we weren't happy with the accuracy of some of the piles, that you can simply unscrew them as easily as putting them in.'

The piles were cut off 600mm above ground level, and the structural steel frame welded to them. Since it is beside the river, the site is potential flood territory, which is why the buildings are raised. Underneath, a network of soakaways has been constructed, not only to take rain water, but also to facilitate the rapid drainage of floodwater.

The sub-frame to the steel structural frame is mostly tanalised 150 x 50mm softwood.

The external walling is of rough-sawn vertical 150 x 12.5mm Douglas fir planks, with 75 x 19mm pieces covering the vertical joins between the planks and screwed through to battens at 400mm vertical centres. These horizontal battens are fixed over a breather paper to the 150 x 50mm studs, which are at 400mm centres. Inside, 40mm thick sheet Celotex is fixed to the inboard face of the studs, using the vertical plasterboard battens which are screwed through the Celotex to the studs. The foil-backed plasterboard is taped and filled and painted, though not skimmed. Harnden says that the 150mm void enhances the capacity of the building to cope with moisture.

The internal flooring is a cork tile on interlocking particleboard panels, floating on 70mm of Celotex over 25mm ply screwed to the joists at 400 mm centres.All this is easy to dismantle. Harnden says there is a good spring to the floor. One child recently said of it: 'I really like my bouncy classroom.'

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