Local knowledge: Green, lively and robust, this branch library adds vigour to a Seattle neighbourhood.

From the client body that brought you OMA's bravura Seattle Library (AR August 2004), this more modest branch facility forms part of the city's 'Libraries for All' development programme. Impressively, since 1998,25 branch libraries have been built or remodelled, adding vigour to local neighbourhoods. Designed by Bohlin Cywinski Jackson (BCJ), this new building also forms part of a regenerative masterplan for the city's Ballard district.

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First settled by Scandinavians in the mid nineteenth century, Ballard is an arty, eclectic neighbourhood ten minutes drive to the north-west of downtown Seattle. Drawing on Ballard's Scandinavian origins and maritime heritage (BCJ partner Peter Bohlin also boasts Swedish roots), the new building combines the functions of library and neighbourhood centre with the aim of reaching out to a younger and more diverse local populace.

The site is a typical block on an urban grid, with three sides addressing different streets. The fourth side adjoins a currently vacant lot, but plans are in play for a retail and housing development. Though essentially a low slung, single-storey structure, the new library is given added civic presence by the powerful gesture of a gently curving roof that unifies the disparate volumes and spaces below. On the short north and south ends, the roof sweeps up to admit daylight through slots of clerestory glazing, while along the long west street edge it oversails to form a generous, welcoming colonnade, supported by a line of tapering steel columns. This creates a zone of shelter and informal interaction on the street frontage, enhanced by planting and robust, custom-designed seats each crafted from a single sheet of bent steel. Certain elements--the steel wrapped blob of the multipurpose room and the sinuous glazed entrance foyer to the neighbourhood service centre--are pulled out and expressed as incidents along this main facade.

At the north end of the site, the double-height reading room forms the tranquil, luminous heart of the building. A narrow zone of staff, storage and ancillary spaces separates the library functions from the more compact neighbourhood centre at the south end. The roof dips and heaves over the tall space, its exposed timber structure propped on mast-like columns. Light floods in through clerestorey glazing and a row of groovy domed bubbles punched into the roof.

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Enveloped in a layer of planting that mimics the natural pattern of seeds spread by the prevailing breeze, the roof is also a key element in the project's strategy of environmental control. The sod clad surface provides insulation, traps rainwater and is a natural habitat for flora and fauna. It also hosts solar panels and an array of anemometers to measure wind speed and direction. (A periscope set in the wall near the circulation desk offers younger library users glimpses of the roofscape and activities.) Together with the solar panels, photovoltaic film on the south-facing windows generates electricity, the output monitored by Seattle's City Light Green Power programme which analyses the effectiveness and potential of such schemes.

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From the yellow pine used in the crisply articulated roof structure to carpets, glass and ceramics, the project is also underscored by a strong enthusiasm for recycled or low cost materials. BCJ took particular delight in designing rustic plywood furniture that simply slots together without the use of adhesives; a small contribution, perhaps, in the overall scheme of things, yet characteristic of the project's prevailing spirit of thoughtful, Scandinavian decency.

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