An apartment block rising a storey every 10 days on the Sundbyberg waterside in Stockholm is making news on several fronts.

Undoubtedly the prime reason the Strandparken development is stirring up the Nordic construction press - and has developers queuing to take a look - is size. At eight storeys and 25m to the ridge of its distinctive pitched roof, the shingle-clad, cross-laminated timber (CLT) and glulam structure will be Sweden's tallest wood building. And this is only phase one. When workers vacate the prestigious plot, it will have three more identical 31-apartment blocks. That will make it one of the biggest residential timber developments worldwide, comprising 28,000m2 of CLT, 180m3 of glulam and 14,000m2 of cedar shingles. But it's not just about scale. Strandparken marks timber high-rise's strategic debut in Stockholm too, and is billed as a departure in design. It also sees firsts for the architect, developer and engineered wood specialist involved, and possibly the start of an influential partnership between them.

Project instigator was developer Folkhem. It has built top-end timber housing for decades but, due largely to objections from Stockholm planners, despite Sweden's block on timber building over two floors being lifted in 1996, had not previously tackled multi-storey. "We've been pressing for multi-storey for some time, but until recently met with resistance," said Folkhem projects manager Stefan Karlsson. "But attitudes are changing due to increasing emphasis on sustainable construction and appreciation of timber buildings' environmental advantages, plus Stockholm's need for more housing generally. Now the authorities are coming to us!" While Strandparken's architects, Stockholm practice Wingårdhs, had worked with so-called 'massive timber', it had not used it in andy high-rise projects before either.

"We and Folkhem decided to use wood here together," said executive architect Hanna Samuelson. "It's sympathetic to the setting and references Sweden's wood building heritage. At the same time we wanted it to mark an evolution; for the design not to be dictated by the system, as you see in many massive wood buildings, with their 'fat' plastered exterior walls and splices between elements showing. We've achieved that by working conceptually with wood, having features such as pitched roofs, no eaves, reduction of plates, huge balconies and the shingles used as a wrapping that hides the element splices. It's a prototype to take us forward."

The third party in the project is timber build system specialist Martinsons Byggsystem. Based in Bygdsiljum, in central Sweden, the company is a subsidiary of Martinsons Såg AB, Sweden's largest private sawmiller and one of its leading glulam and CLT producers and, through Martinsons Träbroar, a major timber bridge builder.

Martinsons undertakes complete shell erection, as well as providing CLT panel, glulam and timber build kits, and had already made its mark in large-scale multi-storey developments. But Strandparken marked a stepping off point for it, too. "It's our biggest project so far," said Byggsystem managing director Mikael Lindberg.

On a bitter January day, their debut joint project was living up to expectations for all three partners. Not only had it captured the imagination of the wider construction sector and consumers, with all the first block apartments pre-sold, it was on schedule. Most time to date had been spent re-routing a road, laying the 3-11m foundations and building the concrete undercroft car park. But from the start of work on the first apartment block, including ultra precise attachment of timber to concrete using hi-tech steel connectors, it had reached three storeys in a month and was on target for completion in May.

Helping it keep to programme through rain and snow was its protection system from Martinsons subsidiary Extoler, a giant awning that rises on steel towers as floors are added and includes hoists and gantries.

Other key factors in the rate of construction, said Lindberg, are the level of prefabrication and size of the CLT panels. Some of the 120mm exterior wall units are 12x2.4m, while interior panels are the same size, but a mix of 120mm and 170mm-thick varieties depending on the degree of support needed. Similarly the floor/ceiling panels blend 145mm and 70mm-thick units, with the former up to 11x2.4m. In places supported by glulam beams going up to 330x450mm, these create open spans up to 11m. CLT is also used for the stairwells and floors in thicknesses of 120mm and 170mm.

"These big panels not only mean rapid construction, but appeal to architects as they create these wide, open spaces without vertical support," said Lindberg. The CLT also has door and window apertures and service channels pre-cut at the factory. "This is very efficient, but means everyone has to be involved from the earliest project design stage so services are precisely pre-planned," said Samuelson.

Rockwool insulation is factory-fitted too; three 95mm layers for external walls, one 145mm slab for interior walls and floors.

The non-structural partition walls comprise two closed timber frame panels with 20mm gaps between. "The gaps give added fire safety and noise insulation, whereas the CLT is pretty sound-proof in its own right," said Lindberg, stamping on the floor to make his point. Most of the pine-framed, triple-glazed windows from WB Trä are also prefitted. But 7.5m French windows leading onto the apartments' 13m2 CLT balconies weigh 500kg, so are installed on site.

For the first time, Martinsons is also prefabricating the roof modules. These will comprise 95mm-thick 9x2.7m glulam panels, with insulation and breather membranes attached in the factory, and just the windows and shingles installed on site.

Interestingly, while the CLT is skinned in fire-resistant gypsum, none of the structural timber is fire retardant treated, even for stairs and lift shafts. The cedar shingles are only treated to the first floor and in larger windows, where jambs are reinforced with glulam and steel columns, it's the steel which is fire-protected.

"This timber is thick and retains its structural strength in a fire, charring at just 0.7mm per hour," said Lindberg. "There's also little concern about fire risk in timber buildings among Swedish consumers. They're trusted."

"In fire tests the panels did so well we only just crossed the threshold for needing sprinklers," added Karlsson. The building really does fit together like a kit, with threaded rods embedded through one panel marrying with 20mm bolts in the next.

"The connection is strong, but simple, so we're less labour intensive than other forms of construction," said Karlsson. "We've only got 22 carpenters here, plus two Martinsons assembly leaders and six interior workers - and, as the shell is rapidly weathertight, interior work can start on one floor, while those above are built."

Most of the shingles also come fixed on pine battens over a breather membrane to the panels, with Martinsons carefully 'mapping' the walls to ensure they line up. They are then being left to weather naturally.

"This avoids re-treating and maintenance," said Samuelson. "But we're also moving away from standard wood cladding associated with timber building. Untreated, this tends to discolour unevenly. The shingles will also weather differently, but the effect will be pixilated, giving a simultaneously rough and slick, modern façade which will age with dignity."

The apartments will all have timber flooring, while underfloor heating will plug into the local district heating network, combining with the insulation and mechanical ventilation to deliver the statutory 55kW/m2 for residential energy use.

The apartments aren't cheap - a 124m2 flat costs SKr5.4m - but, as the level of pre-selling has shown, that's clearly not an issue. And the partners behind the project see more demand to come, with Strandparken acting as a poster development for the aesthetic, eco and quick-build potential of timber multi-storey. All three express a desire to collaborate again, and Folkhem and Martinson already have their next project planned.

"Stockholm is growing at 30,000 people a year and urgently needs more apartment space, but also demands high standards of building performance and design - timber construction is the solution," said Lindberg. "We also believe other Swedish urban centres will be persuaded by such developments here."

"Currently demand here is sufficient for our capacity," said Mr Lindberg. "But the introduction of Eurocode 5 for timber construction makes it easier for us to work elsewhere in Europe."

Strandparken is not seen as the final word for timber building size-wise either. Folkhem and Martinson's next project is 12 storeys and the former's managing director, Arne Olsen, wants to go higher still. "I believe we can go to 20 now," he said.

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Fact sheet
Architects Wingårdhs (Hanna Samuelson executive architect)
Main Contractors Folkhem
Suppliers of wood solutions Martinsons Byggsystem
Construction system CLT, glulam, concrete (ground floor)
Construction time Ongoing: 2011 - 2015
Year of completion 2015
Location Stockholm, Sweden
Strandparken 2
Strandparken 1
Strandparken 3
Strandparken 4
Strandparken 5

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