More cross-laminated timber on the London skyline

Summary
• Eight-storey Bridport House is the tallest all cross laminated timber (CLT) block in the UK.
• The timber panels were made by Stora Enso in Austria and built by specialist timber contractor Eurban. The architect was Karakusevic Carson.
• CLT was lighter and faster to build with than concrete and steel.
•The carbon saving over steel and concrete was 2,113 tonnes

Like the Stadthaus before it, also in Hackney, Bridport House is pushing the boundaries of timber construction up to eight storeys.

Although Stadthaus is marginally taller at nine storeys, it features a concrete ground floor, whereas Bridport House is CLT from the ground up, so it’s a moot point which is technically the taller timber tower.

Designed by Karakusevic Carson Architects as the first part of the regeneration of the Colville Estate, Bridport House replaces an original 1950s block with 41 new homes in two joined blocks, one eight storeys high and the other five storeys.

All elements from the ground floor upwards are of cross-laminated timber (CLT) supplied by Stora Enso Wood Products – including the lift shaft. Below ground level (the raft, foundations and lift pit) are of reinforced concrete.

Often in such projects the ground floor will beconcrete because it is used for an application such as retail, and needs larger openings. At Bridport House, however, the lowest two floors are occupiedby maisonettes which have been oriented in a different direction from the apartments above by changing the direction of the load-bearing walls.

Lightweight and fast

There are several reasons why CLT was specified. One was to do with weight. It is considerably lighter than the alternative structural materials of reinforced concrete or structural steel. This was important at Bridport House, where a large Victorian sewer runs beneath the surface and it was necessary to avoid point loads wherever possible.

Speed of construction is another benefit of CLT which, it is estimated, can take as little as half the time of constructing a conventional reinforced concrete frame. In addition, the construction process is far less likely to be interrupted in bad weather conditions.

Engineering was completed by timber contractor Eurban Ltd, with CLT boards made from PEFC-certified spruce at Stora Enso Wood Products’ 65,000m³ CLT factory and adjacent fully integrated sawmill in Austria.

Integration of factory and sawmill means raw material selection is greatly improved and CO₂ emissions reduced because there is no extensive raw material transportation between the sawmill and the factory. After manufacture, boards are transported by lorry to the UK.

Airtight, sound and fireproof

The accuracy of construction with cross-laminated timber aids airtightness as elements such as windows will fit exactly. Wayne Probert, of Stora Enso Timber UK said Stora Enso’s CLT boards are edge glued, which further increases their airtightness and means that its three layer CLT boards could achieve the same airtightness as non-edge glued five layer boards.As a result, the airtightness of Bridport House, at 3m³ per hour, is 60% better than that demanded by Building Regulations.

Edge gluing also increases the acoustic and fire performance of the CLT boards as well as making them watertight. This technique has the additional benefit of increasing the temporary weather protection when building in the winter months.

Probert said the faces of the visual (exposed interior faces) and non-visual boards are all sanded in the factory, which means any water blemishes incurred on site can then be simply sanded away.

Stora Enso has also completed several other UK projects, including sports halls, swimming pools, student accommodation and university buildings.He described Bridport House as the most challenging so far, due to the long and narrow site, which meant the engineering was of critical importance.

Carbon saving

Probert said CLT was becoming very competitive with UK construction companies, partly due to the carbon sequestration in the structures.

Bridport House main contractor Willmott Dixon's in-house sustainability consultancy Re-Thinking, has been working with the Centre for Sustainable Development at the University of Cambridge calculating the embodied carbon of Bridport House. Their calculation shows that had the building been of conventional reinforced concrete frame, the materials required would have incurred an additional 892 tonnes of carbon. This is equivalent to 12 years of operational energy required to heat and light all the dwellings at Bridport House; alternatively it would take 61 years to save the same amount of carbon as the planning requirement of 20% renewables. When the sequestered carbon locked up in this 1,576m³ timber structure, is added to the carbon avoided, the total figure is 2,113 tonnes of carbon – equivalent to 29 years of operational energy.

Completion is expected at the end of the summer.

This feature first appeared in the Autumn 2011 edition of Timber & Sustainable Building. For copies contact mjeffree@ttjonline.com