• Eight-story Bridport House is the biggest solid cross laminated timber (CLT) residential development in the UK;
• Carbon saving was 892 tonnes over traditional building methods;
• Including the sequestered carbon locked up in the timber, the saving is 2,113 tonnes;
• Air tightness is three times better than Building Regulations

Having achieved the first BREEAM Outstanding for a healthcare building, Willmott Dixon is again pushing the boundaries of construction, this time delivering Bridport House, the largest solid timber residential development in the UK.

It might also be the tallest timber building: although the nearby CLT-built Stadthaus is marginally taller at nine storeys, it had a concrete ground floor, while Bridport House is CLT from the ground up.

Using CLT as the main structural material helped meet sustainability objectives for the project. By building with timber, Bridport House offset the carbon emissions resulting from the construction process as timber acts as a carbon sink. Each of the 41 homes contain approximately 30-40m³ of timber which is equivalent to over 50 tonnes of CO2.

Designed as the first phase of the regeneration of the Colville Estate, Bridport House was commissioned by Hackney Council to replace 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 CLT – including the lift shaft. Below ground level (the piles, foundations and lift pit) is reinforced concrete.

Lightweight and fast

There are several reasons CLT was specified.

The first was weight. It’s considerably lighter than alternative structural materials of reinforced concrete or structural steel. This was important at Bridport House, where a large Victorian storm relief sewer runs beneath the surface and it was necessary to avoid a heavy frame structure.

The unique properties of CLT’s engineered strength combined with its light weight made it the ideal solution to this challenge. The structure has been designed so that the load bearing CLT panels are placed in a variety of positions on each floor, thus spreading the load.

Speed of construction is another benefit; constructing the CLT took 12 weeks on site, six weeks less than a traditional frame solution.

Engineering was completed by timber contractor Eurban, a company Willmott Dixon has worked with extensively on education projects. The CLT boards were made from PEFC-4,300 certified spruce trees from within 100km of Stora Enso Wood Products’ factory and integrated sawmill in Austria - 94 minutes after felling the trees, replacement timber would have grown back!

Integration of factory and sawmill means raw material selection is greatly improved and CO2 emissions reduced as the factory uses all its waste for energy driving the machinery. And there is no material transportation between sawmill and factory. After manufacture, boards are transported to the UK.

Air tight, sound and fireproof

The accuracy of construction with CLT aids air tightness as elements such as windows fit exactly.

The CLT boards are also edge glued, which further increases their air tightness and means the five layer CLT boards achieve the same air tightness as non-edge glued five layer boards. As a result, the air tightness of Bridport House, at 3m3/m2.hr @50 pa is three times better than minimum Building Regulations. We achieved 1.6 on one of the flats, giving us one of our best-ever results and well below our targets.

Carbon saving

CLT is becoming more popular owing to the carbon benefits in the structures. Our in-house sustainability consultancy Re-Thinking worked with the Centre for Sustainable Development at the University of Cambridge to calculate the embodied carbon of Bridport House.

They found 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.

Also, 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 saved provided the timber is reused – equivalent to 29 years of operational energy.

Project facts: