Timber MMC

By Suzanne DavenporT

Learning from experience of timber modern methods of construction (MMC). Published by NHBC Foundation, 2021

Introduction

Timber is perhaps the oldest of all construction materials. In Britain, archaeologists have uncovered timber-framed homes from more than 10,000 years ago. It would be hard to argue that timber construction is ‘nontraditional’ but the potential for timber frame structures to be prepared or pre-assembled and then transported has long been recognised. A stainedglass window in The Guildhall, Londonderry/Derry (1887) is annotated: ‘Framed Houses sent from London 1609’. Other examples include: Henry Manning’s Portable Colonial Cottage for Emigrants, Isambard Kingdom Brunel’s Renkioi Hospital and Boulton & Paul’s (B&P) prefabricated buildings. From 1864 B&P manufactured an extensive range of buildings, described in catalogues, which were sent all over the British Isles and former Empire.

Jointing and fixing techniques for timber steadily evolved over centuries and the survival of Elizabethan buildings is a testament to the longevity and durability of native hardwoods. However, timber technology, particularly during the 20th century, evolved to make use of softwoods in prefabricated elements that could be manufactured without labour-intensive traditional jointing. Trussed structures can now achieve long spans with short sections of timber jointed with plate connectors. From the late 17th century, softwoods imported from the Baltic and later North America (countries with greater areas of forests) increasingly replaced indigenous slow-growing hardwoods.

After the Second World War there was a period of materials shortage but from the 1950s onwards the timber industry secured a growing share of the domestic housebuilding market, backed by the development and innovation of the Timber Research and Development Association (TRADA). In the early 1980s a television documentary exposed decay in timber frame new builds, damaging the emerging timber housebuilding industry. The steady re-emergence of timber since then has been stimulated by new manufacturing techniques presenting new opportunities for prefabrication. Glue- or cross-laminated timber, SIPS (structural insulated panel systems) and modular timber frame have come to the fore. However, following the Grenfell Tower fire, there have been renewed concerns about using structural timber at height, even for midrise buildings.

Origins: Early 20th century

Although there are historical examples of timber structures being exported from England, for example the Manning Portable Colonial Cottage, large-scale prefabrication of wooden houses was mainly developed abroad, particularly in the United States and Germany.

Sears Modern Homes were catalogue and kit houses, with more than 400 home types, sold across America primarily through mail order by retailer Sears, Roebuck and Co. Chicago-based Sears sold over 70,000 homes from 1908 to 1940.

In 1907 a sophisticated technical development of log building in Niesky, Saxony, Germany made the Christoph & Unmack Company (C&U) the world’s leading producer of prefabricated wooden houses. German modernist architect Konrad Wachsmann was linked with and promoted C&U from 1925, designing the director’s house in Niesky in 1929. Through Wachsmann many skills developed at C&U were transferred to North America.

The Tennessee Valley Authority (TVA) was set up in 1933. It was one of the earliest of Franklin D. Roosevelt’s ‘New Deals’, substantial infrastructure projects that received US Government funding in the 1930s. Permanent as well as temporary, portable and demountable houses were factory produced. The design of the permanent houses was based on an interesting concept: they were ‘perfectible’ – let to workers without internal linings to be finished off and upgraded later. However, it was the TVA’s various demountable plywood prefabricated houses that caught the eye of the architectural press and influenced the visiting British Burt Committee.

Wachsmann escaped to America in 1941. There he had a leading role in the massive development of prefabrication that accompanied the Second World War and the years of reconstruction that followed. In partnership with Walter Gropius he developed the Packaged House System and formed the General Panel Corporation.

Like Manning’s cottages, The Packaged House System enabled walls and floors to be locked together without screws or nails, with proprietary ‘universal joint’ connectors. The inventors became so obsessed with perfecting the technology and the dream of a universal system that they lost focus of market changes. The final product was too late to benefit from subsidies through the American Government’s post-war building programme.

Applications: 1940s and 1950s

The Burt Committee, established in 1942, was charged with finding ways of rebuilding Britain’s housing stock. A delegation sent to America witnessed the efficiency of ‘The Packaged House System’ and the Tennessee Valley Authority’s modern prefabricated houses for dam workers. They reported back with recommendations to develop prefabricated homes. The Tarran and Uniseco prefabs demonstrated at the Tate exhibition in 1944 used timber frames. However, timber was in short supply. As a result the Government imported a number of Swedish Baltic pine prefabricated homes.

In the 1950s the Timber Development Association (TDA) developed a prototype prefabricated roof truss with toothed plate connectors. TDA trusses were positioned 1.8 to 2.4m apart with traditional rafters and ceiling joists between. As timber technology developed, today’s prefabricated roof trusses evolved with the efficient use of small pieces of timber in a composite structure.

At the same time as Gropius and Wachsmann were working on The Packaged House System in California, Lennart Bergvall and Eric Dahlberg were also attempting to move the production of houses into a factory in Sweden. They invented and patented the Elementhus system, which used timber in elegantly engineered and economical ways. They built a highly automated, expensive factory in Mockfjärd, which began production in 1952. Over the next 25 years it produced approximately 18,000 houses (about 2.75 houses a day). The homes cost less to build and were more economical to operate than standard houses of the time. Many still make up part of the architectural heritage of working-class Swedish towns.

Innovations: 2000s

The fascination with prefab timber has never diminished, particularly in countries where timber remains the first-choice material for domestic construction. Ranging from the sophisticated Swedish Elementhus and German Huf Haus (1960s), to Walter Segal’s low-tech self-build homes (1980s).

A 1983 television documentary made by World in Action exposed defects in system-built timber houses. The issues were later found to be caused by poor site practices and not widespread, but the damage to the UK timber-frame housing market was profound. However, timber use in Scotland has always held a strong market share (around 80–90%) and since the mid 1990s the timber industry has been re-establishing itself in the rest of the UK. Details evolved to ensure that gaps behind cladding were maintained and ventilated; to prevent condensation in cold roofs; and to accommodate shrinkage. Most of the Design for Manufacture (2005) pilot sites used timber frame or timber structural insulated panel systems (SIPS).

Future: 2020s

The steady re-emergence of timber-based systems has been stimulated by new manufacturing techniques: composites of timber and insulation, SIPS, and large format load bearing panels of cross-laminated timber (CLT) presenting new opportunities for prefabrication. Timber’s potential to lock up or ‘sequester’ carbon dioxide means that it is now being promoted as one of the most environmentally appropriate building materials.

SIPS achieve high levels of airtightness and good insulation, but the panels must be protected adequately from the elements with appropriate detailing. As with most prefabricated systems an inadequate detail is likely to be repeated many times (see NHBC Foundation report NF10, which reported on systemic panel failures in Canada).

Timber frame has also developed increasing panel depth and insulation thickness. Engineered C-studs, made with timber flanges and particle board web, create panels that are conceptually closer to Elementhus than traditional timber frame.

Timber: Summary

The long history of preparing or pre-assembling and then shipping timber frame is perhaps unsurprising. Timber is lightweight and easy to transport and, as probably the oldest construction material, knowledge of joints and fixings have been developed over centuries.

Forested countries in the Baltic and in North America with a wood economy have driven the development of the uses of softwoods. Christoph & Unmack Company, based in Saxony in Germany, industrialised and developed traditions of log building, pre-assembling houses in huge production halls before disassembling them and shipping houses worldwide. Tennessee Valley Authority set out to design mobile houses to house workers on three- to six-year dam building programmes. Amongst other innovations they pioneered demountable sectional volumetric timber houses, which inspired the British post-war Temporary Housing Programme and prefabs.

Financed by Swedish industry, the Elementhus house factory, which operated from 1952 to 1977, was technically very advanced. A sophisticated and expensive factory was established in Mockfjärd, Sweden with unique, automated processes – a plywood box beam (continuously produced and automatically packed with insulation) was fitted together to form walls and floors. However, Elementhus’ traditional appearance made the radical method of production invisible. Calls to cooperate with architects in the 1950s were ignored and history has largely forgotten this ambitious and successful attempt to industrialise the production of houses.

Softwoods are nowadays bonded or formed into composite components, glue-laminated beams and plywood or particle board panels. SIPS (structural insulated panel systems) combining timber and insulation, and CLT (cross-laminated timber) structural panels present new opportunities for panellised and volumetric prefabrication. As the operational energy required for new houses has reduced with more energy efficient designs, previously overlooked embodied energy is coming into focus. Timber from sustainably managed forests is being specified as a unique material that has the ability to lock up or ‘sequester’ carbon dioxide absorbed while the tree was growing. However, the government’s ban on combustible materials in external walls of more than 18m in height following the Grenfell Tower fire is leading even timber proponents to move away from CLT for apartment buildings.