The appearance of historic oak-framed buildings has always appealed to the British public. Unfortunately, this has often resulted in the surface application of thin oak boards to conventional masonry buildings to simulate oak framing in new buildings. This was commonly done by the Victorians and Edwardians in an attempt to mimic Tudor buildings.

However, there is now a new-found interest in using oak framing as the genuine structure for a wide range of buildings, from small residential to relatively large structures. The structural frame may or may not be exposed on the outside of the building, but the appeal lies largely in the scale and nature of the green (undried) oak framing internally. Traditionally, oak frames were exposed on most houses, whereas for barns, factories and warehouses they were concealed behind brick or timber cladding. Both techniques are now used in new buildings, the latter perhaps more typical of less ‘traditional’ designs.

One example of the modern use of green oak framing is the Olivier Theatre at Bedales School, near Guildford in Surrey, by Feilden Clegg Bradley, built in 1993. This building has been framed in large sections of green oak using traditional carpentry joints and dowels for the smaller spans in the two-storey foyer area. For the larger spans in the auditorium, however, the oak is formed into lattice beams, incorporating steel ties and connections. The oak framing is partially exposed externally, but the more conventional softwood framing between the oak columns has been clad externally with larch boarding, which has also been used in a ‘board on board’ form for the roof.

The architects were very aware of a particular feature of green oak which is that large sections will continue to shrink considerably even after the building is in use, because the sections are too large to be fully dried before they are installed. Detailing must therefore always allow for this movement and for any possible distortion of the wood that may occur during the in-situ drying process. This requires considerable care, particularly when the green wood abuts other components such as glazing, masonry or conventional timber frame construction. This shrinkage will also lead to checking or surface cracking of frame members, externally and particularly internally due to heating. These checks are not structurally significant, but need to be accepted as part of the aesthetic of green oak construction.

The interior of the Olivier Theatre

Darwin College study centre at Cambridge, designed by Dixon & Jones and built in 1990, is another example of a large framed non-residential building using a green oak structure. Another more recent structure is the Abingdon School Boathouse, entirely constructed of green oak, but of more traditional appearance than the first two examples. Perhaps the best known example of modern green oak construction using traditional carpentry joints, and with all the frame members exposed, is the Globe Theatre on the South Bank designed by Peter McCurdy, to re-create as closely as possible the 16th century Shakespearian theatre.

There are now many other examples of this type of construction in the UK, some traditional and others entirely modern in design. The building of several oak framed houses has been illustrated in Channel 4’s Grand Designs programmes, which have undoubtedly raised interest in the use of oak in this way.

Besides new buildings, there is the common use of oak framing for repairing and converting old barns into desirable residences, as well as large conservation and restoration projects, where historic buildings may have been damaged by fire, decay, or general lack of care. The great roof of York Minster had to be repaired in this way using substantial sections of oak, but on a smaller scale this is typical of the repair of many parish churches and chapels throughout the country.

In the majority of these projects home-grown oak has been used, and despite the huge loss of old oak forest, largely for ship building up to the middle of the 19th century, there is now plenty of oak available from regeneration and commercial planting that took place in the 18th and 19th centuries. These trees were then not required for the original purposes such as housebuilding, shipbuilding or even for mining props and they have now matured and are ready for use in quite large sections.

Inside the Globe theatre

There is also an increased use of home-grown oak for other building components such as windows and doors, cladding boards in various profiles, or external decking, whether these are associated with oak structural frames or not. Used green, these components need careful detailing to allow for the shrinkage and possible distortion of the wood. It should be remembered that while using undried wood may save the cost of kiln drying, or the time required for air-drying, there may be extra costs in the fixings necessary to allow for such movement. A particular feature of oak is the high quantity of tannin in the wood, which will exude slowly from wood when exposed to sunlight and weathering. This will be most evident in ‘green’ wood, and will gradually dissipate throughout the bleaching process. Besides possibly leading to unsightly staining of other materials, unless these surfaces are protected, tannin is also very corrosive to mild steel. Any corrosion from mild steel connections can lead to unsightly staining of the wood, and for this reason it is always advisable to use corrosion resistant metals such as stainless steel for any fixings into the oak.

Many people assume that oak exposed to the weather requires a protective coating. This is a misunderstanding that is probably due to the Victorian habit of tarring exposed oak black to emphasise the contrast with other materials, such as white rendered walls. Left unfinished, oak will generally bleach to a silvery grey, and the absence of a coating will have no bearing on its durability. However, the wood may blacken if continually exposed to a high moisture level, airborne pollution, or if it is in close proximity to heavy foliage, which can lead to the growth of surface mould or algae. In these locations there are ways of preventing this effect by surface treatment, but occasional re-treatment may be necessary.

Modern green oak structure – completed frame before enclosure

Home-grown oak has many environmental advantages. Individual trees can be removed from woodland, rather than clear cut like conifers, thereby maintaining the use and appearance of the setting. It is obviously renewable, and there is now much greater emphasis on replanting native hardwoods in the UK. Oak requires little energy in conversion or transport, if it is home-grown. Its natural durability means that there is no need for preservatives or coatings, and consequently little maintenance, and finally it can be re-used in many ways after its original use has been finished, as shown by the frequent historical use of ship’s timbers and other oak components in building construction. • More case studies, including the Downland Gridshell (illustrated) can be found in ‘Green oak in construction’, recently published by TRADA Technology Ltd, in partnership with the Forestry Commission and Scottish Enterprise. Visit www.trada.co.uk/bookshop for details.