Robin Lancashire of Exova BM TRADA discusses Britain’s enduring appreciation of green oak framing and offers some points of good practice.
The popularity of green oak goes back centuries, with oak structures being used as a robust form of construction in Britain since the Middle Ages. While the use of oak has declined due to cheaper alternatives and fewer people with the specialist skills required to build with it, this material remains in fashion as a premium product thanks to its longevity, versatility and unique charm.
Today, much of the joy of oak comes from the stunning hand craftsmanship that is visible in the exposed structural elements that characterise it. The fact that it is a renewable, home-grown and traditional material is also appealing. It is often used to
add eco-friendly extensions that work in harmony with historic properties, but is also used for buildings such as theatres, galleries, restaurants, museums and visitor centres – as well as new homes.
One of the major differences between building with oak and most other timber construction methods is that it is used when green – i.e. recently felled and still carrying a relatively high moisture content. The large section sizes used means that it takes several years to dry. This happens when the frame is in service, and so this is when it develops the cracks and texture that are characteris- tic of the material and give it its aesthetic.
Green oak construction is a niche form of construction and if engineers are employed on the project who aren’t oak specialists, problems can easily arise. When thinking of building with this unique material, there are a few things which should be considered.
Used undried or ‘green’ with an initial moisture content of 30 per cent or more, the timber shrinks considerably as it dries, which may result in splits or distortions. While careful selection can reduce this risk, it is not always possible to predict how the wood will behave when it is in its green state. Care must be taken in the design to ensure that any shrinkage or distortion will not cause damage to other components or reduce the weather resistance of the building. If it is simply aesthetic, it may just need to be accepted as part of the “charm” of using this material.
Many of our clients have heard about the need to design and construct allowances for timber movement, but do not know how to do this. Others are concerned that work may require guidance through on site consultancy. On some occasions, clients become aware of a problem with the building in use, but do not know what has caused it, if it is important, or who should be responsible for it.
Tannins and extractives
Oak has a high extractive content (commonly known as tannins), particularly when the wood is used green. Tannins appear as a dark brown exudation on exposed surfaces as a result of exposure to weather and may run down onto surfaces below – so protection is always advisable below exposed oak frames. These extractives are also very corrosive to mild steel, so fixings into oak used externally should always be of stainless steel or nonferrous metal.
A primary decision in the structural design of a green oak frame is to use traditional carpentry joints such as mortise and tenon, dovetails and scarfed joints with traditional pegs and wedges, or modern joints comprising steel plates and screws, connectors and tie rods.
Where steel connections are to be used, it is preferable to use the wood at a lower moisture content than for traditional carpentry joints and this is best done by limiting the size of the sections. Small sections can be dried more easily and the amount of shrinkage that will occur is therefore reduced.
Although traditionally an oak structural frame would be exposed on the inside and the outside, today it is not possible to do this while also achieving the air tightness and thermal insulation required under Building Regulations. To get around this, the easiest way is to conceal the frame on the outside with a suitable cladding system and expose the oak frame on the inside of the building only – but this changes the traditional appearance of the building, which is often the motivation for using the material in the first place. Alternatively, a thinner separate secondary oak frame or decorative timber sections can be incorporated into the wall outside the thermal insulation layer.
Several green oak building manufacturers have developed systems that use either prefabricated insulated panels, or insulated softwood stud framing to meet the current regulations. These systems generally increase the wall insulation well above the required level to compensate for any heat loss through the structural columns or at the junctions between infill wall and column.
Robin Lancashire is the senior timber frame consultant at Exova BM TRADA