The popularity of green oak goes back centuries, with oak structures being used as an attractive and robust form of construction in Britain since the Middle Ages. While the use of oak declined in the 17th century, this material continues to come back into fashion thanks to its longevity, versatility and unique charm.
These days, the joy of oak comes from the stunning hand craftsmanship, but also the fact that it is a renewable, home-grown material. It is often used to add eco-friendly extensions that work in harmony with historic properties as well as new homes with bespoke timber solutions.
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 while still carrying high moisture content. As the frame dries out in service it develops the cracks and texture that are characteristic of the material and give it its beauty.
Green oak construction is very niche, and if engineers are employed on the project who aren’t oak specialists, problems can easily arise, increasing the importance of working with a timber frame consultancy. When thinking of building with this unique material, there are a few things related to timber engineering which should be considered:
Used undried or ‘green’ with an initial moisture content of 30% 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.
Tannins and extractives
Oak has 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 non-ferrous metal which your timber frame consultancy will be able to recommend.
A primary decision in the structural design of a green oak frame is to use traditional carpentry joints such as mortice 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 and retain timber design details, 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.
Several green oak timber engineering 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.