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Last edited 11 Nov 2022
Green wood timber
 What is green wood?
The term green wood (or green timber) refers to a type of timber which has been freshly cut, and because of its high moisture contents, it is softer to work with, more flexible and less brittle. It relates to the forest term 'greenwood' because it is considered to be living, green or active and thus containing more water, but it is also prone to greater movement and deformation as it looses that water, dries or seasons over time. The term greenwood refers to a living forest during the season when the leaves are lush and green. It was often used to describe an idealised forest setting for historical stories such as Robin Hood.
 Moisture in green wood explained
Moisture content, or %MC, is the measurement of how much water is present in wood compared to the dry matter of the wood. Although not very common, technically, it is possible to have a moisture content of over 100% and up to 200%, this happens when there is more water than wood in the sample being tested. Green wood has significantly more free water in it than seasoned wood, this means it is less stable and likely to move as it seasons.
A guide to the use of urban timber (FB 50), written by Geoff Cooper and published by BRE on 31 July 2012, defines green timber as: 'A term used to describe wood which has been freshly cut and still contains a high percentage of moisture (free moisture).'
Free water or free moisture refers to water that is stored in the cell cavities, water that is stored within the cell walls is referred to as bound water. The higher the %MC of the timber, the more free water it contains between the cells, and green wood tends have very high levels of moisture, up to 100% or more. This is significantly higher than the Fibre Saturation Point (FSP) which is around 30%, under this percentage the bound water is within the cells. The drying of bound water generally has a greater impact on the shape or form of timber.
Once seasoned, timber becomes more stable and harder, so most timber products are seasoned before use, however traditional green wood or green oak framing makes use of non-seasoned wood to ease cutting and preparation of the timber frames, particularly of the joints. Traditional timber frames are a common part of UK and European vernacular buildings, most would have been built using green wood joints with timber dowels that removed the need for metal fixings.
Timber dowels are small round pins, usually of oak or a similar material that has been seasoned. The dowel is dry when it is inserted into a joint of green wood, so it soaks the moisture from the freshly cut wood and expands. The expansion of the dowel creates a tight fixing for the green wood. Over time and exposure to the elements, the frame and the dowel season, become drier and harden and create a rigid connection and stable frame.
Many traditional houses of the oak frame tradition were constructed this way. Often a frame would be left for a year without cladding to allow it to season and become rigid. It would then be clad and walls in-filled with wattle and daub.
Innovative use of green wood in contemporary architecture has included the construction of timber gridshells, in particular the Weald and Downland Living Museum gridshell built by Edward Cullinan Architects, Buro Happold and the Green Oak Carpentry Company. This gridshell structure used the flexibility of green wood oak, through a diagonal grid of laths laid flat on supporting scaffold, the edges lowering gradually over weeks, forming a three hump shell. Once in position the laths, secured at the edges to curve base frame, created a stable structure which over time dried and seasoned, hardening to becoming a rigid curved building frame.
For more information and photographs of the construction visit https://www.cullinanstudio.com/project-downland-gridshell.
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