The use of timber as a construction material throughout the ages, and what technological advancements have brought it back to the forefront of the construction industry?
Edwin Westwell - N0521458
Earthfoot and sill mounted structures
Up until around 1100 AD, in areas of Britain that had a healthy supply of wood, structures were commonly constructed of wooden posts, also known as earth posts, pushed into the ground allowing them to stay upright. Crossmembers spanned between the posts giving the structure rigidity. Between these, a weave of wooden strips called wattle was inserted and daubed with a sticky material usually made of a combination of wet soil, clay, sand, animal dung and straw (fig 1). Lastly, a thatched roof made of straw would have been added to protect the interior from the elements. However, the posts inevitably rotted where they contacted the ground, requiring the dwelling to be repaired or rebuilt at least once a generation. These types of structures are now known as earth foot structures and it was their short life span that brought about the first technological breakthroughs in timber construction (Stevekir, 2005).
Fig 1, Wattle and daub. (Nair, A, 2005).
These breakthroughs came in the form of stone plinths, timber sills and strong timber joints, resulting in the birth of what are now known as sill mounted structures. Stone plinths were made up of a row of stones that sat partially below ground at the foot of the structure’s walls. This provided a solid foundation away from damp ground that the sturdy wooden sill sat on (fig 2). Usually made from oak, the sill would have been cut to a rectangular shape using a broad axe and then refined somewhat using an adze which allowed the sill to sit flat on the plinth. Posts, also cut to a rectangular shape, were then inserted into the sill using mortice and tenon joints, held together with oak pegs. The joint was used to connect the crossmembers, also cut into rectangles, to the posts that increased lateral support. This created the skeleton of the dwelling. Like this structure’s predecessor, this timber frame was then infilled with wattle and daubed with a thatched roof to top the structure (Stevekir, 2005).
Fig 2, Timber sill sat on stone plinth. (Westwell, E. 2017)
The mortice and tenon joint (fig 3) is remarkably strong and rigid; it is testament to its quality that it is still widely used by carpenters and joiners today. There are five main ways in which a structural component can fail: bending, compression, extension, torsion and shearing. The mortice and tenon joint prevents most of these but if it were to fail it would be due to shear, occurring if the pegs failed whilst resisting the pulling out of the tenon. This, along with the addition of the plinth, eliminated the main source of damp from the ground and extended the life of sill mounted structures indefinitely, given reasonable maintenance (Stevekir, 2005).
Fig 3, Mortice and tenon joint. (Byrne, L, 2013).