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
Cruck frame construction
The predominant feature of a cruck frame building is the structural A-frames that carry the whole weight of the roof and the walls. These A-frames, also known as crucks, consist of two transverse trusses that meet at the apex where the ridge beam sits, the individual trusses are known as cruck blades. The most common forms are not jointed at the apex but held together, either by a yoke situated at the top where the ridge beam sits or a collar fixed slightly lower, and the blades butt up against the ridge beam (Brown, R. J, 1986, p66).
A tie beam that spans the mid width of the truss also secures the cruck blades together, adding stability. The tie beam projects past each end of the cruck blades, until the ends are directly above the base of the cruck blades. This allows room for the vertical studs to be pegged to the tie beam and the bottom of the cruck blades, forming the vertical external walls. Wall plates running between and connecting each cruck frame are fixed to the tie beams (fig 4) (Brown, R. J, 1986, p67).
Once erected, the base of each cruck blade, that make up the cruck, would sit, depending on the time of construction, on a pad stone or in a timber sill resting on a stone plinth, keeping the timber out of contact with the damp ground. Cruck buildings would need at least two of these crucks to form the gable ends of a building, commonly spaced at around sixteen feet apart. Additional crucks could be added in between, if required, depending on the length of the building. These crucks were commonly made of oak and usually curved using the natural shape of a tree. Cruck blades were often cut from the same tree by splitting the trunk in two and creating identical lengths of timber to make both sides symmetrical. This symmetry not only worked aesthetically and structurally for the building but it also ensured that the timbers would dry and distort in a similar fashion due to them being two halves of one whole (fig 4) (Brown, R. J, 1986, p67).
Fig 4, Cruck frame construction diagram. (Brunskill, 1999)
Crucks then served as a roof truss to support the purlins and ridge beam that spanned between, connecting the crucks along with the wall plate. In turn, this supported the common rafters that were topped by a thatched roof covering. The gable ends and walls of the cruck frame building once constructed, would either be infilled with wattle and daub or clad with timber (Angus, 2007).
The above briefly describes the full or true cruck; there are several other variations that are commonly related to the cruck family but this style is deemed to be the original.
Crucks were predominantly used during the medieval period for large structures, such as barns. The earliest constructions uncovered by archaeologists date back to as early as the 5th century. However, late into the medieval period the bent timbers used for the blades became increasingly rare due to the high demand in the ship building industry. Crucks were still being used for new buildings in the 19th century, although not as abundantly. Over two thousand cruck framed buildings still stand today in the UK, although they are not always easily identifiable due to alterations. The English midlands boast the best examples of these ancient structures (Brown, R. J, 1986, p67).