The bulk of the carpentry work is carried out in the workshop using mainly traditional specialist hand tools in addition to modern, state of the art machinery where applicable.
All joints throughout a frame are fitted carefully, making skillful use of classic pegged mortise and tenon joints of a range of shapes and types. This tried and tested method of fabrication is known for its strength and durability.
Let's take a look at how we do it...
First of all, with a design approved and a schedule established, the timbers are ordered from the sawmill... often amounting to many tonnes. Upon delivery to our workshop, they are sorted and inspected individually to ensure they are free of defects such as excessive knots, bowing, shake, sap-wood or cross-grain.
With each timber allocated a specific position in the frame, they are re-stacked to allow best access as the fabrication progresses.
For example, a roof truss will typically be laid flat for working on trestles. A wall frame may be laid with its sole and wall plates positioned parallel to one another, with posts placed across them for marking. Accuracy is paramount. Each plane of the oak frame takes shape on the trestles as the joints are shaped and fitted.
Each component frame will often be assembled, dismantled and re-assembled two or three times as it progresses through the various stages of marking and jointing.
Occasionally, we are commissioned to design and fabricate an innovative structure which requires its own individual approach to laying out, marking and working.
In such instances, techniques can be developed to accommodate the frame's features and allow us to mark and joint the timbers. Custom made brackets and even modified tools may be as novel as the structural design itself.
Whilst largely determined during the design stage, the finer carpentry details are often decided upon in response to the features of the timber to make best use of the particular strengths of every piece of wood - positioning, sizing and shaping accordingly.
Green oak, by its nature, is a dynamic material and, as such, demands a high standard of craftsmanship and a thorough understanding of the material. It weighs around a tonne per cubic metre so handling it can, at times, also be physically demanding. Strength and durability are often as important in a framer as in the frames they make.
This doesn't harm the timbers in any way at all in wet weather since, being 'green' and of a high moisture content, they won't absorb any additional moisture. However, during prolonged periods of hot and dry weather, it may be necessary to protect them from drying too quickly.
There are many different types of joints, which are central to any oak framed structure. Those most commonly used in timber framing include a variety of mortise and tenon joints, dovetails, bridles, scarfs and housings. On some frames, however, the design may require a number of novel and complex joints conceived and shaped for that single application.
Each is applied according to the eventual position and purpose of the joint within the structure and the forces acting upon it. In its shaping, precision is of the utmost importance. An inappropriate or loose fitting joint has the potential to shift the stresses it has been designed to withstand onto another part of the frame not necessarily designed to take it. This is where the knowledge and skill of the carpenter comes to the fore.
You can read more about the various tools we use to cut and shape the joints of a green oak frame on our Carpentry Hand Tools page.
In Roman numerals, carpenter's marks ensure the correct positioning of each part of the structure. Whilst we like to keep them proportionate and decorative, they are essential as no two timbers in a frame are ever the same. Each piece will only ever fit correctly in its own special place.
They are cut to a variety of sizes and shapes, depending upon their position within the frame. The deflection of their curves are marked and cut in line with the grain of the wood before being positioned across the laid out frame for marking, shaping and fitting.
Where braces are required to take additional load, such as arch or sling braces, their dimensions are increased accordingly - sometimes as large in section as any other timber in the frame.
All joints are secured with oak pegs, sized according to their position. Through off-set holes, they draw a joint tight when driven in.
Even a relatively small sun room can use as many as a hundred pegs - several hundred for larger projects. In the workshop, joints are held temporarily with podgers.
When all the timbers have been shaped and marked, they are stacked carefully to ensure their stability against warping and bowing, ready for transport to site and construction.
Sometimes, a frame may be ready for fitting before the site is ready. Whilst it is generally held that an oak frame is best erected as soon as possible after fabrication in order to minimise any effects of drying, its timbers can be stacked for a period either in the yard or on site.
You can read more about the process of delivering and erecting a green oak frame on our Oak Frame Construction page.
A brief look at some of the traditional carpentry hand tools we use in our workshop.
As well as the different types of joints used in oak framing, there is a wide variety of forms within those types. Their shapes are often adapted to suit their individual positions and roles within a frame and, although they can be grouped together by overall type, there is often some commonality of shape and function between types.
This is the most common joint used in oak framing, with variants in pretty much every frame. In its simplest form, a tenon on the end of one timber will slot into a mortise cut into another, fixed in place with one or more pegs, with the purpose of holding the timbers together tightly. However, the shape of each part must account for the size and shape of the timbers, the position of the joint and the forces acting upon it.
Very occasionally, we are asked to create a structure from reclaimed timbers of irregular shape, where jointing can be very much complicated and often precludes the use of modern framing tools - requiring reversion to more ancient methods.
A bridle joint is very similar to a mortise and tenon joint in many ways, but is open ended. They are often found at the apex of a truss where a pair of principal rafters meet, though also sometimes where they join with posts.
The principle of a dovetail is that it widens within a joint to prevent it pulling apart. As with other joints, there are may different forms.
Simple socketed dovetail.
Socketed clasp dovetail.
Dovetail tenons often secure collars to principal rafters, used in tension to resist roof spread and further secured with opposing wedges.
The purpose of a housing or socket is to carry the load of a timber from beneath without bearing that load upon the part of the joint securing it in place.
Longer frames often require longitudinal timbers such as ridge beams and wall plates to be joined end to end with scarf joints. There are many forms of scarf joint, their use dependent upon their application and the nature of other joints nearby.
A simple mitred scarf.
A complex table scarf shaped to permit nearby housing and jointing of numerous other timbers from various directions and angles.
A question we are asked from time to time.
The answer is: "it depends."
Look around our oak frame gallery to see the green oak building projects we've completed over the many years we've been crafting them.
This is a brief overview of workshop processes at Sessile Oak, rather than a guide to the science and art of timber
framing. Structural timber framing should not be practised without proper training and experience or professional guidance.
Green Oak Frame
Green Oak Frame
