Timber Frame Joints and the Science of Joinery

The term “joinery” refers to the practice of attaching two separate pieces of wood to form a more complex structure. Joinery is used in numerous arenas, most notably in making wooden furniture and building structures like houses, churches, and many other types of buildings because timber frame wall construction is extremely popular. Joinery has been practiced for centuries. In fact, furniture dating as far back as ancient Egypt incorporated complex joinery.

Joinery is common in the context of timber frame joinery. Timber frame wall construction is complex, and a single wall may have hundreds of timber frame joints. Timber frame joints are any place in the frame where two or more boards come together. A timber frame corner joint is particularly complex as it tends to incorporate three or even more pieces of wood.

Broadly, there are two types of timber frame joinery. Traditional timber frame joinery tends to include only wood as a building material, without incorporating glue or metal fasteners like screws or nails. Traditional timber frame joinery is beautiful, useful, and simple, and it also has many other benefits, which include the following:

  • added structural integrity
  • durability in fluctuating weather
  • ability to incorporate resinous wood types

In some climates, nontraditional joinery techniques may not be as durable as traditional joinery. The reason for this is that wood contracts and expands depending on the temperature and humidity levels. Metal fasteners, which do not expand and contract, may have difficulty accommodating these changes. Traditional timber frame joinery is also preferable for certain types of wood that have a high resin content. The resin means that glues cannot successfully adhere to the wood.

There are many ways to join multiple pieces of wood, and each method has its own strengths and weaknesses. Three of the most common types of traditional joinery used for timber framing include

  • dovetail joints
  • mortise and tenon joints
  • tongue and fork joints

Dovetail joints are used for many timber framing connections, typically at a point where the end of a timber meets the side of another timber. The end piece has wood chiseled out an angle, so that the end piece looks like it has a triangle with one point driven deep into the wood. A triangle of the same shape is cut into the side of the other timber, and they are joined together. These timber frame joints are extremely sturdy because their shape makes it difficult to disconnect the two pieces once they are joined. Dovetail joints are frequently used in timber framing at connections including

  • rafter and purlin (the long beam that forms the top angle of a peaked roof)
  • the ends of joists (the horizontal pieces that support a floor or ceiling)

Mortise and tenon joints are commonly used for the timber framing joints that bear the heaviest load. They consist of one piece of wood that has a slot or hole cut into it (sometimes referred to as the female end), a second piece that has a cube or block cut into the end (sometimes called the male end) to fit perfectly into the slot. Often, holes are drilled into the block (male end) on the second piece and the sides of the slot (female end) on the first piece so that a joiner can add wooden pegs. These wooden pegs further stabilize and strengthen timber framing joints.

Tongue and fork joints are similar to mortise and tenon joints. However, mortise and tenon joints tend to be used when one end meets the side of another piece, while tongue and fork joints tend to be used when two ends come together at an angle. For example, a tongue and fork joint may be used when two rafter timbers come together to form a peak. In this type of timber frame joinery, one timber has blocks chiseled from the outside to form a “tongue” at the end of the timber. The opposite timber becomes a “fork” when a block is cut out of the center. These timbers are joined together by slotting the tongue into the fork with the timbers at a 45-degree angle. Like with a mortise and tenon, a hold is sometimes created through the connection so that a wooden peg or dowel can slide through both timbers, further strengthening the timber frame corner joint.

Timber frame wall construction on certain buildings requires additional reinforcement. Traditional timber frame joints are beautiful and sturdy, but cannot always bear the heavy loads required by modern buildings. In other cases, a builder might choose to use a slightly weaker wood and strengthen the frame with timber frame brackets. Timber frame brackets are sheets of metal that are shaped to connect to both (or all) pieces of wood that are joined. They are typically fastened to the wood with large, sturdy steel bolts. If a bracket is used, a traditional joinery technique may or may not also be used. In some instances of joinery with brackets, builders may simply lay boards side by side and fasten them with the steel brackets and bolts without doing any additional cutting of the timber.

In some cases, if a timber frame joint will remain exposed after the building is finished, a builder may choose to use timber frame bracket for purely aesthetic reasons, even if it is not required to support the structure. Timber frame brackets have become popular to lend an industrial look in buildings with exposed timbers. They are also sometimes used at a timber frame corner joint, which is more likely to require reinforcement.

Timber frame joinery is a beautiful and complex process, and the plethora of options available ensure that builders can find an optimal solution for every one of the thousands of timber frame joints that make up a single building. Traditional and nontraditional timber framing joints all have their own benefits, so the “right” one to use depends on the particular building and the particular joint in consideration.

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