If the essence of a timber frame house is the timber, then the joining of the timber is the intrigue. I build timer frame homes for a living and the strength and beauty of the method never ceases to impress me. Since there are no nails and next to no hardware, the placement and accuracy of the joinery are the cornerstones for the integrity of the structure.
Commonly used joints in timber framing are: mortise, tenon, lap and scarf. Architectural aids, such as triangulated braces help to keep the timbers from racking (moving) and add strength.
Mortise/tenon joint: Even if you’ve never picked up a saw or a router, you’re familiar with the mortise and tenon joint. Take a look at nearly any piece of furniture you own. Mortise and tenon joints are the strongest and cleanest choice for joining two pieces of wood at a 90-degree angle.
Interestingly, mortise and tenon joints have been uncovered in archeological sites from around the world, dating back to 2500 B.C. The theory as to why this joint has stood the test of time is that no additional fasteners or glue are needed, if you achieve a good cut. In humid areas, the swelling of the tenon would “lock” even an imperfect cut.
Lap joint: In its most basic form, a lap joint is two pieces of wood cut to overlap one another. There are different forms of lap joints that offer different strengths, including a half lap, center lap and miter lap. In many applications, a lap joint will require an additional fastener. In the image below, a very simple full lap joint uses just gravity.
Scarf joint: A scarf joint is a joint that aligns two pieces of wood or timbers, while keeping the joint in uniform size with the wood being joined. Scarf joints have a low strength and typically require additional fasteners.
With regard to the corner braces and triangulation I mentioned above, timber frame techniques will employ multiple joints in order to create additional strength from the geometry of the timbers. For instance, in the image below, mortise and tenon joints were used to create an angled, corner brace. A triangle is the strongest geometric shape in construction (since most materials prevent the use of full circles).
If you’re working on a smaller project and need help choosing which joint to use, check out this video of a strength test performed by www.finewoodworking.com.