To understand the difference between German framing and American framing we need to understand the most fundamental variation. In an American frame, the single most important joint is the tying joint. The entire frame is wholly reliant on the proper execution of this joint to function. (There are notable exceptions where trussing action is relied on to accomplish this goal so that the ceiling space is open) In German framing, this joint does not exist at all, except in a few barns built according to the older style. There is no such thing as a tying joint in this tradition; it has been totally discarded.
But we still must resist outward thrust.
German frames use 2 techniques simultaneously to deal with this issue. First, there is a joint system in every frame that resists thrust, and does so quite well. But it is not a tying joint. Instead they use the floor or ceiling joists which always run across the structure to do this. These joists are sandwiched between two horizontal timbers, with simple cogged or dovetailed joints extending about an inch into both of them. These joints are known as kammverbindungen, or comb joints. The timber below the joists is the top plate, and the one below is a special sill that supports the roof framing known as a Stuhlschwelle (literally chair sill, although the roof framework is known as Dachstuhl, or roof chair) This sill generally supports leaning posts that hold up purlins, which in turn support collar beams which tie rafter pairs together and give them support.
The other system used to resist thrust is the Dachstuhl, or roof support, itself. This system effectively transfers most of the roof load into a downward force, bringing outward thrust to a minimum. German roof framing relies exclusively on purlins for roof support, and not on complex trussing. Although at times the purlin support framework can take on the form of a complex, truss-like system. The two types used on houses are the Stehender Dachstuhl, which relies on directly posted purlins, and the liegender Dachstuhl, which relies on leaning posts with special bracing to support the purlins. It should be noted that these posts lean toward the center of the structure, however, and not outward like the leaning posts in many American barns. They rely on struts to convert loads from horizontal to vertical. They also have proven their effectiveness over some very large open spans.
As far as rafters and joists go, spacing varies from region to region. It would appear that in the north it is common for rafter and joist spacing to match post spacing (and post spacing in these regions is perhaps closer than in others), with rafters joining directly into the joists, while in southern regions it is far more common for the rafters and joists to be much smaller and much closer together, with the rafters passing over a special sill to form wide overhangs. Post spacing in the southern rural areas seems to be the widest, at times perhaps exceeding 4 feet. The large Bernese Swiss Bauernhaus seems to employ the widest post spacing out of any of the styles I have seen, averaging around 4 feet or a little less, while joist and rafter spacing seems to be the closest (around 2 feet to perhaps 32 inches) These same Bauernhäuser also tend to be the largest fachwerk buildings.
