The first force I will discuss is compression. Loads bearing on posts are resisted by an equal reaction on the opposite end, resulting in compression force of the timber. The second force is tension. Tension is a force pulling a timber from either end, which causes tensile stress, which amounts to two forces playing tug of war with a timber. Finally there is force of shear and bending movement.
Shear movement is when to forces slide by each other and destroy the material in-between.
Horizontal shear, which is along the grain, is a much more common problem and is caused by adjacent wood fibers being pulled and pushed in parallel but opposite directions. The principal rafter and purlin roof is probably the most popular, so I will discuss this style only. With this type of roof the rafters are joined directly to the tops of principal posts and are connected to each other with a series of purlins spaced about four feet apart. I like this system because it eliminates the need for top plates. It is difficult to get timbers that are long or stable enough for these plates, so they often have to be scarfed together.