Last month, we looked at detailing cathedral ceilings that are supported by structures running lengthwise. This month, we will consider the other case, where the main structure spans the width of the space. Exposed Trusses When the span is longer than it is wide, trusses are in order. (In some cases, trusses are also attractive in short, wide spaces.) First, let's consider trusses that are entirely visible within the room. In this case, these trusses carry purlins that run lengthwise in the room and support the roof. Such purlins have two problems. First, when you set the purlins on the trusses, do you set them vertically – at an angle to the roof plane – or do you set