From a carpenters point of view, this variation of square rule, with connecting girts and common purlins, seems to further simplify the prefabrication process, which appears to be one of the primary criteria of square rule as opposed to scribe rule. (I am assuming that the common purlins are tenoned into the principal rafters of the trusses, and are flush with the top surface of the rafters.) The design of the structure is now a series of regularly spaced bents. All the connecting girts and purlins are a common length. This simplifies the cutting list and the actual cutting. There are now no long timbers to procure, or scarf joints to cut,just a large stack of equal length girts and purlins with tenons at both ends. When it comes to rearing, it is simply start at one end and stop at the other, temporarily propping the first and second bents and then fitting the connecting girts and purlins. The next bent is simply repition of this procedure. This variation can be viewed as reducing the design to a simpler modular format, that is still in use today in, using steelwork or reinforced concrete sections rather than timber. It is fascinating and illuminating that this secondary leap of conciousness took place within a decade or two of square rule first emerging as a "new" framing system. It could be interpreted that once the scribe rulebook had been torn up that all elements of design were under re-evaluation. These original designers/carpenters should be celebrated for their contribution to the evolution to the craft.
