My assumption of tenoned purlins, was based on contemporary English carpentry, so it will be interesting to find out if this detail was retained in the "new improved square rule" or if the purlins were all housed. Presumably there must be some fixings between at least one of the purlins and the rafters to prevent the other purlins falling out of their housings. How closely spaced are these common purlins? about 4 foot apart or closer?
