Why is the total length of the ancient building girder the sum of the total span plus the diameters of two purlins?

When building a house in the countryside, the roof truss is made of beam-column wood, and the two rooms are not connected by partition support, so a girder is supported by two pillars, and wood is placed on both sides of the girder. In order to increase the height and firmness, a shorter and thinner small beam (secondary beam) is sometimes placed parallel to the main beam.

As the saying goes: the beam is crooked, which is the beam.

The big stilts and the second stilts above are the main beams and secondary beams here.

The wood on the roof is covered with straw and grass, and there are eaves in front and back (north and south) for rainwater to come out. This unearthed piece (30-40 cm) should be supported by small pieces of fine wood, each about 30 cm. These small bars are "rafters". As the saying goes, "the rafters in the early days rot first", that is to say, the rafters in the early days are exposed to the wind and the sun, and of course they are corroded first.

Purlins are not "purlins", but also called purlins, or logs on beams. The so-called "column" is the main support when building a house, and the "building" here is the purlin.

There are probably some proverbs and allusions related to these words, such as the gentleman on the beam, so I won't say more.