The purlin was the horizontal beam that ran between the rafters. Where the rafter went up and down the roof the purlin went side to side. The purlin tied the rafters together the way the rungs of a ladder tie the rails. Without the purlin the rafters were just a row of sticks leaning against each other. The purlin made them a roof.

The purlin carried the sheathing. The boards that covered the roof sat on the purlins and the purlins sat on the rafters and the rafters sat on the walls and the walls sat on the foundation and the foundation sat on the earth. The purlin was in the middle of the chain but the chain does not work without the middle. The middle is where the load transfers. The purlin was the building's translator. It took the weight from one direction and sent it in another.

Nobody talked about the purlin. The rafter got mentioned. The ridge beam got mentioned. The purlin sat between them doing its job and nobody said its name. The purlin was the building's middle child. Not the oldest beam and not the youngest beam. The beam that held everything together while the other beams got the credit. Every building has a purlin. Every family has one too.

The purlin was mortised. The carpenter cut a rectangular hole in the rafter and the end of the purlin fit into the hole and the joint was tight and the joint held for a hundred years without a nail. The mortise was the carpenter's signature. A tight mortise said this man knew his trade. A loose mortise said this man was in a hurry. You can tell how much a carpenter cared about a building by looking at the mortise joints in the attic where nobody would ever look. Care that nobody sees is the only care that counts.

They do not use purlins anymore. The truss system spans the whole roof in one piece and the purlin is not needed because the truss is its own purlin. The truss eliminated the middle. The truss is faster and cheaper and the truss does not need a man who knows how to cut a mortise. The mortise took ten minutes. The metal plate takes ten seconds. We saved nine minutes and fifty seconds and lost the signature.

See also: Rafter, Joist