Purlin

The lean-to roof had begun to sag where the rafters ran too far without support, and Nyra meant to set a purlin to carry them. She and the girl raised the long seasoned timber up onto the braced posts and laid it level across the backs of the rafters, so that it crossed each one in turn. Nyra pegged it at every meeting place until the whole slope of the roof came to rest along that single line.

The girl had thought each rafter should be made thicker to hold its own share of the weather. She could not see why one timber laid crosswise would do more than many stout ones standing alone. It seemed to her that strength lived in each separate piece, not in the line that joined them together.

Nyra told her that a rafter left alone must bear whatever falls upon it, and the longest ones bear the most and sag the soonest. The purlin gathers the rafters into one span and shares the load along its length. No single rafter carries more than it can, because the timber laid across them lends from the strong to the weak.

What the purlin gives is not more wood but a path for the weight to travel. The roof holds now because the burden is spread wide and laid down evenly, each rafter leaning a little on the line that crosses it. A load carried along a whole span presses on nothing hard enough to break it.

Later, writing the day into her book, Nyra set down only that the lean-to was braced and the roof made sound before the rains. A book gathers what can be set down in a line and counted, and leaves the rest to the one who was there. The book keeps the line, and you keep the spanning of it.