The Town Lattice

In 1835, Ithiel Town — an architect better known for Gothic Revival churches — patented a timber truss that required no skilled joinery whatsoever. The Town lattice is exactly what it looks like: two layers of diagonal planks crossing each other at angles of roughly 45 degrees, pinned at every intersection with treenails and lapped at the top and bottom chords. No mortises, no tenons, no specialist carpenter. If a community had a sawmill and a supply of wooden pegs, it could build a bridge.

The geometry does the work. Each plank runs from chord to chord on the diagonal, and because every intersection is pinned, the lattice distributes load across dozens of members simultaneously rather than concentrating stress in a handful of large timbers. There is no single critical diagonal to fail; the redundancy is built into the pattern itself. This made the Town lattice unusually forgiving — damage one member and the rest carry on.

Construction required quantity rather than precision. Planks were typically two to three inches thick and perhaps ten inches wide, sawn to length and stacked into the lattice in two layers offset from each other. Treenails — hardwood pins driven through pre-bored holes — swelled with moisture and locked the connections tight. Because each plank was a standard commodity dimension, a contractor could price the job from a lumber yard list and raise the truss with ordinary labor. Town reportedly collected a toll of one dollar per foot from builders who licensed his design, and two dollars per foot from those who used it without asking.

The truss was widely popular through the middle of the nineteenth century, particularly in New England, and survives in several covered bridges in the Northeast today. Its weakness is the chord connections: where the diagonal planks meet the top and bottom chords, the accumulated shear across many small pins can open up over time. Inspection of a Town lattice concentrates on those chord laps and on the condition of the treenails themselves — a loose pin is a working pin, and enough working pins signal a connection that is redistributing load it should be holding still.

This made the Town lattice unusually forgiving — damage one member and the rest carry on.

The elegance of the design is that it made structural engineering a matter of carpentry. The patent sold that simplicity.