The Plank-lattice Truss (often referred to as the Town Lattice Truss)
The plank-lattice design was first patented in 1820 by architect Ithiel Town of New Haven, Connecticut. Mr. Town promoted the use of his invention, sending agents all over New England. If a builder chose to use the design, Town's agent would license him for a one-dollar royalty per foot of bridge. If a builder was found using the design without permission, the royalty fee was doubled. Town made his fortune not by building bridges, but by selling the rights to use his design.
The timber bridges were fastened together with mortise and tenon, tediously drilled, chiseled, and--finally--pegged. The lattice bridge is constructed entirely of planks instead of the heavy timbers used in the Queenpost and Kingpost trusses. The planks, usually of spruce or hemlock, are simply fastened with hardwood pegs called treenails (pronounced trunnels). A lot of treenails are needed however--it was noted by a bridge carpenter that over 2,500 holes must be drilled to receive nearly a thousand treenails to assemble a 100-foot lattice highway bridge.
The treenails are used to pin the chord planks together on both sides of the lattice and to pin the lattice planks together where they cross. The chords usually consist of four layers of planks-- two layers on each side of the lattice. A strong chord requires that the planks should not be less than thirty feet long and that the plank pairs should overlap one another by half their lengths. The chord-plank pairs are pinned together at a lattice-plank crossing with three or four treenails. The lattice-plank crossings are secured with two treenails except where the chords cross.
The plank-lattice design calls for four chords on each truss--the lower chord and secondary lower chord and the upper chord and secondary upper chord. The bridges built with the four pairs of chords have held up well over the years against wind, water, and heavy loads. Only three of Vermont's plank-lattice bridges built without upper secondary chords survive--the School House Bridge in North Troy, the Scott Bridge in Townshend, and the Worrall Bridge in Rockingham. All three have required additional bracing.
"The main thing about the plank lattice was, it was the cheapest and easiest to build," says Paul Ide, framer and joiner of bridges and barns. "Any carpenter would be able to build it quickly, without large dimension timber. You didn't need anything thicker than three inches for your chords, and generally, you didn't need a plank longer than thirty-two feet."
The usual angle in the lattices constructed in Vermont is 45 degrees, with some as steep as 55 degrees. The average length of Vermont's surviving lattice truss bridges is 105 feet. Plank sizes are typically three by eleven inches with the lattice spaced at three-foot intervals.
From Spanning Time Vermont’s Covered Bridges, by Joseph C. Nelson