A Howe truss is a structural framework patented by William Howe in 1840 that uses vertical metal rods for tension and diagonal members for compression. It became the dominant bridge design in America from the 1840s through the 1870s and remains one of the most popular truss forms for timber roof construction today.
How a Howe Truss Works
The defining feature of a Howe truss is the direction of its diagonal and vertical members. The diagonals slope inward toward the center of the span and handle compressive forces (pushing together), while the vertical members handle tensile forces (pulling apart). This arrangement is the opposite of a Pratt truss, where the diagonals carry tension and the verticals carry compression.
Howe’s original design paired timber diagonals with wrought iron threaded rods for the verticals. Because wood performs well under compression and iron excels under tension, each material was doing what it does best. This combination of wood and iron, called a “combination truss,” was a significant step in the eventual transition to all-metal bridge construction. By 1845, builders were constructing entirely iron Howe trusses.
The vertical iron rods gave the design another practical advantage: builders could tighten the nuts on the threaded rods to prestress the entire structure, pulling the joints snug and stiffening the bridge. Earlier all-wood trusses required driving wooden wedges to achieve a similar effect, which was slower and less precise.
Why It Dominated 19th-Century Bridges
William Howe was the first truss designer to base his layout on the stress analysis methods available at the time. Previous truss designs were difficult to analyze mathematically, which made them harder to engineer confidently for heavier loads. Howe’s approach, combined with a few key structural innovations, made his truss the clear favorite during America’s railroad boom.
Two improvements proved critical. First, each diagonal spanned only one panel of the truss rather than crossing multiple panels. This simplified both analysis and construction. Second, Howe introduced iron castings at the joints with webs that passed through the horizontal chords, transferring force directly from the rods to the diagonals. This eliminated a weak point where loads would otherwise press perpendicular to the wood grain, a direction in which timber is far weaker.
The result, as the American Society of Civil Engineers described it, was a truss “wonderfully adapted to the properties of wood, statically indeterminate, prestressed, easily adjusted and maintained.” Its comparatively simple erection process also mattered: railroad companies building hundreds of bridges across the expanding network needed designs that crews could assemble quickly and repair in the field by simply tightening a rod or replacing a timber member.
Howe Truss vs. Pratt Truss
The easiest way to tell a Howe truss from a Pratt truss is to look at the diagonals. In a Howe truss, the diagonals form a pattern that slopes upward toward the center, creating a series of “V” shapes when viewed from the side. In a Pratt truss, the diagonals slope the other way, forming an inverted pattern.
This difference isn’t just visual. Because the Howe truss puts its diagonals in compression, timber works well for those members. The Pratt truss puts its diagonals in tension, which favors steel or iron. When steel became cheap and widely available in the late 1800s, the Pratt truss gained an advantage for metal bridges, since thin steel rods handle tension efficiently but need to be bulkier to resist compression without buckling. For timber construction, though, the Howe layout remains the more practical choice.
Famous Howe Truss Bridges
Many surviving Howe truss bridges are covered bridges, since the wooden members needed protection from weather. The Hartland Covered Bridge in New Brunswick, Canada, is one of the best known, with a main span of 51 meters. Knight’s Ferry Bridge in California, built in the 1860s, is another well-preserved example at about 39 meters.
The design also spread internationally. The Russeintobel Bridge in Switzerland has a main span of over 56 meters, and the Akkagawa Bridge in Japan spans 46 meters. Even a pedestrian bridge in Spain, the Pasarela Santa María, was built using the Howe configuration as recently as 1997. Several American covered bridges from the 1880s, including the Ballard Road Covered Bridge (1883) and the Glen Helen Bridge (1886), still stand as historical landmarks.
Modern Roof Applications
While its bridge-building heyday was the mid-1800s, the Howe truss is far from obsolete. The Timber Frame Engineering Council calls it “the most popular, practical, and efficient truss form for moderate span applications” in roof construction. It is by far the most common parallel chord truss type, used ahead of both Pratt and Warren configurations.
The same principle that made it effective for bridges applies to roofs: steel rods handle the tension in the verticals while timber handles the compression in the diagonals and chords. This keeps the design lightweight and cost-effective. Parallel chord Howe trusses are often used for long spans exceeding 50 feet, making them a common choice for warehouses, event halls, churches, and other buildings that need wide open interiors without columns. The design traces a direct line from the 19th-century mill buildings, which featured timber roof trusses modeled after patented bridge designs, to the engineered timber structures built today.