The Context of 19th-Century Bridge Building

Most covered bridges in Canada were built between 1850 and 1920 — a period when timber was abundant, iron was available but expensive, and the engineering profession was still formalizing. The builders who erected these structures were not civil engineers in the contemporary sense. They were master carpenters who had learned bridge framing through hands-on work, often under another builder, and who adapted standard truss configurations to local conditions based on accumulated experience.

Provincial and municipal governments contracted bridge construction through a tender process, and the winning bidder typically organized the entire operation: timber procurement, site preparation, abutment masonry, truss assembly, and enclosure framing. The builder was responsible for the structure standing and carrying traffic; formal load calculations and drawings were rare. Construction was guided by rules of thumb, pattern books, and the builder's own judgment about what worked.

Timber Procurement and Preparation

The primary constraint in 19th-century bridge building was not design knowledge — it was timber supply. A large covered bridge required straight-grained logs in lengths up to 18 metres for chord members, and in consistent cross-sections for truss diagonals and posts. Builders typically contracted with local sawmills or operated their own portable saw operations at the building site.

White pine and eastern spruce were the preferred species in the Maritime provinces and Ontario. Both were available in the large dimensions required and both were workable with the hand tools of the period — broad axes, adzes, chisels, and hand saws. Timber was typically cut in the winter when snow provided a transport surface for moving heavy logs out of the bush, then milled in the spring and allowed to dry partially before installation.

Green — freshly cut — timber was often used in bridge construction because there was no practical alternative. Waiting for full seasoning would have delayed construction by a year or more. Builders accommodated the shrinkage of green timber in their joint designs, leaving space for the wood to move without splitting mortise walls or crushing tenons. The iron tension rods in Howe truss bridges could be retightened after the timber dried and shrank, bringing the truss back into its designed geometry.

Abutment and Foundation Work

Before any timber framing began, builders had to establish the abutments — the masonry or timber crib structures at each end of the span that transferred the bridge loads into the riverbank or bedrock below. In New Brunswick, abutments were typically dry-laid or mortared fieldstone, built by masons working independently of the timber crew.

In locations where bedrock was close to the surface, abutments were founded directly on stone. Where the riverbank was alluvial — sand, gravel, or soft clay — builders drove timber piles or constructed timber crib foundations filled with stone ballast. Crib foundations were particularly common in Quebec and Ontario, where the glacial till underlying many riverbeds made pile driving practical.

Intermediate piers, where required for multi-span bridges, were more complex. The Hartland Bridge required pier construction in the Saint John River, which meant working in moving water. Builders used cofferdams — temporary enclosures of driven planks that were pumped dry — to allow masonry work below the waterline. The stone piers at Hartland, completed in the 1880s during an earlier bridge construction, were retained and incorporated into the 1901 Howe truss structure.

Truss Assembly and Erection

The sequence of truss assembly varied by bridge type, but the general approach involved prefabricating truss panels on the ground — or on a flat work platform adjacent to the bridge site — and then lifting or sliding them into position over the abutments and piers.

For a Howe truss bridge, construction typically began with the bottom chord. Long timber members were joined end-to-end with scarf joints, secured with bolts and iron straps, and laid along the full span length. Vertical posts were mortised into the bottom chord, and diagonal timber compression members were fitted between the posts and the top chord using a combination of mortise-and-tenon and iron connection hardware. Iron tension rods were then threaded through the structure and tightened to bring the truss into its design geometry.

Erection was done without cranes in any modern sense. Heavy timber members were moved using block and tackle rigged from temporary timber A-frames or from the bridge structure itself as it rose. The labour was intensive and the risk of injury was significant. Historical records from New Brunswick bridge construction contracts note injuries during truss erection as a common occurrence, though systematic accident reporting was not required until later in the 20th century.

Deck Construction

The bridge deck — the running surface for vehicles — was typically constructed of transverse timber planks, 50 mm to 75 mm thick, laid directly on longitudinal timber stringers that ran along the length of the bridge. The stringers transferred wheel loads from the deck planks to the truss bottom chord or floor beams.

In some bridges, diagonal planking was used to improve the distribution of point loads from horse-drawn wheels across a wider area of the deck. The diagonal pattern also provided some shear stiffness to the deck plane, contributing to the lateral stability of the structure.

The deck was the most heavily loaded and most frequently maintained component of a covered bridge. Even with the protective enclosure, the deck surface was exposed to tracked-in moisture, road salt (in later decades), and the abrasive wear of iron wheel rims. Most surviving covered bridges have had their decks replaced at least once, and some have had multiple deck replacements while the original truss structure remained in place.

Enclosure Framing: Roof and Siding

The roof and siding were framed after the truss and deck were complete. Roof framing used common rafter construction — pairs of rafters meeting at a central ridge board — with the rafter feet bearing on plates set on the top chord of the truss. The roof pitch was typically between 1:4 and 1:3, sufficient to shed snow without making the structure excessively tall.

Siding was applied horizontally or diagonally in board-and-batten or lap siding patterns. The siding served as weather protection but, as noted in the structural engineering discussion, it also functioned as a shear diaphragm, bracing the truss against lateral loads. In some bridges, the siding boards were applied diagonally specifically to increase the shear stiffness of the wall plane.

Windows — typically narrow horizontal openings cut through the siding — were positioned to provide light to the interior without creating large apertures that would admit driven rain or snow. The spacing and size of windows varied by builder preference and was not standardized across the industry.

Knowledge Transmission and the End of an Era

The knowledge required to build a covered bridge was not written down in any comprehensive way during the era when it was most actively used. Builders carried the knowledge in their heads and hands, transmitting it through apprenticeship. When the bridge-building trade moved to steel and concrete in the early 20th century, the practical knowledge of timber truss construction began to dissipate.

By the mid-20th century, there were few builders in Canada who could construct a timber truss bridge from scratch using traditional methods. Rehabilitation of existing covered bridges increasingly relied on structural engineering consultants who understood the principles of truss mechanics but not always the practical constraints of working with large timber — joint geometry, grain orientation, shrinkage allowances, and the tolerances appropriate to hand-worked connections.

Several Canadian provinces have since developed specialist contractor pools capable of traditional timber bridge work, often in collaboration with heritage programs. The knowledge is being reconstructed from the physical evidence in surviving bridges, supplemented by the small number of published builder's guides and pattern books from the 19th century that have been located in provincial archives.

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