Early bridges, built before sawn timber was available, were built of timber cut at or near the bridge site. The timbers were hand-hewn with the broad axe and adze, cut to size and drilled for fastening together with wooden dowels. About 1870, Michigan pine was shipped in and widely used. The arch rings for the Burr truss were cut from poplar which was easy to work, and the wide boards were cut in the segment of a circle, Oak, being very hard and durable, was used for the floor boards. Poplar was used for the siding as it has the ability to form a pulpy surface coating which is resistant to weathering. The roof was covered with hand-split wood shakes before wood shingles became a standard mill product.
The basic materials used were stone for the abutments and piers and timber for framing the structure. Later, iron rods were used in many of the bridges to tie the trusses together and bolts replaced the wooden dowels for joining the members. Concrete for building abutments and piers did not come into general use until the 20th century.
The first step in building was the construction of the abutments at each end of the span and piers in the stream for multiple span bridges. Abutments and piers were built of stone, quarried and shaped in large blocks and hauled to the site for erection. Excavation for the abutments and piers was carried to below the low water level of the stream to solid earth so the timber and wood piling, when required, would not rot.
A grillage consisting of two or three layers of heavy timbers placed crosswise to each other and covered with a layer of heavy planks was placed in the excavation to form a platform for the stone work. When poor soil was encountered, wood piling was driven to solid bearing and the grillage set on the piling. Contracts were awarded for the entire structure, or separate contracts were awarded for the abutments and the structure.
When they were awarded for the entire structure, the builder had the option of building the entire structure or awarding a subcontract for the masonry work to contractors skilled in this type of construction.
The timbers for the trusses were either laid out on the ground at the bridge site in the general shape of the truss, then scribed to proper size and cut, or this work was performed in the builder’s work yard, and the timbers hauled to the site for erection. Trusses were built with a slight comber to prevent the floor from sagging. This was done by forming a slight upward curve in the bottom chord of the truss so that when the loads were applied, the floor would deflect to nearly level.
Before the erection of the bridge, "false work" was erected between abutments. This "false work" consisted of poles cut at or near the site, or hauled from the builder's yard, driven into the stream bed, braced and covered with planks, providing a working platform for erecting the bridge. The bridge was then erected, and upon completion the false work was removed.
A bridge of approximately 100 ft, span, after the abutments were in place, could be built in approximately three months, using three to four skilled carpenters and one or two unskilled helpers. If the site was too far to travel back and forth, the men boarded with a nearby farmer or pitched a tent and did their own cooking.
Numerous bridges built at a curve in the road had a window at the end in order to see oncoming traffic and windows built into the side to let in light. An open space was left between the top of the siding and the eaves of the roof to let in light and to relieve the air pressure of the tunnel effect of the bridge. Various theories have been advanced as to why bridges were covered. The real reason was that as long as the timbers were kept covered, they were protected from the elements and would last for many years. This is evidenced by many bridges over 100 years old still in daily use. J.W. Britton (deceased], Midland, Michigan.
Wayne M. Weber, Covered Bridges in Indiana (Midland, Michigan: Northwood Institute)