Durability and Wood Protection for Historic Covered Bridges in the United States

Covered bridges are iconic structures in the United States. Over half of the 16,000 covered bridges found across the globe are located in the US. Most were built in the mid- 19th century and found mostly east of the Mississippi River. The unique cultural and architectural qualities of covered bridges drive efforts to protect them from biological and physical deterioration as well as structural damage by vandalism and arsonists.

Portland Mills Bridge, Parke County, Indiana

Portland Mills Bridge, Parke County, Indiana

The National Historic Covered Bridge Preservation Program, sponsored by the Federal Highway Administration, was established to preserve these unique historic structures through research to restore, rehabilitate, and protect them. Vina Yang and Carol Clausen of the Forest Products Laboratory’s Durability and Wood Protection research group, presented a poster paper for the International Research Group on Wood Protection. Their poster, Durability and Wood Protection for Historic Covered Bridges in the United States, was presented at a spring conference in St. George, Utah.

The Portland Mills bridge was constructed using a Burr Arch truss.

The Portland Mills bridge was constructed using a Burr Arch truss.

Protecting covered bridges from decay and insect damage is a top goal and typically done through in-place remedial treatments. Naturally-durable locally-sourced wood species for above-ground replacement components are suitable alternatives to treated wood during bridge rehabilitation. Likewise, guidance for selection of replacement fasteners is available.

Fire is a leading cause of loss and damage for covered bridges, sometimes accidental but also commonly by arsonists and vandals. Traditional fire prevention measures such as sprinklers, alarms, and fire retardant treatments have been evaluated along with the development of new technologies based on flame detectors, fiber optic sensors, and infra-red camera systems that could be used to alert authorities to possible acts of vandalism.

Three-dimensional laser scanning is being used to document as-built design details to authenticate restoration efforts. A variety of new remote sensing technologies are also under development, focusing on continuous remote monitoring of biological and physical conditions in bridges.

In-Place Preservative Treatments for Covered Bridges

Most covered bridges are made of wood and can be vulnerable to damage from fungi and insect attack. A recent paper from FPL forest products technologist Stan Lebow and Oregon State University professor Jeff Morrell describes treatment options that help prevent or slow down biodegradation.


Stan Lebow, a Research Forest Products Technologist at FPL.

“Controlling exposure to, and protection from, moisture is one of the best ways to prevent biodeterioration in covered bridges,” says Lebow. “We have found that regular maintenance and attention to needed repairs is extremely helpful for keeping these bridges in working order.” Lebow says that county or local governments charged with maintaining the structural and aesthetic integrity of covered bridges can benefit from the research on in-place preservative treatments.

Deterioration of bridge beams tend to be more common wherever beams contact abutments, are near the ends of bridges subject to wetting from splashing, or are below windows or other openings that allow wind-blown precipitation access to the interior bridge space.

Improved adhesives will help reduce costly repairs to historic covered bridges.

Covered bridges are an important part of the historic fabric in rural communities across the nation.

In-place preservative applications can help limit deterioration when moisture cannot be eliminated. The goal of in-place treatment on a covered bridge is to distribute preservative into areas that may easily get wet from exposure to precipitation. In-place treatments include surface coatings, pastes, rods, gels and fumigants. Some preservative treatments may cause a color change in the treated wood and/or present safety and handling concerns.

One limitation of all these treatments is that they cannot be forced deeply into the wood as is done in pressure-treatment processes. However, some can be applied into the center of large members via treatment holes and can move through the wood by vaporization or diffusion.

Lebow and Morrell used laboratory and field research to compare the movement of water-diffusible and fumigant treatments. The wood in some covered bridge timbers, they found, may be too dry to promote the effective spread of diffusible preservative treatments. Water diffusible treatments must be applied in locations where moisture accumulation is suspected and fumigants have greater potential for movement in dry bridge timbers and wood species that resist moisture movement.

Saving the Past: Restoring and Preserving Historic Covered Bridges

Public Roads, the bimonthly magazine of the Federal Highway Administration, featured an article on their National Historic Covered Bridge Preservation Program, which the Forest Products Laboratory (FPL) has been a part of since its inception in 2000.

Portland Mills Bridge, Parke County, Indiana

Portland Mills Bridge, Parke County, Indiana

According to the article, titled Saving the Past, there are 800 covered bridges in the United States today, but taking care of the structures is challenging. Preserving their unique engineering and construction is historically important, but their designs cannot always handle modern traffic loadings.

The Portland Mills bridge was constructed using a Burr Arch truss.

The Portland Mills bridge was constructed using a Burr Arch truss.

The research and development arm of the preservation program is where FPL got involved. Research focused on developing ways to combine new and traditional techniques to maintain the bridges.

In the article, FPL Assistant Director Michael Ritter explains that the program “provided the opportunity to formulate practical solutions for rehabilitating, repairing, evaluating, and protecting historic covered bridges that are truly national treasures.”

Several publications were produced as a result of this research, including the Covered Bridge Manual and the Guide for In-Place Treatment of Wood in Historic Covered and Modern Bridges.