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.