Covered bridges are designed for protection from the elements, but it is difficult to protect all wood components from moisture exposure. Some areas of the structures, such as weatherboarding and wood members near the ends of the bridges, are particularly susceptible to wind-driven precipitation. Consequently, biodeterioration from decay and insects is frequently responsible for replacement of aboveground covered bridge components. Naturally durable domestic wood species, particularly underutilized or invasive species, should provide a suitable alternative to chemically treated wood products for repair and replacement of aboveground components of covered bridges. Chemical extractives have long been recognized as key features that impart natural durability in the heartwood of certain wood species. The extractives themselves have been evaluated for use as environmentally friendly preservatives or additives to coatings, but little is known about the chemical makeup of the extractives. Research is needed to isolate, identify, and characterize the chemical extractives believed to be responsible for high resistance to decay and insects in well-known and lesser-known durable domestic wood species. A comparison of chemical properties of well-known and lesser-known wood species will improve our understanding of the differences in physical properties and offer opportunities as suitable materials for repair and rehabilitation of covered bridge components in protected outdoor exposure. The chemical component(s) responsible for imparting durability could also provide new environmentally benign wood protection systems for above ground components of covered bridges.
The overall findings of this study indicate that several of the species evaluated have benefits for use in above-ground replacement components for covered bridges. For complete information please read the final report by Kirker, Clausen, Blodgett, and Lebow