Evaluating Wood Species and Preservatives for Highway Sign Posts

Wooden sign posts have been around about as long as roads. While most people take such roadside attractions for granted, researchers at the Forest Products Laboratory – as usual – take the time to investigate potential improvements.

Lebow

GTR-231 lead author Stan Lebow.

FPL research scientists Stan Lebow, Robert Ross, and Sam Zelinka, in cooperation with the Wisconsin Department of Transportation (WisDOT), have worked to evaluate options for alternative species used as sign posts as well as the preservatives used to enhance durability and lengthen service life. Their work is now available as a General Technical Report (FPL-GTR-231) titled Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts.

Most wooden sign posts are made of red pine or Southern Pine. Treated with chromated copper arsenate (CCA), these posts generally experience solid performance and a satisfactory service life.There are some concerns, however, including warping, sourcing of the wood, and whether CCA is the optimal treatment solution for various alternative wood species.

Warp is sometimes a problem over the long term service life of some pine posts. Use of locally sourced wood species could decrease transportation costs associated with shipping posts from longer distances while benefiting regional wood industries supplying locally sourced products. Although CCA is an effective preservative, it may not be the optimum treatment for non-pine wood species. The report reviews engineering properties and characteristics of alternative wood species such as ash, aspen, hard and soft maples, basswood, hemlock, and spruce, among others.

The report concludes that WisDOT’s current practice of using red pine or Southern Pine posts treated with CCA may be the best combination of wood species and preservatives currently available. Red pine and Southern Pine are readily available and relatively strong compared with many other softwood species. Red pine is also an important commercial resource for Wisconsin and the upper Midwest. Other potential regional species to consider include eastern white pine and soft maples. However, strength may be a concern with eastern white pine, whereas cost may be a concern with soft maples.

CCA is an effective preservative, readily treats red pine and Southern Pine, and is compatible with aluminum signs. Copper naphthenate in oil solvent appears to be one of the most logical alternatives to CCA, and would be a strong candidate for treatment of hardwoods. Copper naphthenate is non-corrosive to aluminum and would avoid warp associated with re-drying after treatment with water-based preservatives.