Treated lumber is a popular building material that plays a key role in the construction of many structures in the Pacific Northwest. Our good friend, Dennis McWhirter of Exterior Wood, is an expert on pressure-treated wood, with nearly three decades of experience in the industry. In this series, Dennis provides answers to some common questions and reminds us why treating lumber is one of the best things we can do to help preserve our greatest renewable resource.

In today’s episode, Dennis explains the difference between pressure-treated lumber and railroad ties.

What is pressure-treated lumber?

Pressure-treated lumber is lumber that has been chemically treated to resist rot and decay. It’s a fairly common building material in the Pacific Northwest, as the moisture in our climate makes wood particularly susceptible to rot.

What are railroad ties?

Railroad ties are pieces of lumber used to support railroad tracks. Laid perpendicular to the track, railroad ties keep tracks spaced properly, hold the rails upright, and help to transfer loads from the rails themselves to the track ballast and subgrade.

What’s the difference between pressure-treated lumber and railroad ties?

The biggest difference is the way they’re treated. Pressure-treated lumber is treated with a water-based preservative. A mixture of water and the chemical treatment (copper azole) is inserted into the wood, and when the water dries, the treatment stays in the cell structure of the wood and doesn't migrate out.

Railroad ties are treated with an oil-based preservative (creosote). As opposed to water, which evaporates, oil doesn’t dry. This can result in the creosote migrating out of the wood timber, especially if it’s a large, vertical piece. A prime example of this is telephone poles. Most telephone poles are treated with creosote, and over time, the treatment begins to creep toward the bottom of the pole. Next time you’re out, look towards the top of a telephone pole. Chances are, the top will be more gray than the bottom because the treatment has seeped out of it. You may also be able to see a puddle of creosote pooling on the concrete at the bottom of the pole.

When should I use pressure-treated lumber vs. railroad ties?

Because the treatment in railroad ties migrates out of the wood, pressure-treated lumber is a much safer option. There’s no worry about chemicals leaching out of the lumber. Especially if you’re building a raised garden bed, pressure-treated lumber is a much better option. Railroad ties or creosote-treated lumber options can leach chemicals into the soil and the vegetables in your garden, whereas pressure-treated lumber will not.

Railroad ties should only be used for industrial purposes where there is no risk of human consumption.

We hope this video will help you understand more about pressure-treated lumber so you can decide on the best materials for your next project. For more on treated lumber, check out this post about how pressure-treated lumber is manufactured, this history of the chemicals used in treated lumber, and this breakdown of the differences between professional- and premium-grade treated lumber. And be sure to stay tuned for the rest of our series with Dennis.