Lumber Used for Retaining Walls

Natural Lumber

• Typical lumber woods, such as pine, hemlock or spruce, need to be either painted, coated with a wood preservative or stained and coated with a water repellent finish before installation. Even with treatment, these woods will not last long in contact with the ground. Woods that are naturally rot resistant, such as cedar, redwood and cypress, are a better choice, especially if food crops will be grown nearby. All natural woods will decay over time.

Treated Lumber

• Preservative-treated lumber works well for retaining walls due to its longevity in contact with wet soil. Four commonly used treatment compounds are creosote, Penta-cholorphenol, Chromated Copper Arsenate (CCA) and Copper-8-quinolinolate. Creosote-treated wood has an oily surface and odor due to its coal tar base. CCA-treated lumber is widely available and often labeled as "Wolmanized." Lumber treated with a water- or solvent-based preservative can be stained or painted. Sealing treated lumber with urethane or shellac will help prevent preservatives from seeping into adjoining soil.

Plastic Lumber

• Plastic lumber (RPL) is manufactured from recycled waste plastics. High-density polyethylene (HDPE), the same plastic as milk jugs, comes in a variety of colors. Wood-filled RPL is made from low-density polyethylene (LDPE) mixed with wood fiber and accepts painting better than HDPE plastic lumber. RPL may cost more than wood lumber, but it lasts longer and is maintenance free. It is less rigid than wood lumber and so may need more support.

Precautions

• Check with your local building permit department for code requirements. Retaining walls more than a few feet in height may require a permit and structural engineering to ensure public safety. When working with treated woods, wear a dust mask or respirator when sawing or sanding. Never burn scraps of treated wood, and keep them away from water sources to avoid contamination. Treated woods, with the exception of those treated with Copper-8-quinolinolate, should not be used near where food crops are grown.