Wood can be a budget friendly solution for a DIY retaining wall. However, wood is subject to fungal rot, termites, and water damage. When following all of the best recommendations, a wood retaining wall can last up to 20 years. Without proper techniques, a wood retaining wall can fail in as soon as 5 years. Compare this to a properly constructed segmental block wall which can last 50+ years.
Types of Wood Retaining Walls
There are a few basic styles of wood retaining walls. In later articles, we will discuss all of the construction techniques involved with these walls.
Timbers
When referring to a retaining wall built of timbers, this consists of 6″x6″ or 8″x8″ lumber stacked on top of each other. The bottom timber is secured by driving steel rebar through the timber and into the ground. Each subsequent timber is tied to the one below with long skews, nails or spikes. Wall greater than 2-ft require deadman anchors which extend back behind the wall.
Railroad Ties
Railroad ties can make a great short height retaining wall if you can find them. Functionally, railroad tie walls are built just like a timber wall. Railroad ties are impregnated with creosote, a known toxin that leaches into the soil. Remember, the railroad ties that you buy have reached the end of their life for supporting trains, and there is no good way to determine how much longer they will last in a retaining wall.
Post and Panel
A post and panel retaining wall consists of a wood post embedded in concrete. The posts support wood boards (sometimes called panels or lagging). The lagging is installed behind the posts and attached with screws or bolts. Taller post and panel retaining walls also require deadmans or cantilever footings to keep them from leaning outward.
Pressure Treated Wood
All wood used in retaining wall construction must be pressure treated. But there are many grades of pressure treatment. The pressure treated wood must be rated for ground contact and outdoor use. Even though pressure treated wood may have a lifetime warranty, this is voided if the wood comes in contact with the ground.
The pressure treated timbers found at any hardware store are typically category UC4A and rated for general use ground contact. The treatment is Micronized Copper Azole (MCA) or Copper Azole (CA-C). Although UC4A is rated for ground contact, there are better treatments for a retaining wall application.
It is best to use UC4B wood which is typically a Copper Azole, Type C (CA-C) treatment. Pressure treatment category UC4B is for heavy duty ground contact. You can still order this at most hardware stores, but you must go to their contractor desk and specifically ask for UC4B pressure treated wood. A Lowes or Home Depot will not have UC4B wood out for the the typical customer to buy. You must special order it. UC4B wood will cost about 20% more than UC4A wood, but it is worth it.
Treatment category UC4C (extreme duty ground contact) may also be used, but it has to be approved by the EPA for use on a project which will only happen on commercial projects. Category UC4C woods are typically chromated copper arsenate (CCA) which can leach arsenic into the soil over time.
For more on pressure treated wood, check out these guidelines by the USDA.
Should I use “Eco Safe” Treatments?
In the 20th century, pressure treated wood contained arsenic. That is no longer the case. In 2002, the EPA pushed lumber manufacturers away from treatments that contained arsenic except for special circumstances. Now, most pressure treated wood uses copper as described above. Copper is considered less toxic and is used throughout the food industry as a fungicide on crops.
You might encounter “Eco safe” products. Eco safe products use non-metallic treatments, but they do not last nearly as long as traditional pressure treatments and they are not rated for ground contact. The eco safe treatments cannot resist termite damage and fungal rot long term like those treated with copper. Although today’s traditional pressure treatments will leach out copper, this process is slow and is lessened with proper drainage design.
If you build a retaining wall properly out of long lasting materials, the wall will last at least 20 years. Eco safe treatments will last closer to 10 years in the same application.
Wood Treatments for Vegetable Garden Beds
If you are building a vegetable garden bed, you might be concerned with copper based wood preservatives. Although copper is used as a fungicide on crops, it is not an organic product. For organic gardening, install a heavy plastic moisture barrier between the retaining wall and the garden soil to reduce the copper from leaching into the plants. Alternatively, use decay resistant wood like cedar and redwood. Note, even cedar and redwood will need to be replaced every 5-10 years. You can avoid wood all together by building a segmental block wall and be confident it will last.
Treating Cut Edges
Pressure treatment only extends an inch or so into the wood. So, when you cut through the wood or drill a hole, the cut edge will be untreated. Treat any cut edge with a paintable copper based wood preservative*. In addition, wear a mask to avoid breathing in any dust from cutting pressure treated wood.
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