Wood Retaining Walls
Although fading in popularity in some areas, wood retaining walls still complement many yards and homes. This is especially true if the home has a rustic appearance, often including natural or stained wood, rock and/or stone features. Wood is also a popular choice when looking for an economical retaining wall solution.
Depending partly on the height of your wall, you can use either standard dimension lumber (such as 2x4s, 2x6s, etc.), or timbers (4×6 and larger).
If you’re a reasonably competent carpenter, a wood retaining wall may be a perfect DIY project. Wood is easily worked to include angles in your wall, steps, or other custom features, as desired.
Wood Retaining Walls Are Affordable & Easy
In most cases, a wood retaining wall is relatively simple to construct. Low costs make timber retaining walls a common choice for do-it-yourself home improvement.
Landscape contractors may also offer this type of wall as an affordable way to protect shorter slopes from erosion and/or the loss of stability over time. There has also been a recent trend toward raised bed flower or food gardens. As a result, wood retaining walls have become increasingly attractive to many homeowners.
Construction Methods for Wood Retaining Walls
Post and Panel Retaining Wall
A post and panel retaining wall often includes wood posts embedded in concrete. The posts support wood boards (sometimes called panels or lagging) that span the width of the wall with this construction method. Lagging (generally 2x6) is installed behind the posts and attached with screws or bolts. Taller post and panel retaining walls generally require deadman ties (see below) or cantilever footings to provide stability and keep them from leaning outward. It may be best to consider a contractor to install these, as this type of site work benefits from an experienced contractor.
Xpress Engineering offers wood post and panel retaining wall designs specifying a type of cantilever design for each post.
Horizontal Stacked Wood Retaining Walls
Using horizontally stacked timbers or railroad ties is a common way to build shorter walls, and can be reasonably strong. For a retaining wall built of timbers, multiple 6″x6″ or 8″x8″ timbers are stacked on top of each other and connected with long screws or spikes.
As with other types of retaining wall materials, timbers should be placed on a level gravel base with good drainage behind the wall. The bottom timber should be secured by driving steel rebar through the timber and into the ground. Each additional timber should be fastened to the others using long spikes or screws.
Walls greater than 2 feet high should be fitted with deadman anchors extending back behind the wall. A “deadman anchor” is a tie installed perpendicular to the wall face and attached to both the wall face itself and a timber cross plate at the opposite end. The length of the deadman anchor should be the same as the height of your wall. Such ties, or anchors, will add substantial stability to your wall.
Types of Wood for Retaining Walls
Railroad ties can make a great short 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 will leach into the soil. Normally, 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.
Recently milled timbers have become more common for stacked retaining walls because of availability and concerns over chemicals used to treat railroad ties. Choose timbers that are rated as suitable for ground contact, and seal any cut ends with a good quality preservative to prevent rotting. Pressure-treated pine and fir that are rated for ground contact should survive 40 years; western red cedar or redwood lasts about 20. Timber walls can usually be stained or painted to match your landscape aesthetic.
Pressure Treated Wood
Ideally, wood used in retaining wall construction should be pressure treated, which will significantly increase their useful life.
There are many grades of pressure treatment, but such wood used for retaining walls should be rated for ground contact and outdoor use.
Even though pressure treated wood may have a lifetime warranty, this type of warranty is often voided if the wood comes into direct contact with the ground.
Composite timber may be a good material choice for a stacked retaining wall, since they offer a sustainable option.
They are composed of a wood and recycled plastic composite material, making them strong, earth friendly and impervious to damage from insects or rotting.
They’re also light weight, splinter free, kid and plant friendly, and do not use hazardous chemicals. Interlocking versions are available for walls up to 4 tiers tall with minimal excavation.
Using Timber Retaining Walls in Your Landscape
In the right setting, a timber retaining wall can blend into your landscape beautifully. In a garden, they are particularly appropriate and beautiful, blending with the trees, shrubs and other landscaping that surrounds them.
The best kind of lumber for these walls is Douglas fir pressure treated with preservatives to discourage rot. It will likely be green or brown and should be rated for earth-to-wood contact. For timber walls, large timbers can be expensive, which is why railroad ties are a common alternative.
Building Timber Retaining Walls
A timber wall, if well built with proper materials, waterproofing and preservatives can last for about 20 years or more. The primary weakness is the wood itself, which is subject to rot where there is extensive wood to earth contact. Normally there will be moisture in the soil which increases lateral earth pressure. It's the combination of weight and rot that often causes retaining walls to fail. This vulnerability will shorten the lifespan of wood retaining walls compared to block or concrete walls.
Preventing Moisture Problems
To maximize your timber wall’s lifespan, you (or your contractor) should pay close attention to water, where it gathers and pools, both on top and behind the wall. The accumulation of water will speed the decomposition of wood, so drainage structures are very important, as well as the overall grades of the slope both at the top and behind the wall, and at the toe (front) of your wall.
In many cases it's wise to apply a layer of waterproof sheeting behind a timber retaining wall to protect it from direct contact with the soil. There may also be gravel packing behind the wall, including the bottom, where water may accumulate on rainy days.
The gravel immediately beneath the wall should extend the length of the wall and may also contain perforated tubing for drainage to drain water to one or both ends of the wall. While some walls may include weep holes at the bottom, potential benefits should be considered, since weep holes can potentially shorten the life of the wall.
Preservatives & Soil Health
For railroad ties there is concern that chemicals can leach into the surrounding soil. This is especially troubling for raised bed food gardens. In this case, foundation grade redwood or cedar would be a better choice.
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 40 years. However, a wood retaining wall can fail in as little as 5 years if proper techniques are not used. Compare this to a properly constructed concrete or CMU block wall which can last 50+ years.
Pressure treated wood found at any hardware store are typically category UC4A and rated for general use ground contact. 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 treatment. Pressure treatment category UC4B is intended for heavy duty ground contact. You may be able to order this at your local hardware store. However, you will likely need to specifically ask for UC4B pressure treated wood and special order it through their contractor desk. You should be aware that popular stores such as Lowes or Home Depot will probably not have UC4B wood displayed on the floor. It is generally about 20% higher in price.
Wood preservative 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 generally will only be available for commercial projects.
At one time, pressure treated wood contained arsenic. However, arsenic is no longer used for this purpose. Currently, most pressure treated wood uses copper as described above. Copper is considerably less toxic and is used throughout the food industry as a fungicide for crops.
You might also find “Eco safe” products available, which use non-metallic treatments. However, they do not last nearly as long as traditional pressure treatments and they are not rated for ground contact. Also, eco safe treatments usually cannot resist termite damage and fungal rot long term like wood products treated with copper. Although today’s traditional pressure treatments may leach out copper, this process is slow and is lessened with proper drainage.
If you build a retaining wall properly out of long-lasting materials, the wall should last at least 20 years. Eco safe treatments will last closer to 10 years for the same application.
Professional Engineering Retaining Wall Design - Fast and Affordable
Whichever material you choose for your retaining wall, keep in mind that most local building codes require an engineering design for any retaining wall of 4 feet or more in height.
And even for shorter walls, a design from a trained structural engineer is highly recommended to insure the security and longevity of your wall. We can provide a certified engineer design for you within 24 hours (M-F) for just $350 for your concrete, block, or wood retaining wall in most cases. Submit your Design Request here, or contact us if you have any questions or unique requirements.