Building a wooden retaining wall is a great do-it-yourself project. Here’s some hints to help
Wood products of various types are common choices as building materials for retaining walls. They are relatively inexpensive, durable and readily available and fairly easy to use if a few simple guidelines are followed.
Before beginning your project, you should determine if there are any special requirements. First, how large a wall are you planning? In general, walls that are over 3 ½ feet high require that a licensed engineer approve the design. Walls of this height require a foundation that is stable and will not shift and allow the wall to fall.
Check with your local building permits department to prevent problems that could become costly. Next, some neighborhoods restrict what kinds of building materials can be used. In particular railroad ties are prohibited in some places because of environmental concerns. Some areas prohibit certain materials because of concerns over appearance. Another consideration is location. Wood is generally not a good choice in areas that are wet or might have populations of termites or carpenter ants. Even treated lumber will succumb in a few years to wood rot and insects under these conditions.
The most common wooden building materials are railroad ties and pressure treated lumber that is available in 2” x 4”’s, 4”x 4”’s and a special flattened oval shape known as a landscape timber. This wood is specially treated to resist wood rot and insects. Railroad ties are timbers about 12”x12” that are treated with creosote to resist decay.
Although retaining walls can be built by stacking timbers on top of one another and nailing them together, this technique is really only effective with railroad ties. Because of their weight, railroad ties can be used in this way for small walls. Most wooden retaining walls, however, consist of upright poles with cross bars placed between them. This is the technique we’ll discuss.
Assuming that you’re building a 3 or 4 foot wall, the first step in actual construction is to prepare the surface. The ground on which the first row of crossbars is laid must be level. If the ground slopes, you will have to dig out a level trench and create terraces that are one crossbar deep if necessary. Ideally the wall will tilt ever so slightly back toward the soil behind it. There should be about 6 inches of space between the wall and the soil behind it. You may need more space if you need to swing a hammer to nail the crossbars to the uprights. We’ll fill this space later with gravel.
Now that the ground is prepared, use a posthole digger or auger to dig holes for the uprights. The holes should be from 12” to 18” deep. The uprights should be spaced so that the crossbars go from the center of one upright to the center of the next. Now put the upright poles into the holes and fill the holes with mixed concrete. Use a level to make sure the poles are straight up and down and lean very slightly back toward the soil to be retained. Once these are in place, let them stand overnight so the concrete can set.
The next day, begin to set the crossbars in place. The crossbars should go behind the uprights, closest to the soil to be retained. This provides maximum strength. Place the first crossbar, make sure it’s level and nail it at each end. Continue this process until the entire first row is in place. Now if the level looks good, you can add more rows of crossbars until the wall is at the desired height. For added strength, you can drive nails down from a crossbar to the one below it if desired.
Now you’re ready for the finishing touches. Use a large drill bit (1 “or so) and drill holes about every two feet between the first and second row of crossbars. These weep holes will prevent water from being trapped behind the wall and stressing it. Fill behind the wall with gravel up to about 6 inches of the top. Fill the rest with dirt and top it off with grass. Your wall is ready to enjoy.