Landscape timbers are an effective, easy to build and inexpansive solution for creating garden retaining walls or raised beds. Properly constructed, they‘ll last up to 20 years. Retaining walls can save a eroding slope. They can also transform a sloping yard into a series of terraces for lawns, gardens, or patios. For areas with poor soil, timber walls can help create a raised planting bed.
Before heading to the lumber yard, design the wall on paper. By doing so you’ll be able to accurately determine how many timbers you’ll need as well as the placement of rebars, joints, and anchors. The following is a general overview of timber wall construction.
The Base
Excavate the base of the slope in order to create a level surface for the wall’s first row of timbers. Allow 2-3 feet of space, for working room and drainage installation, between the wall and the slope. After the initial excavation, dig a trench at the base of the slope about 12” deep and spread a 6” layer of gravel as a subbase. Compact the base thoroughly.
Wall Construction and Reinforcement
The bottom row is the key to the strength and stability of the whole wall system. Place the first timber so it is level. Add or remove subbase material to compensate if necessary. Use a sledgehammer to drive 2-foot lengths of rebar into holes drilled through the timbers and into the ground until they are flush with the tops of the first row timbers. Once the bottom row is in place, level and secure begin placing the second row of timbers. Its important to cut the timbers so that you can stagger them, so the joints do not line up. Use a circular saw to cut the timbers and ALWAYS be sure to wear a dust mask, safety goggles and work gloves. Set the rows with a 1/2″ batter. That is, each row should sit back 1/2″ from the previous row. Secure the courses together using 12-inch galvanized spikes placed about 2 feet from each joint. Use additional spikes every three feet along the length. Drilling pilot holes will make the job easier. Be sure to continue to check to see if the remains level.
Installing an anchor (or deadman) timber is essential to the strength of your wall. An anchor is a 3’ long piece of timer which is set perpendicular to the wall and into the slope. The weight of the soil filed in around and on top of this timber helps hold the wall in place and prevent tipping. Place anchor timbers every third row up a wall and every 4-8’ along its length.
Once all the courses are in place, drill long holes for the rebar. Place the holes about 8 to 10 inches on either side of the joints in the top of the wall and continue down until they exit the bottom timber. Be sure, however, to avoid drilling into the rebar which secures the first row of timbers or the spikes which hold the courses together. Drill a hole through each anchor timber and into the ground. Drive 24-inch lengths of rebar through the anchor and into the ground. Cut the rest of the pins so that they’re 2 feet longer than the wall is high. Then using a sledgehammer, drive the pins through the timbers and into the ground.
Back Filling
Back filling behind the wall with gravel is essential to providing adequate drainage. Start by placing a layer of 4-6 inches of gravel behind the first timber. Then place a perforated drain pipe at least 6 inches away from the wall. Then it is only a matter of filling behind the wall and tamping as you add rows of timber up the wall. Stop back-filling six inches below the top timber. Fold the landscape fabric over the gravel and fill the last six inches with top soil.
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