April's Tip of the Month

Building a Wood Timber Retaining Wall

Timber walls will last for 15 to 20 years if built correctly. They are the easiest wall to build and are excellent for the first time wall-builder. They can fit into a formal or casual landscape and  Use pressure treated lumber at 5x6 in six. Small timber will not retain the load of earth. I have divided the construction of the wall into three steps:

The Base
Excavate the bottom of the slope to create a level base for the wall. Allow 2-3 feet of space between the wall and the hillside. Use stakes at either end of the front edge of the wall. Connect stakes with a mason's string and adjust till it is level. Remove a trench of soil 12 inches below the string. Line the trench and hillside with landscape fabric, overlapping pieces by at least 6". Spread a 6" layer of gravel subbase. ( When ordering gravel for the base let the dealer know what you want it for and they will help you choice the right type.) 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 that the top is even with the string. Add or remove subbase to compensate if necessary. Once the bottom row is in place add the second row keeping in mind that you do two things. First, never install timbers so that end joints align. You will most likely need to cut several timbers so that you can stagger them. Secondly, you will need to set the rows with a 1/2" batter. That is, each row should sit back 1/2" from the row below it. See picture. Use galvanized spikes  or reinforcement bars to anchor the ends of each timber to the underlying timbers. Use additional spikes every three feet along the length. Drilling pilot holes will probably make the job easier.

Installing a deadman in a wall can seem like a lot of extra work but it is vital to the strength of a timber wall. A deadman is basically a horizontal anchor that holds the wall to the soil it is supporting. Place a deadman about every 4-8' feet apart depending on the height and width of the wall. The anchor itself is made of two 3' long lengths of timber connected in a 'T'. See picture. Place deadmen every third row up a wall.

Back Filling

Back filling behind the wall with gravel is vital for good 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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