How to Build a Wood Retaining Wall

Wood Retaining Wall

Simon Brown / Flickr / CC BY-SA 2.0 

Building a wood retaining wall helps to reshape slopes on your property to create level areas for driveways, gardens, paver patios, children's play areas, and decks. Retaining walls also keep soil and vegetation away from established structures. Retaining walls that use masonry retaining wall blocks or those made from thick timbers can be difficult to build because of the overly heavy materials.

Yet there is another type of wood retaining wall that uses lighter weight dimensional pressure-treated lumber. This retaining wall does not need unwieldy tie-backs dug back into the slope, like many of the four-by-four timber retaining walls. And it doesn't rely on sheer weight to hold back the slope, as do masonry walls. Instead, post holes are dug in the same way as you would dig holes for fence posts. Pressure-treated boards are then nailed across the back of these posts, much like fence boards. This project will produce a 16-foot long straight wood retaining wall that is 24 inches high.

Materials

  • (4) two-by-six ground contact pressure treated lumber boards, each 8 feet long
  • (8) two-by-eight ground contact pressure treated lumber boards, each 8 feet long
  • (8) bags of 50-pound fast-setting concrete mix
  • (8) bags of 0.8 cubic foot 7/8-inch drainage rock
  • (2) wood stakes
  • Ordinary twine or yellow braided nylon mason line
  • 16d hot-dipped galvanized nails
  • Wood preservative, such as Copper-Green Wood Preservative

Tools

Plan the Retaining Wall

Stake out the area where you plan to build the wall. Pound a stake at one end, then another stake exactly 16 feet away. Run the line from one stake to the next, pulling it tight.

Cut Back the Slope

With your shovel, dig about 2 foot back into the slope. You are only digging sideways, not down. You need to dig far enough to give yourself enough room to work on the retaining wall.

Prepare the Retaining Wall Area

With your shovel, follow the line and dig out a groove about 4 inches deep where the retaining wall will rest. Make sure that this groove is level from end to end by laying down one of the boards with the bubble level on top.

Mark the Other Post Locations

Pound six more stakes between the two end stakes. These stakes should be exactly 2 feet apart, each following the twine. You should now have a total of eight stakes in a perfect line, each 2 feet away from its neighboring stake. Each stake represents the position of a post hole.

Dig the Post Holes

The rule of thumb is that the height of the soil you are holding back should roughly equal the depth of the post holes. So, you will be digging down 24 inches to account for the depth of the post, plus another 4 inches for a bed of landscape gravel.

Remove one stake at a time, then run the one-person auger at each stake point. With your tape measure, make sure that each hole is 28 inches deep.

Add the Gravel to the Holes

Open the bags of landscape gravel and pour 4 inches of gravel into the bottom of each post hole. Be careful not to let soil drop into the holes. You should use four bags of gravel between the eight holes.

Set the Posts

Cut each of the two-by-six posts in half, so that you have eight posts, each 4 feet long. Daub the cut ends of the lumber with the wood preservative. Place the posts in each hole. Working one post at a time, use the laser level to make sure that the post is aligned perfectly plumb. Pour one bag of dry quick-set concrete into the hole, followed by enough water to fill the hole. Hold each post firmly in place until it is set before moving onto the next post.

Add Gravel Under the Wall Location

After four hours, the concrete should be fully set. Add 2 inches of gravel to the groove where the wall will rest. The gravel will help water drain away and prevent premature rotting of the wood.

Create the Retaining Wall

Run four rows of two-by-eight boards horizontally across the set two-by-six posts. The two-by-eights should be nailed onto the slope side of the posts for greater strength. While nailing them on the visible side would be more attractive, the soil on the slope side of the wall would eventually force the boards loose.

The lowest row will be about 2 inches below ground level. One hint for perfect placement is to run the rows starting at the top. That way, the retaining wall is guaranteed to match the tops of the posts. Nail each board into place with the hammer and the 16d nails.

Back-Fill the Slope Side of the Wall

Pour the remaining bags of landscape gravel along the bottom of the retaining wall on the slope side, distributing it evenly. The gravel promotes drainage along the bottom of the wall. Back-fill the rest of the cavity with soil.