How to Build a Solid Wood Retaining Wall

1. Plan Out the Retaining Wall

   Stake out the area where you plan to build the wall. Pound a stake at one end, then drive another 186.5 inches away if you plan to extend the wall longer than 16 feet. Run the line from one stake to the next, pulling it tight.

   Tip

   Instead of making the distance 192 inches (16 feet), you'll make it 186.5 inches to account for the width of the two end posts. If the wall is going to be longer than 16 feet, account for each end post being 5 1/2 inches wide.

2. Cut Into the Slope

   With your shovel, dig about 2 feet back into the slope. You are only digging sideways, not down. You must dig far enough to give yourself enough room to work on the retaining wall. At the same time, the more soil you remove, the more you need to replace it with gravel backfill.

3. Prepare the Retaining Wall Area

   With your shovel, follow the line and dig out a groove about 5 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. The flat-bladed shovel or spade is the ideal width for scraping down areas of the groove or replacing soil in other areas.

4. Mark Other Post Locations

   Pound six more stakes between the two end stakes. The stakes should be equally spaced apart from each other or roughly 2 feet apart on-center, each following the twine. You should now have eight stakes in a perfect line, each 2 feet away from its neighboring stake. Each stake represents the center position of a post hole.

5. Dig Post Holes

   With wood retaining walls, the rule of thumb is that the height of the soil you hold 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. The hole for the 4-by-4 posts should be about 12 inches in diameter.

   Remove one stake at a time, then dig a hole with a post hole auger or manual digger (clamshell digger) at each staking point. With your tape measure, ensure each hole is 28 inches deep.

6. Add Gravel to Post Holes

   Pour 4 inches of gravel into the bottom of each post hole. Be careful not to let soil drop into the holes. You should be able to spread one bag of gravel for every two holes (or four bags of gravel between the eight holes).

7. Set Posts in Holes

   Cut each of the 2-by-6 posts in half so that you have eight posts, each post 4 feet long. Daub the cut ends of the lumber with wood preservative. Place the posts in each hole. Working one post at a time, use the laser level to ensure the post is aligned perfectly plumb. Use scrap lumber for stakes to hold it in place.

   Tip

   While the faces and factory-cut ends of pressure-treated wood are protected, the ends of site-cut wood are not protected. A copper-based wood preservative is commonly used on cut ends of pressure-treated lumber. The copper solution effectively replaces the preservative injected into the wood mill's wood.

8. Pour Concrete

   Pour one bag of dry quick-set concrete into the hole, followed by the recommended amount of water per the product instructions. Leave the scrap lumber stakes in place until the concrete is set before moving on to the next post. Fast-setting concrete sets in about 30 to 40 minutes. Even so, it's usually best to let the concrete fully harden for at least four hours.

9. Add Gravel Under Wall Location

   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. Water can rapidly wick upward through the wood and damage the entire retaining wall.

10. Install Retaining Wall Boards

    Install three rows of 2-by-8 boards horizontally behind the posts, starting at the top and working down. The top row of boards should be flush with the tops of the posts. Nail the 2-by-8s to each post with two 16d nails, nailing through the slope side of the boards and into the posts.

    Install a final row of boards along the bottom, using 2-by-6 boards. The bottom edge of this row will be about 2 inches below ground level.

    Warning

    Install the boards on the slope side of the posts. If you install the boards on the opposite side of the posts, wall strength and stability are significantly compromised. When installed on the slope side, the boards can press against the posts as it gets pressure from the slope.

11. Back-Fill Slope Side of 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. Backfill the rest of the cavity with soil.

    It's vital to backfill with gravel and not with soil or even a combination of soil and gravel. If soil is used as a backfill, it will become water-logged and excessively heavy, potentially collapsing the retaining wall.

Tips for Building a Solid Wood Retaining Wall

Wood naturally decomposes over time. You must get pressure-treated wood that resists rotting, such as wood rated for ground contact. However, these wood types are more expensive. It's worth the expense to extend the life of your wall. The average life of a wood retaining wall is about 10 years, although high-quality wood and double-treating it can help you keep your wall in good shape for about 20 years.

Wood preservatives, insect repellents like creosote to deter insect activity, and weather sealants are effective wood treatment options. Wood stain is another way to go; however, that will need reapplication every year or two.

If opting against using pressure-treated timber, another alternative is creosote-treated railroad ties, which you can usually source at big box home improvement retailers or lumber yards. Railroad tie lumber is usually a less expensive option.

To keep soil in place behind your retaining wall, consider using plantings. Plant roots help keep the soil in place, reducing erosion. However, if the plants die in winter, replace them in the spring. Voids left behind by dead foliage can loosen the soil.

When to Call a Professional

Hire a professional to build a wood retaining wall taller than 30 inches. The project becomes more complex the taller it gets since adding height usually means more pressure and natural forces will be exerted on the wall.

Also, contact an expert if the area where you want to build the wall experiences drainage issues. Professionals have engineering skills to account for significant runoff issues and a strong understanding of those related engineering considerations.