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.
Yet 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. As long as the height of the retained soil stays fairly low, you can build another type of wood retaining wall that uses lighter weight dimensional pressure-treated lumber.
Basics of Low Wood Retaining Walls
A low wood retaining wall does not need unwieldy tie-backs dug back into the slope, like many of the four-by-four timber retaining walls. Nor does this wall rely on sheer weight, plus a slight tilt, to hold back the slope, as do masonry walls.
The wall and the soil behind it must remain low. Heights over 24 to 30 inches will result in too much pressure against the back of the wall, and the wall may eventually bend over and fail. Check with your local permitting office for information about recommended retaining wall dimensions.
- Working Time: 4 to 6 hours
- Total Time: 1 to 2 days
- Skill Level: Intermediate
- Material Cost: $200 to $400
What You Will Need
- Post hole auger or manual post hole digger
- Power miter saw or circular saw
- Bubble level
- Laser level
- Measuring tape
- (4) two-by-six pressure-treated lumber boards, each 8 feet long
- (6) two-by-eight pressure-treated lumber boards, each 8 feet long
- (2) two-by-six pressure-treated lumber boards, each 8 feet long
- (8) bags of 50-pound fast-setting concrete mix
- (8) 0.8-cubic-foot bags of 7/8-inch drainage rock
- (2) wood stakes
- Scrap two-by-fours
- Ordinary twine or yellow braided nylon mason line
- 16d hot-dipped galvanized nails
- Wood preservative
This project will produce a straight, 16-foot long, wood retaining wall that is 24 inches high. The construction method used is suitable only for low walls of a maximum of 30 inches. Taller walls require a different type of structure. For durability, use only pressure-treated lumber rated for ground contact.
Plan Retaining Wall
Stake out the area where you plan to build the wall. Pound a stake at one end, then drive another stake 186.5 inches away if you are planning on extending the wall longer than 16 feet. Run the line from one stake to the next, pulling it tight.
Cut Into Slope
With your shovel, dig about 2 feet 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. At the same time, the more soil you remove, the more you need to replace with gravel backfill.
Prepare 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.
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 a total of eight stakes in a perfect line, each 2 feet away from its neighboring stake. Each stake represents the center position of a post hole.
Dig Post Holes
With wood retaining walls, 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. The hole for the four-by-four 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 stake point. With your tape measure, make sure that each hole is 28 inches deep.
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 out one bag of gravel for every two holes (or four bags of gravel between the eight holes).
Set Posts in Holes
Cut each of the two-by-six 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 make sure that the post is aligned perfectly plumb. Use scrap lumber for stakes to hold it in place.
Pour one bag of dry quick-set concrete into the hole, followed by the recommended amount of water according to the product instructions. Leave the scrap lumber stakes in place until the concrete it is set before moving onto 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.
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.
Install Retaining Wall Boards
Install three rows of two-by-eight boards horizontally behind the posts, starting at the top and working down. Nail the two-by-eights to each post with two 16d nails, nailing through the slope-side of the boards and into the posts. The top row of boards should be flush with the tops of the posts.
Install a final row of boards along the bottom, using two-by-six boards. The bottom edge of this row will be about 2 inches below ground level.
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. Back-fill the rest of the cavity with soil.
It's vital to back-fill with gravel and not with soil or even a combination of soil and gravel. If soil is used as a back-fill, it will become water-logged and excessively heavy, potentially collapsing the retaining wall.
When a Permit is Required. Fairfax County Government Land Development Services.
Information for Building Code Officials: How do I make sure the treated wood on the jobsite meets code? American Wood Protection Association.
Browder, John O. Fragile Lands of Latin America: Strategies for Sustainable Development. Taylor & Francis, 2019.