To create a terrace held in place with wooden retaining walls, it’s essential to build walls that won’t lean or shift. Use concrete to anchor support posts every 2 feet along the path of the retaining wall. Then, attach your wall timbers to the supports and build to the desired height. Backfill the retaining wall with gravel, followed by dirt, to promote good drainage. Once you have your first retaining wall in place you can repeat this process for as many walls as you need to terrace the entire hillside.
For this job you will need:
- 2 stakes
- String level
- Hammer or mallet
- Posthole digger
- Bagged concrete
- Retaining wall lumber (railroad ties, planks, logs, etc.)
- Carpentry screws
- Electric drill
10 Steps to Terrace a Hillside with Wood Retaining Walls
Wood terrace walls provide a natural, rustic look to your terrace garden. You can inexpensively and easily build strong retaining walls for your terrace using railroad ties, landscape timbers, or logs, depending on the style you prefer. Simply follow these steps and you can turn a slope into a terrace garden.
Find the Rise and Run of Your Slope
Before you put your retaining wall in place, it’s essential to know how big your project will be. To properly plan a terrace project, find the rise (change in elevation) from the foot of the slope to the top of the hill. Then, find the run (the horizontal distance between the foot of the slope and top). To find these numbers, perform the following steps:
- Drive the first wooden stake partway into the ground at the foot of the hill.
- Drive the second stake into the ground up the hillside, at a point where you can tie a level string between the stakes.
- Tie a string between the two stakes. Make sure it is level using the string level.
- Measure the distance between the ground and the point where the string is tied to each stake. This is the rise. For example, if the level string is tied to the first stake 25 inches above the ground and it is tied to the second stake 1 inch above the ground, the ground rises 24 inches between the two points.
- Measure the distance of the level line. This is the run.
- Repeat this in sections as necessary, moving up the slope to the point where the terrace will end.
- Add the rise of all measured sections to find the total rise of the slope.
- Add the run of all measured sections to find the total run of the slope.
This may seem like a lot of math, but it makes planning a professional terrace project much easier. On a gentle slope, you can measure the rise and run in a single step. On a steep slope, the ground may be too inclined to tie a level string between the top and bottom stake. This necessitates measuring in sections.
Plan Your Project
Now that you have the rise and run of your slope, you know how tall your hill is and how much horizontal ground it covers. These numbers are critical to planning the height of your terrace walls and the distance between them. Let’s take a look at an example:
- The rise of the slope is 6 feet.
- The run of the slope is 18 feet.
- On this slope, you can construct a terrace with 3 walls that are each 2 feet tall and 6 feet apart.
- Sketch out your terrace design for a visual.
- Terrace wall height should be no more than half the distance between the walls. 2-foot walls must be spaced at least 4 feet apart.
- Improperly spaced terrace walls are prone to erosion and collapse.
- If desired, add a set of stairs to your terrace plans to make tending to your garden easy.
As long as your retaining walls are not too close to one another, there are no hard rules on how many terrace levels you can have. You can create a few tall retaining walls or several low retaining walls.
Dig Your Retaining Wall Trench
Now that you’ve planned your terrace project, begin by digging a 10-inch deep trench at bottom of the hill, where the first retaining wall will be. Use stakes and string to mark a straight line. Follow this as you dig.
- Begin your first retaining wall at the bottom of the hill.
- Dig a 10-inch deep trench along the path your retaining wall will follow.
- The trench should be 6 inches wider than the timber you will use for your retaining wall.
- Mark your trench path to ensure it follows a straight line.
Pile the soil you excavate nearby. This will be used to backfill your retaining wall in later steps.
Pour a 4-inch layer of gravel into the bottom of the trench. This will allow water to drain below the wall, rather than collecting water that will rot your timber wall. Once the gravel has been laid, tamp it down firmly.
- Pour 4 inches of gravel into the bottom of the trench.
- Tamp the gravel into place.
- Use landscape gravel, such as 3/4 inch gravel.
Large, angular gravel is the best type for this purpose. It provides good drainage, while small or round gravel is prone to compaction.
Set Support Posts
Use a posthole digger to dig holes in the trench every 2 feet along the path of the retaining wall. The holes should be placed in the front of the trench (downhill side). These are the holes for your support stakes. The depth should equal the height of the wall. So, if you plan to make a 24-inch tall retaining wall, the support postholes should be 24 inches deep.
- Use a posthole digger to dig 1 support post hole every 2 feet along the ditch.
- Hole depth should equal intended wall height. A 24-inch wall needs 24-inch deep postholes.
- Add 4 inches of landscape gravel to the bottom of each posthole.
- Cut 4×4’s to post size. For a 24-inch wall, cut posts that will extend 24 inches out of the ground when the lower ends are placed in the hole.
- Place the posts and fill the surrounding hole with concrete.
Once your posts are cut to size and placed, mix bagged concrete and pour it into the hole around the posts. Posts set in concrete are far sturdier than posts buried in soil. This support system will work for a wall up to 3 feet tall.
Add Wall Timbers
Once your posts are in place and the concrete has set, place your timber behind the posts. Make sure it is level, and then attach the timber to the post with screws. To hide the screw heads, drive the screws into the back (uphill side) of the timbers. This will lead to a clean finished look on the visible side of the wall.
- Beginning at the bottom of the trench, build your retaining wall up by attaching timbers to the posts.
- Place the timbers behind the posts (on the uphill side) for increased sturdiness. This way, the pressure of holding back the soil is on the posts, not the screws holding the timbers in place.
- Pre-drill screw holes in your timbers to prevent wood from splitting.
It’s a good idea to drill pilot holes in your timbers and posts before driving screws. This helps to keep the wood from splitting and leads to a much stronger, more attractive finished wall.
Backfill the Retaining Wall
Once you’ve built your wooden wall, backfill the area directly behind it with additional landscape gravel. Again, this helps improve drainage and prevents moisture from being trapped against the wood. Once the gravel backfill is in place, complete backfilling with dirt.
- Backfill the wall with a 2–4-inch wide mound of landscape gravel.
- Landscape gravel should come up within 4 inches of the top of the wall.
- Complete backfill with dirt.
Don’t worry if the dirt from the trench excavation isn’t enough to completely backfill the wall. In the next step you will be excavating part of the hillside. This additional soil can be used as backfill.
Level the Terraced Area
Now that your first retaining wall is in place, it’s time to create a level garden bed behind the retaining wall. Referring to your terrace plan, excavate and level the hillside to the desired width behind the wall. Use a carpenter’s level and measuring tape to check the level and terrace width.
- Use your shovel to create a level terrace garden area of desired width.
- Use excavated dirt to backfill your retaining wall.
Once complete, you should have a level “step” cut into your hillside. Congratulations! You’ve completed your first terrace.
Repeat for Additional Terrace Levels
Once you have one retaining wall in place, repeat the same process of trench digging, setting posts, and building walls as desired for as many terrace walls as you have planned. The level terrace you already created will serve as a firm foundation for you to work from—this is why we terrace from the bottom of the hill to the top.
Plant a Terrace Garden
A garden of flat terrace steps is the perfect place to plant small trees, such as fruit trees, herbs, bushes, ornamental grasses, vegetable plants, or flowers.
- Flat terrace steps make for perfect garden beds.
- Plant your terrace with trees, ornamentals, or vegetables of your choosing.
- Terraces soak up water and the soil retains nutrients, whereas slopes shed water and the runoff nutrients from the soil.
Terraces not only stop hillside erosion, but also make the hill much more plant-friendly. The flat terrace steps absorb rainwater and retain nutrients. In comparison, a sloped yard does not absorb water and the water that runs downhill robs the soil of nutrients plants need. Once your terrace garden is in place, you can turn a hillside into a thriving orchard or vegetable garden.
How Do You Terrace a Slope with Wood?
You can transform a hillside into a terraced garden with wood retaining walls by following these simple steps:
- Find the rise and run of the slope you are terracing.
- Plan the height of your terrace walls and the width of the garden beds between them.
- Start your first terrace wall at the foot of the hill by digging a 10-inch deep trench.
- Line the trench with 4 inches of landscape gravel.
- Dig postholes in the trench at 2-foot intervals along the trench.
- Set 4×4 posts in place and secure them by pouring concrete into the postholes.
- Build your wall by attaching lumber of your choosing to the posts.
- Backfill the wall with gravel and dirt.
- Level the garden bed behind the wall to the planned width.
- Repeat for additional terrace walls.
- Plant your terrace with varieties of your choosing.
By following these professional steps, you can turn your outdoor space into a gorgeous terrace that will withstand the test of time. Once the work of building the walls is done, you can move on to planting a terrace garden.