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How to Build a Dry-Stack Stone Retaining Wall the Right Way

C.R. Stone owns and operates C.R. Stone Enterprises in East-Bolton, a landscape and construction company specialising in natural stone.

Two-tiered retaining wall in the backyard. Planting flowers and small brushes can soften the look of the stones.

Two-tiered retaining wall in the backyard. Planting flowers and small brushes can soften the look of the stones.

Building a Stone Retaining Wall

As a landscaper and contractor who has built stone creations my whole life (think hundreds of thousands of square feet of stonework), I've handled over a million pounds of stone (so far), and I have found that even some of the most educated advice on how to build a stone wall is not always accurate. I

n this article, I'll share my secrets to building a dry-stack stone retaining wall that will last a lifetime and fit in perfectly with your landscape and home design.

How to Build a Dry-Stack Retaining Wall

  1. Plan out the wall's height and base thickness. For every one foot in height, you'll want to lay the base a foot in from the wall's face.
  2. Prepare a solid foundation or base. Dig out a trench of at least 6-10 inches. This firmly plants the base into the ground below, which helps prevent movement over time. It is also important to consider the material that will hold the base layer and whether it will allow enough drainage.
  3. Lay the stones, starting with the largest stones on the bottom. The bottom layers will bear the weight of the stones above them. Make sure to lay the stones so that they angle in slightly toward the back of the wall. This will help the wall resist the pressure of the backfill (ground soil behind the wall).
  4. Protect your wall with backing. Completely cover the back side of the wall to prevent soil and other debris from getting into the wall's crevices. Geotextile fabrics are my go-to; they allow water to drain through but prevent larger materials from getting into the wall.

1. Planning Your Build

Figure Out the Ratio of Height to Base Thickness

Survey the area where you want to build the wall and draw out a plan. How high do you want the wall to be? The rule of thumb is that for every foot of height, you want the same amount of thickness in the base. If your wall will be over five feet high, you can most likely get away with keeping a consistent thickness of 4–5 feet—if you are careful of how the stones in the back are placed.

However, if you are going to be building an 8–12-foot high wall, you're going to want to increase the thickness at the base to at least six or seven feet, keeping it consistent until you reach about half the height of the wall. At that point, you can start tapering in towards the face, ending with a width of 2–3 feet at the top. In all cases, the width at the top of your wall should be around this width—or a little less if you want to do some planting.

Remember that the place you first think of for your retaining wall may not be the only place it can go. Moving a wall out a few feet from a bank may allow you to save material. You can build a shorter wall and keep the same slope, or you can make a more gradual slope by pushing the wall even further out. You can add more soil/gravel behind the wall to fill the gap.

Note: For most DIYers, walls three feet high and below will be manageable. For greater heights, you may want to call in professionals to ensure the wall is built correctly. You may also need to get city approval for walls above three feet in height, depending on where you live.

Slope the Wall Towards the Back

To help the wall hold up to the pressure from the backfill, plan to dig out the foundation and lay the stones so that the stones slope towards the back of the wall—at least a two-inch drop for every foot of height.

Draw a Cross-Section of the Wall

When you draw a sideview of your wall, you should see a triangle shape. Ideally, the area of the triangle should be at least the same as, if not bigger than, the area behind it from the back of the base up. This way, the weight of the stones, along with the inward slope, will allow your wall to withstand the pressure of the backfill.

A Second Tier Needs Additional Planning

If you are planning to build a second tier, you should also draw out where your wall faces and backing will end up, as well as how the upper wall's weight will be distributed to the area below it.

For instance, if you have a four-foot wall in the front, and a second wall in the back that is only three feet behind the finished face of the front wall, the entire weight of the second wall will push against the center of the first wall, causing the wall to bow outwards and eventually crumble. This is because the upper wall is spreading its weight over the area below at an angle of 45 degrees from its edges.

2. Preparing the Base for the Wall

Dig the Foundation

Mark the area where your wall's base will be. You'll then want to dig a trench at least six inches to a foot into the ground, making a slight incline towards the bank (2-inch drop for every foot of wall height). This helps make sure the wall will withstand the pressure from the backfill over time.

Also make sure the surface is even all along the base. Using a square spade can help with this. For larger projects, a tractor may be necessary. You want the foundation stones to be able to lay flat with no room to wiggle.

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Choose the Foundation Material

You'll then want to figure out what sort of material will form the base layer. Depending on the area, you may find either regular ground soil, clay soil, or gravel. You may also choose to dig deeper and fill the foundation up to the desired level with one of these materials. I will discuss these materials in more detail below.

Gravel

Gravel is best for water drainage—something you'll want in order to prevent the wall from shifting around. if there is nothing to obstruct the draining process. It should not need much done to it other than a good compacting with a vibrating-plate compactor or jumping jack, depending on the size of wall you're planning to build.

If there are areas that will obstruct the drainage, take care of them by laying a perforated drain tile from the low spot that water will pool in—even if the spot isn't visible on the surface—sloping downwards away from your future wall to an area where collected water can flow away freely.

You won't want to direct drainage from the wall towards your house; steps were taken when your house was built to gather and evacuate water from the edges of your house, so don't compromise what should work. With your gravel base compacted, and potential wet spots taken care of, you're ready to start laying your base rocks.

Clay

Clay will not drain internally. It will need to be either sloped away from the face of your wall so it can drain freely or sloped to the right or left along the face of your wall so that it may drain past one end.

Many landscapers install drains under their walls when clay is found, but this creates an area for water to collect under your wall, which is the last thing you want. But if there is no other way to drain the water that simple sloping methods will collect, you can install a drain behind your wall.

Put it far enough behind the wall so that your wall will not be resting on the tile, and slope the base very slightly towards the back of your wall to ensure water makes it to the drain and so that the tile you install will direct the water into some area that will never get restricted. Always take steps to prevent sediment from infiltrating your drain (see below).

Soil

Earth and other materials are not the most desirable bases on which to build large stone applications. However, in the following cases, they are acceptable:

  • The area to be built on has been undisturbed for many years, has had time to naturally settle, and has an existing slope in front of the area the wall will face.
  • The retaining wall will be very short in height and will not weigh down significantly on its base.
  • There is mature lawn under the area where the wall will be placed, which has been there for at least a few years and slopes away from the face of your wall. A well-structured lawn is great at evacuating surface water.

However, in most other cases, consider taking steps to ensure that your wall will lie on a solid base. If your wall will be higher than around 24," it will apply a lot of force to the base it lies on, which over the first few years of the wall's life will cause unstable ground to absorb the weight more in some areas and less in others. This will cause your wall to shift or sag and lose its structural integrity.

Because of this, I suggest de-turfing the area, removing light soil or lawn, and adding "clean" drainage material, that is, material that doesn't include pieces under a given size. Many other landscapers don't use clean materials, meaning they include sand and silt that can cause clogging. I use "3/4 clean," which has only 3/4" pieces.

The problem with material that is not clean is that it creates a false base over your problem area by floating on top of existing materials and sealing them off. Clean material, applied in thin layers and compacted in stages, will be absorbed into the existing materials which lie underneath, adding to their ability to perform as an appropriate base.

In most cases, when a two-inch layer of 3/4" clean material is added to and compacted into an earthy base area, the two inches will easily be driven down into the existing material. Additional 3/4" clean material should then be added and compacted to create at least an additional 2" rise from existing levels in front of the future wall face to ensure water can evacuate freely.

In All Cases:

You should ensure that your wall will rest on an area that will slope in such a way that water can naturally escape. The face of your wall will have at least 18" of stable ground in front of it. Erosion will eat away at areas of the ground in front of your wall should there be a significant slope. If there is a slope of up to 25 to 30 degrees, you should have 18" of area directly in front of your wall sloping no more than about 10 degrees.

On any slope of 30 degrees or more, you should have a couple of feet of strong stable ground in front of your wall if you would like it to last any amount of time. You have to keep in mind that the weight of your wall will be forcing down on its base—and not only straight down, but at an angle up to 45 degrees away from your wall's face. You have to be sure that there is sound material in that area in front of the wall to accept the load.

Common Mistakes to Avoid:

  • Placing a drainage tile under your wall without complete drain preparation. This allows water to collect and move your wall if even the smallest shift should occur in underlying terrain.
  • Burying your base stones beneath the future levels of your finished landscape terrain. This will definitely allow water to gather around and under your base stones and move your wall.

Video: How to Build a Dry-Stack Retaining Wall

3. Laying Stone

Stone Choice and Placement

Your base stones should be your largest stones. They will bear the weight of all the other stones above them and behind them. You should also use similarly large stones behind your base, as well as behind the center sections of your wall. In other words, spread the weight of the stones over each other so that each one helps hold the others in place. This is to create a stronger wall whose stones are tied into each other.