A retaining wall is a structure that you use to provide lateral support to a soil mass. This means that the soil mass on different sides of your wall gets retained at different levels, and this is why your retaining wall materials are so important. You use retaining walls to bond two soil loads on two different terrain levels in areas around your landscape where slopes wouldn’t be desirable. This can happen when you build a basement, but it’s also extremely common when you build up your landscaping or gardening projects.
Many people assume that retaining walls function to hold a specific type of soil, but this isn’t nearly as simple as it seems. For example, normal walls will have a lateral support along the top of the structure, and retaining walls don’t have it. Normal walls also get built to support a vertical load, and a retaining wall deals with supporting a horizontal load. This puts more stress on the retaining wall materials.
However, in most situations, the vertical pressure applied to this wall isn’t as much as a vertical load-bearing wall holds. So, the ability of the wall to support this load is usually determined by the materials you pick out. So, we’re going to look at the most popular retaining wall materials, characteristics to consider, drainage, and more below.
Retaining Wall Characteristics to Consider
When it comes to your retaining wall materials, there are a few characteristics that you want to consider. This will help ensure that you get the strongest materials possible so your wall lasts for years.
Depending on whether you plan on building a large retaining wall next to your driveway or a short one for your plants, the retaining wall materials can change. When you have a smaller wall, it doesn’t have to be extremely solid. You could make it out of stones, wood, or stacked concrete blocks that use gravity instead of mortar to stay in place. A larger retaining wall that is part of your infrastructure will most likely need thicker boards, poured concrete, or boulders that can support heavier weight loads.
Maybe the design style or aesthetic is important for your retaining wall. With smaller projects, you have more design freedom because function and safety aren’t as pressing as with a larger wall. With big walls, you have to prioritize safety over looks because they have more force and weight pressing on them. W
here it’s possible, using stone or wood retaining wall materials can give the wall a pleasing, natural look that blends with your landscape and can boost how the nearby buildings look. You can also improve how your finished retaining wall looks by applying an outer finish in some instances. For example, concrete is a very popular retaining wall material that you can cover with stone veneer.
The amount of water your retaining wall materials get exposed to will be something to keep in mind. Water can be precipitation, waves, or humidity in the air. The retaining wall may function to protect the coastline, or you might put it in a place where there is no water around it but humidity is a factor. For a coastline retaining wall, poured concrete is usually the longest-lasting option. Also, concrete is ideal in this situation because you can create a nice slope in the design to help deflect the force of the water hitting it easier than you can when you make the wall out of stacked materials.
For humidity or frequent snow or rain, you want to avoid using wood as your retaining wall material since it can’t survive rot in an environment that is moist or wet. Stone, brick, and concrete blocks will withstand water better as long as you have good drainage in your retaining wall.
13 Retaining Wall Material Choices
As we touched on, retaining walls are popular landscaping tools to help keep soil at a specific elevation. Typically, the wall will be vertical and divide two levels of ground that can be sloped slightly or flat. You will generally see smaller retaining walls in residential gardens or landscape designs, but there are also large-scale ones at the bottom of hills that protect roads or buildings.
Due to the massive issues you could run into if your wall failed and the earth slid down past it, you need to carefully consider your retaining wall material. The material you pick out should match your project’s limitations and requirements. You also have to look at the material characteristics and material weight before you make your final decision.
Using boulders as your retaining wall material is, without a doubt, the best-lasting and oldest kind of manmade structure. Unlike manufactured modular blocks or quarried stone pisces, boulders are very easy to obtain. They’re also ready to go without any refinement. At the same time, boulders keep most of the benefits you get with furnished stone, and they can last for a minimum of a century. Boulders are great for any colonial, country, or English-style garden design.
Brick is a very traditional and popular retaining wall material, but the composition of the brick can vary wildly. Lime, clay, and sand can get mixed into your bricks. In spite of this, you’ll get shared features through most brick types. You won’t need to worry about moisture if you use brick, and it’s a decently durable choice. Unlike stone, you will have to apply mortar between the bricks to ensure they stay together. Additionally, trying to replace a damaged section of your retaining wall with brick as your retaining wall material can be very time-consuming and challenging because it’s easy to challenge the wall’s integrity.
Brick retaining walls are a very popular choice for a more formal, English-garden style look and feel. Brick can easily last for years, and it’s very low-maintenance. Retaining Wall Going Up by Shawn Henning / CC BY-SA 2.0
You can get poured concrete or concrete blocks for your retaining wall material. Concrete blocks are usually better if you plan on building a shorter wall because making it too high can weaken the wall. They’re very simple to put into the correct place, and there is a huge selection of bricks you can choose from. Additionally, concrete bricks are tough, so while a large rock wall can slowly lose stability, a short block wall will stand up to the force of the soil or the elements.
You can transfer poured concrete in the liquid stant into molds at your retaining wall build site where it will harden into the mold’s shape. Since you’ll have to worry about fewer pieces, this retaining wall material can support much larger amounts of soil than blocks can, and poured concrete is very popular in infrastructural retaining walls. Also, you don’t get stuck with just a straight vertical wall. Some poured concrete retaining walls get reinforced with a horizontal concrete base underground that prevents it from shifting. Others will feature triangular columns that strengthen the wall. However, concrete does have a very cold look and feel.
Gabion is a very, very old wall building method. This name comes from gabbia, and this is an Italian word that means cage. This describes this retaining wall design very well. For the retaining wall materials, you’ll get wire or steel rods that you set up and fill with rubble or rock. They were very popular throughout the Civil War as they were great for protecting soldiers, but their ability to work with a host of recycled materials made them popular today among eco-friendly builders and homeowners.
5. Interlacing Concrete Block Retaining Walls
Any retaining wall that uses interlacing concrete blocks also needs you to install a crushed stone footing to give it more stability. There are heavy-duty mesh anchors placed against the ground at a set distance apart, and it comes with a batter of one inch for every foot of wall.
Limestone falls into the sedimentary rock category, and it’s made up of skeletal fragments that marine organisms left behind, including mollusks and corals. The primary materials in limestone are aragonite and calcite. You may mistakenly think that limestone is much more fragile than traditional types of rock, you have to remember that some of the most long-lasting buildings around the world like Giza’s Great Pyramids and the Taj Mahal feature limestone as the building material. However, you have to keep in mind that limestone has disadvantages and advantages.
If you want to create a retaining wall using this retaining wall material, you’ll need the help of a professional mason. The footing has to go below the frost line, and the footing should be made out of rebar that is reinforced with concrete. This will be a steel bar or steel wire mesh that gets embedded into the concrete to help strengthen it. There must be weep holes placed every four to six feet along the wall. If you have mortar-free walls, you’ll only need to have a footing made out of crushed stone instead of reinforced concrete. You should also add a batter of one inch per one foot of wall.
Masonry walls require specific materials and knowledge to make them structurally sound and safe, so you’ll have to call in a professional to create it. Redi-Rock-Retaining-Walls by Redi-Rock International / CC BY 2.0
8. Plastic Lumber
Plastic lumber allows you to have very durable retaining wall materials with a much more natural look. HDPE plastic gets molded into boards that can get the surface grain texture of wood, and you can choose from a range of colors. Different shades of gray or brown are very common, and the material fits well with different organic surroundings.
As a bonus, the plastic makes this retaining wall material resistant to the usual weaknesses wood has, like decomposition and water damage. You may find fiberglass mixed into the plastic lumber, and this increases the hardness to the point where you can safely use it for bigger projects. There are reinforced types of plastic lumber that can withstand coastal or crashing waves and the weight of supporting bigger chunks of earth.
A segmental-style retaining wall is one that gets made by interlocking identical pieces of precast concrete before anchoring them and backfilling them with soil. This retaining wall material is most commonly used with geotextiles when your slope’s grade is greater than 45-degrees and it’s not recommended to have a reinforced soil slope.
Stone has been around as long as wood for retaining wall materials because it’s very durable. It also has a very close natural appearance and composition, and this works to enhance the visual appeal. To create retaining walls, you can cut stone into even blocks that smoothly fit together or you can fill a wire framework with stone to create a gabion-style wall. The way you choose to use the stone can change how you work with it.
Blocks can enhance the beauty of the stone, and you can arrange it to fit into whichever shape you need very easily since you don’t need mortar to stick them together. The process to place the stones can be time-consuming though. Putting stones into a gabion-style retaining wall is quicker, but it has a more rustic look. The wire can also corrode due to water exposure.
11. Stone Veneer
This retaining wall material is protective, and you can use it as a decorative covering for vertical surfaces and walls. The veneer itself is purely an aesthetic choice, and it needs a solid core to work as well like CMU blocks. However, it looks very nice on your walls, and it adds a hint of luxury to the wall.
Even though you may think that wood and timber are the same things at first glance, they’re not the same building material. When you talk about timber, you’re describing the wood’s stage after you cut the tree. Any supposedly finished timber is the wood after it gets processed and cut into different sizes, but it’s in a column shape. So, timber is a different building material from wooden planks that we’ll touch on below.
Wood gives you a very rustic and classic look and feel as a retaining wall material. The texture and grain are visually familiar, and this lends a very desirable aesthetic to your finished wall. This is because it comes from a once living source, so that makes it easy to blend into the outdoors with the surrounding earth and plants. Wood is also easy to cut and handle to form whatever shape you need, and it’s a lighter weight with a higher degree of flexibility.
However, the downside of this retaining wall material is that it’s not as sturdy or resilient as many other choices. It can’t take on too much weight without failing, and it’ll decay from insect and water exposure. You have to routinely perform maintenance and treat it with chemicals to keep it in good condition year in and year out.
Wood isn’t a hugely popular retaining wall material because it’s more prone to decay and breaking down with exposure to the elements. Backfill behind the backyard retaining wall by LISgirl / CC BY 2.0
Retaining Wall Finishes
Once you pick out a retaining wall material and build the wall, you may want to add a finish to give it a more elegant look and feel. Stain and veneer are two options that many people like.
You can give your interlocking retaining wall materials a new look by adding a penetrating finish in a golden brown or reddish brown coloring. You can apply water-based stains with a roller or sprayer directly onto an unsealed, clean surface.
To bring life to a bland surface, you can adhere thin slices of stone or brock, or you can add concrete molded to mimic brick or stone. You get a traditional look for less than you would pay for regular mortared masonry.
When to Replace Your Retaining Wall
Even though a solid retaining wall can last for decades, there will come a point where you need to replace them. If you see any of the following, they’re clues that it’s time to consider a new wall and new retaining wall materials. They include:
- Bulging – Bulging in your wall is a sign that there is water pressure buildup behind the wall. A lack of anchoring with interlocking blocks can also cause it. Carefully excavating the site can save your retaining wall materials, but you may have to scrap it and rebuild it.
- Cracking – You can fill in a range of small cracks in the wall, but any cracks that are more than a quarter of an inch wide and deep and that extend more than two feet signal that you have structural damage to your retaining wall materials.
- Leaning – Failed footing, poor drainage, and tree root growth can cause your retaining walls to lean.
- Sagging – Sagging is one sign that your footing failed in a certain spot. A contractor might be able to rebuild the section. However, depending on how bad the failure is, you may need to replace the whole wall.
DIYing Your Retaining Wall
Interlocking concrete blocks and timber retaining wall materials are great DIY options. Poured concrete and mortared masonry are usually left to a professional mason. If you want to build a retaining wall yourself, most places have codes that state that any wall over four feet tall has to be built by a professional and have an engineer design in.
How to Pick the Best Retaining Wall Backfill Material
What the best backfill materials are for your retaining wall will depend on how much rain you receive in your planting zone, how good the drainage in the landscape is, and a few other factors. There are generally three popular backfill materials, including:
- Native soil
In order to get a drainage-optimized, reliable foundation for your retaining wall materials, you have to ensure that it gets backfilled correctly. The following five steps will help you backfill the retaining wall correctly. They include:
- Lay a three-inch base of compacted native soil
- Compact the soil to ensure that it’s firm and secure
- Layer the next 6 to 12 inches with gravel or aggregate
- Compact the aggregate or gravel to ensure you get a very sturdy base
- Fill in the remaining six inches with compacted native soil to let plant growth and grass around your retaining wall’s base
The Importance of Backfilling a Retaining Wall
Adequate backfill is critical to help ensure that your retaining wall drains well since water can’t pass through most retaining wall materials. Insufficient backfill can cause poor drainage, and it can result in warping in your wall. In turn, cracks or bulges can form, and hydrostatic pressure can build up and cause the wall to fail.
To ensure that your retaining wall will last for years at a time, you should invest the energy and time necessary to install backfill properly. If you don’t have time to do this yourself, you can hire a company to do the retaining wall backfill for you.
Retaining Wall Drainage Systems
Your drainage system has many key components to it, including a filter fabric, drainage stone, outlets throughout the wall face, and perforated pipe. Before we get into this section, you should know to never use grout between the blocks when you use segmented blocks as your retaining wall material. Water should be able to drain between the blocks, and grout in these small gaps would trap the water. Segmental retaining wall blocks that are properly installed will have pins or lips to keep it from shifting.
When You Need Drainage Pipe iBehind the Wall
Every retaining wall and retaining wall material should have drainage stone in the back. It’s a good idea to install a drainage pipe too, but there are certain situations where having a perforated drain pipe is necessary. The following are instances where you’d add a drainage pipe behind the wall:
- Buried Water Sources – Any retaining wall that has a buried water source within 50 feet of your wall site, like a water main, irrigation, or a hose line will need a drainage system.
- Groundwater is Present – Groundwater can be slightly more challenging to detect. If the area is wet when you start to excavate it for the wall project, or if water starts to collect by the wall location when it’s dry, it’s a good clue that you have groundwater close to the surface.
- Higher Than Four Feet – Retaining walls higher than or equal to four feet when you measure from the top of the wall to the foundation will need a drainage system. Segmented blocks can’t hold much water weight by themselves before failing. Additionally, walls higher than four feet can cause a huge amount of damage if they fail.
- Poor Draining Soil – If you have clay-based soil behind the walls, you’ll need a drainage system incorporated into your design. When clay gets wet, it’s very weak. So, this means that it’s essential to give water a way to escape from behind the wall.
- Poured Concrete or Cinder Block Walls – If you choose these for your retaining wall materials, they don’t come with natural joints in them for the water to drain like wood walls or segmental blocks do. These walls will need a very robust drainage system, no matter how high the wall is.
- Sloping Ground Toward the Wall – It’s a fact that water will naturally drain downhill. If the ground slopes toward the wall, you’ll need a drainage system to remove the water.
Finally, you have to account for surface water. Double-check for any gutter downspouts by the retaining wall and check where the water drains are within your property to ensure you can divert water away from the wall. If you have a downspout behind the wall, you should install additional pipe to help drain the water from the wall. Don’t use a perforated pipe to transport water from your downspout behind your retaining wall.
You can use berms or swales to redirect surface water away from your retaining wall materials. When it’s possible, put a berm or swale at least twice the height of the wall away from your wall surface. If it’s not practical to install a pipe or a swale, you’ll need a drainage system installed.
You now know the most popular retaining wall materials you can use for this project, and you can decide which one is going to work best for your needs. Once you pick one, you can create your retaining wall to last and provide structure and support for your landscape or garden.
Jen is a master gardener, interior designer and home improvement expert. She has completed many home improvement, decor and remodeling projects with her family over the past 10 years on their 4,500 sf Victorian house. She is also a passionate farmer who keeps goats, chickens, turkeys cows and pigs on her farm, and an instructor for her community’s Organic and Sustainable Farming project.