16 Apr Retaining Walls: Natural Stone vs. Blocks
Whether you are building a retaining wall or having one installed for you, the materials used to build the wall need to be the best. There are a wide variety of blocks and natural stone to use. We’re going to cover some of the major differences (pros/cons, essentially) of each type of retaining wall for Minnesota. You can also learn more about retaining walls at our page.
A retaining wall is an integral part of any sloped yard. It allows a homeowner to have a garden, build a beautiful patio, control rainwater drainage, have some flat space for grass, or any other number of uses.
However, the real point of a retaining wall is to hold back soil. Soil is heavy. Not a huge enlightening statement there, but it’s important to remember that fact while you choose the materials for you retaining wall. The amount of force pushing on the wall can vary drastically depending on the length and height of the wall you’re building.
Retaining Wall Blocks
What are they: These are the most popular materials used for retaining walls. They’re the poured concrete blocks that you see pretty much everywhere. They come in two different sizes. Smaller blocks (about 12″ wide, 4″ tall, and 25 pounds) are used for small walls. They’re best used for plant beds and terraces or shorter-length walls. There are also larger blocks available (about 18″ wide, 6″ tall, and 50 pounds) for long and/or tall walls. If you’re doing anything more than a small wall or a plant bed, use these large blocks. The blocks also come in solid blocks (made out of solid concrete, so they weigh more) and hollow blocks (which will be filled with gravel after stacking).
Stay-Together-ness: Retaining blocks use a variety of different technologies to stay together and hold back the weight.
- The most common method is the lip method, where the back end of the retaining wall block has a lip hanging off which locks it behind the block beneath it. This is a tried-and-true method but does not allow for straight-faced walls like in an outdoor living space. The lip is always set so you cannot adjust the set back.
- The other common method for holding the wall together is the pin system. The blocks have holes in them where the installer uses fiberglass or tough plastic pins to anchor the blocks together. The advantages to this system is that you can control the set back so straight walls are possible. However, the holes are pre-drilled so if they get filled with dirt, it makes it difficult to pin. Corners are also tough.
Ease of Use: Blocks are extremely easy to use. The weight of the blocks can be a minor deterrence for some people but the stacking systems are reliable and easy to understand.
Advantages: Easy to learn how to use. Cheaper. Wide variety of color choices. Different stacking methods. Reliable for keeping dirt contained.
Limitations: Corners and decorative walls are harder to obtain and need special pieces or to be cut in half. Curves are difficult. Can be extremely heavy and labor intensive. Not as unique.
Natural Stone Retaining Wall
What are they: Natural stone pieces (like boulders) stacked to create a wall. There are different natural stone materials available. Boulder walls are a popular option, created by stacking very large (and very heavy) boulders in a line. Dirt is stopped from coming through the gaps using landscape fabric. There are also rubble walls. These walls are made from a random assortment of boulders and rocks to create a unique wall. You can use gravel to fill in the cracks behind the rocks. Cut stone is used for one other type of natural stone walls. In this method, stone is cut down to thinner pieces, which are then stacked on top of each other. Every stone is cut differently so the wall has a unique, natural look.
Stay-Together-ness: No mortar or locking systems needed here, just patience and heavy equipment or big muscles. When building these types of walls, you pick out each individual rock or boulder to fit in each spot as needed. This is isn’t so bad with a cut rock wall where each piece isn’t that expensive, but it can be hard when moving around 80+ pound boulders.
Ease of Use: Takes a very creative touch to find an individual stone that looks good and fits in the spot that’s needed. As touched on before, the rocks and boulders are really heavy. Each row has to be completely built up and filled in as you go so this is a time-consuming, patience-testing situation.
Advantages: Unique. More natural look. Much more varied color and design. Can fit any situation (wall height or length).
Limitations: More labor intensive because boulders/rocks are heavy. More expensive. Corners are tough. Time consuming to find the correct rock or boulder for the right spot.
Really Important Retaining Wall Details
Put down a good base
Regular old dirt expands and contracts with the extreme weather in Minnesota and other northern climates. If your retaining wall is built on the ground, the base layer will move up and down with the contraction and expansion of the dirt. This causes the wall to become unstable and fail within 2-3 years. To mitigate this risk, put down a good layer of base material (class 5 preferably). The different sizes of rock and gravel form almost a concrete-like layer at the bottom of the wall that resists expansion and contraction and keeps your wall stable for years to come!
Level the bottom layer (and every layer)
The wall should be extremely stable in order to hold back all that dirt. A wall that’s not level is much more susceptible to push back and failure. The base layer is absolutely the most important layer to get level. The rest of the layers rest on that one, so it’s absolutely crucial. For a block wall, we use a small level and get every individual block level, then use a 36″ level to check long rows.
Have good drainage!!!
Many retaining walls fail because water rushes down the back side of the wall, making the dirt weigh more and push out on the wall. Unless it’s a wall that’s less than 30″ tall, you need to have gravel on the entire back of the wall to allow water to run down the back of the wall so it does not push. Also, include a drainage tube with fabric around it at the base of the wall so any water that gets to the bottom can drain away.