Why Does the Cost of A Retaining Wall Vary So Much Between Contractors?

How can the same retaining wall vary so much in cost between different contractors is a very good question.

I applaud you for taking the time to understand where your money is going so you can make an informed buying decision.

In this article I will answer this question as honestly and transparently as I can. First, please allow me to introduce myself.

My name is Sam Maerz, and along with my wife Natasha we own Back 40 Landscaping.

Personally, I am a level 2 Allan Block trained installer, and our entire team is trained and certified by Allan Block.

Allan Block training and certification is a great foundation for building strong structurally sound retaining walls.

The real learning and understanding of how to build good retaining walls comes from building, being obsessed about how they should be built, and how one can improve themselves.

This is where being German, with an eye for detail, and maybe a bit of a perfectionist really helps us out.

Building strong retaining walls is a minimum, and our real goal is to build a structure that looks so good it becomes a center piece or focal point in your landscaping.

Retaining walls in my opinion can be the holy grail as they have height, depth, color, and curves which are all key elements in a beautifully designed landscape.

To build a high-quality retaining wall that looks perfect is no easy task. After every project we submit lots of photos to Allan Block for coaching.

We debrief as a team and talk about what, if anything, we would change if we were to build this wall again.

In the beginning we learnt a lot, and incorporated those changes into every wall we built.

Eventually we worked to a position of building near perfect retaining walls.

If you are like most clients so far, you are on board with what I’m saying. You are following the logic that to improve you need to take an honest hard look at your past, identify weakness, and make changes.

The path to building great retaining walls is simple, but are most retaining wall companies putting in the same time and effort to improve?

In January of 2023 my team drove around Abbotsford, and Mission. We took note of failing retaining walls in people’s front yards.

Of the over 500 retaining walls we observed, approx. 50% of them were concrete block walls.

Concrete block retaining walls are supposed to last a lifetime…

While the blocks themselves are not failing the installation of them has failed. Retaining walls are unlevel, sunken and wavy, they are leaning over, blocks or caps have come loose, and they generally look bad.

The structural integrity of some of them may be still be there, but a lot of them look terrible (no offense!).

When we try to understand how 2 retaining wall companies can price the project different, we need to understand the broad strokes.

In this section we will cover the biggest factors that will influence your retaining wall project. How much weight you put into each section is entirely up to you.

You may value a lower price today, but keep in mind that will affect the longevity and overall cost long term. This cost may be in dollars, or it may be in losing out on a retaining wall focal point that could make your landscaping look unbelievable vs. run down and in disrepair.

The main components we will cover answering why the cost of a retaining wall can vary so much between contractors are:

• Planning
• Concrete retaining wall block brand
• Excavation
• Soil Disposal
• Gravel
• Drainage
• Strength
• Embedment depth (buried block)
• Craftsmanship
• Cleanup
• Follow up
• Overhead
• Warranty

Planning

Retaining walls surprisingly take more planning than would be expected. Because of their cost, and lifetime durability, it’s important to put them in the right spot the first time.

Planning – Location

Site surveys should be conducted if it’s not clear where property lines are to ensure the retaining wall, its drainage system, and the supporting reinforcement components are all on your property.

Retaining walls are used to create flat useable land. If your backyard is on a hill carving out the hill to install a retaining wall further back will increase your flat usable land.

However, if you simply placed the retaining wall Infront of the hill you would not need to excavate as much or any soil. This saves labor, soil hauling and disposal, but it does not increase your flat usable land.

While it may be tempting to save money up front you won’t be getting the same resale value as if you increased and made your yard larger. Moving a retaining wall down the road is much costlier and not a practical option in most cases.

Planning – Fences

Prior to building a retaining wall you should decide if you’re going to be putting a fence on top, or behind the wall.

Ideally as the wall is built Fence anchor systems like “Sleeve-It” or Concrete forming tubes like “SonoTube” should be installed.

This allows the fence post to go deep underground, while not disturbing the compacted drainage and structural reinforcement behind the wall.

On walls over 4’ this planning has to take place prior to construction as the geogrid reinforcement will physically block a fence post from being dug into the ground.

Constructing a fence on top of the retaining wall caps alone is a bad idea.

The caps are glued on, and do not have enough strength to hold a 3-6’ high fence that will exert leverage on the glued cap.

If fences must be installed on top of a retaining wall the steel posts will have to be installed through the blocks as they are built.

Planning – Site Services

Digging under a retaining wall in the future could compromise its structural integrity so it’s important to plan for reasonable services or things that may need to be run under or behind it in the future.

Site services include things like electrical, conduit for electrical, internet or stereo wires, plumbing pipes for future buildings, drainage, water or irrigation lines.

Throughout the course of talking to your contractor these types of questions are brought up, answered and planned for.

Planning – Retaining Wall Add-Ons

Retaining walls serve a practical purpose of holding back soil, but they also can serve more functions like design and be a focal point in your landscaping.

Optional add Ons like lights, irrigation, flowers, stairs, and handrails all are add-ons that can truly help you get the most out of your space.

Discuss these with your contractor in the planning phase to ensure optimal efficiency, and unexpected costs when building your retaining wall.

Concrete Retaining Wall Block Brand

There are several options available when choosing a concrete block for your retaining wall. The physical size, color, texture, availability, and manufacture support should all be taken into account when choosing a block.

The largest option would be the big commercial style lock blocks that are 2.5 feet wide, 5 feet long, and 2.5’ high.

This commercial looking block is solid concrete, and must be set by a large machine. If your home has not been built this may be an option as you will probably have enough room to have this large block installed, if you like the look, color, and texture options of this block.

Typically, though, this is not used in a residential setting as the site isn’t large enough to place after the home is built. While it is concrete, the standard grey color can look a little drab, and not usually considered “nice to look at”.

There are smaller options available that can come in a flat finish, or textured. On projects where you want a different look they are new finishes that may be exactly what your looking for.

The blocks that can be lifted by hand come in large or small block sizes. The large ones are about 75lbs, while the small ones are half that at about 40 lbs.

The larger 75lbs blocks are the Allan Block classics and are 18” wide (long), 12” deep, and 8” high.

This block size is what most contractors should be using for a residential retaining wall. Its large enough to provide a great structural retaining wall, and light enough to not need a machine to install it. It comes in popular color options, and has a nice uniform split face (rough).

The half size blocks are not as common among contractors, but are very popular among DIYers These blocks are significantly lighter and easier to carry. If you drive around town and look for this type of retaining walls most of them are failing in some way. This is not because of the block, but due to lower quality installs, less gravel used, little to no compaction, and poor drainage systems reducing the water weight behind these walls.

If built correctly these walls are just as strong. Because these blocks are half the size, you need twice as many.

The additional blocks create more opportunity for a block to be out of place, or to be crooked. Time saved in lifting is often spent straightening them.

A contractor may choose to use this half size 9” block if tight radius curves are required as they curve much tighter than the bigger 18” wide blocks.

The 2 most commonly used concrete block brands are Allan Block (Belgard) and Valley stone (Basalite). Both are similar, with Valley stone being about $1 cheaper per block. On a large project this effects the over price by about 0.01%.

This math is based on a $1500 block difference on a $150,000 project.

Here at Back 40 Landscaping, we work with Allan Block for this size and style of block. Their contractor training, and field support is worth every tiny additional cost. Every project has unique challenges, and Allan Block is very quick to get on a video call for support, or even stop by the site. Before, during, and after every project they coach, train, and provide invaluable feedback.

This trusted partner ensures our retaining walls will actually last as long you’re expecting.

Excavation

Excavation is a large part of building a proper retaining wall. This excavation includes digging the dirt, hauling it to the front yard, loading a dump truck, hauling the soil offsite, and then paying for the soil to be disposed.

In almost every project the amount of soil excavated will be considerably more than you are thinking. Soil expands 1.3-1.4 times the volume when excavated as it uncompacts.

The biggest challenge most contractors face is the additional cost and labor associated with dealing with this amount of soil. It is very common to limit the excavation, or to cheat it entirely.

If you are having a tiny wall built it may be possible to excavate the required amount of soil by hand.

On walls over 10-20’ long mechanical excavation should be done. This reduces labor, time, and ensures that the excavation zone is not cheated due to fatigue.

The most common method of moving soil to the front yard is with a wheelbarrow. This is not easy work, and will often times mean your retaining wall builders are laborers and not craftsman.

The weight of the wheelbarrow, distance travelled, and having to go uphill all make wheelbarrowing a poor choice for a competent retaining wall builder.

In recent years narrow track machines have become more popular, as they can reduce the fatigue of the operator, and move material in and out for days or weeks at a time.

Rentals of these machines are also an option if your contractor does not own them.

We used to own 2 of the common track dumpers that you ride on and after using them for a while we realized they really aren’t that great, as they only dump off of the front. This means you need 6 plus feet of room to work.

We looked at a new model called the Wacker DT10. It’s a stand on machine that can carry 3000lbs of material, and it swivels 180 degrees. This allows the operator to drive alongside a narrow area and dump off of the side.

For retaining wall work this machine is indispensable. Without these specific machines you simply cannot move the material as fast or efficiently

What happens if my contractor does not excavate enough?

The simplest answer is that there will not be a drainage zone of gravel behind your wall. This will add weight (water and dirt) and will put pressure on your retaining wall and it will fail.

Gravel

Retaining walls built to industry best practises have a minimum of 6” of gravel underneath, and 12-16” of gravel behind them.

The wall rock under and behind should be clear, angular, and contain less than 10% fine material. This allows water to quickly drain through, and to get away from your retaining wall.

Allan Block recommends using angular clean rock ranging in size from .25” -1.5”. The exact size is dependent on the purpose the rock is serving as well as local availability. I’ll cover this next.

The gravel base for a retaining wall is used to provide a stable, level bed for the first course of retaining wall blocks.

Just like a home the gravel foundation is crucial for a level, long lasting retaining wall. The larger the rock is the more “bridging” capabilities it has. Meaning if a soft spot develops, or the ground settles a little bit, larger stones will be able to compensate, and bridge this.

Locally ¾” clear gravel is readily available so this is most often the subbase gravel. We use this for the first 5” of the 6” gravel base.

Basing the first row of Allan Block is time consuming and tricky. The larger the stone is, the harder it is to make fine or micro adjustments.

Instead of basing on the ¾” subbase we use a 1” thick bedding layer of ¼” clear material. This allows us to base perfectly level blocks, while maintaining a clean draining gravel base.

Typically, we see contractors basing blocks on sand, or very fine gravel. This is a big problem because as it rains the fine sand is washed down into the coarse gravel.

A 1” thick bedding layer of sand can and will literally settle down and disappear. This causes your retaining wall to settle at sink into the ground.

The gravel behind the retaining wall should be ¾” clear material. This is the same rock that is used as the subbase for underneath the leveling layer of ¼”.

The ¾” clear gravel is a great choice behind the wall as it drains water very quickly, and compacts in only a few passes.

Drainage

Without drainage the wall that is built won’t look any different from the front, until a few months or years have passed.

Then it will become apparent that there is a lot of water seeping through the block, and that the wet heavy soil behind the wall is pushing really hard on the wall.

Lack of drainage has and will cause retaining walls to fail.

The wider the drainage zone is behind the retaining wall the less surcharge will be exerted on the wall from wet soil.

Drainage is all about reducing the weight of the soil the retaining wall has to retain. The less weight a retaining wall has to hold the longer it will last.

If you expect your new retaining wall to last as long as the concrete blocks your using make sure the drainage zone meets the minimum standards of 12-16” wide.

Strength

Retaining walls do not retain the weight of soil with just the blocks.

Imagine standing with your feet together, and being pushed. It doesn’t take a lot of force to move you. Now if you stand with your feet shoulder width apart you will have a lot more balance, and will be able to resist a lot more force.

This width or depth is what actually retains the soil, the combination of the blocks and the drainage gravel added together is what hold back the soil.

Now, if you simply stack blocks and pour gravel behind the wall you won’t be combining them in any way.

If it’s a small wall under 4’ you can combine the weight of both the blocks and gravel by compacting the gravel in the blocks and behind the wall.

You do this by core filling the blocks, and drainage zone behind the wall, and running a steel compactor directly on-top of the wall extending towards the back of the excavation on subsequent passes.

If it’s a taller wall than 4’ you will do this, but you will also add a layer of Geogrid (plastic netting) to this mix that will help this to lock together even more.

Compacting directly on top of the blocks is industry best practises, required by Allan Block, and yet rarely done in the field.

Most contractors are scared to do this step as its seems like it will break the blocks, while some may not have the correct equipment to do this.

Not compacting your retaining wall blocks and drainage gravel will reduce the strength of your retaining wall by my estimation 35-50%.

Its significantly harder to pull a block out when it’s been compacted vs. when no compaction has occurred.

Curved retaining walls are stronger than straight walls, and in most cases look better. They do require more work to install the caps, but when they last a lifetime, this added labor and cost really doesn’t add up to much.

Allan block retaining walls should be either straight or curved. They ideally should not have angles.

When a contractor installs an angled wall, this joint is significantly weaker as the “running bond” is from the bottom up the top with no block overlap. This can almost always be avoided by doing a curve instead of a sharp angle.

If an angle is required one leg of the angle should extend into the hillside to protect the other leg of the angle.

This actually looks better, and is more structurally sound. Unfortunately, most contractors do not take this extra step to build a good looking structurally sound angled wall.

*A structurally sound angle can only be done on an “inside” angle. An “outside” angle cannot have one leg extend into the bank. For outside retaining wall angles, use a curve.

Embedment Depth (Buried Block)

Embedment depth is how much of the retaining wall bottom row is buried. Typically, you will want at least .5 – 1.5 blocks buried. This is to stop the bottom of the wall from being pushed out over time.

If the ground below the retaining wall is sloped downhill you will want more buried as this is more likely to slide out.

I looked at 10 photos on Facebook marketplace, and most of those retaining walls did not have any buried blocks.

This will save you money upfront, but will cost more later when the bottom of the wall blows out. Buy, and bury the required amount of block so your retaining wall only has to be built once.

Craftsmanship

Like any service sector retaining walls can be built only as good as the craftsman building them. Attention to detail, skill, and effort will all effect how structurally sound and how long your retaining wall lasts.

The blocks used in building your retaining wall are not all the same size. This means that we simply do not stack them up if we are trying to make your wall look level, straight and without waves.

As the blocks are made some burrs of concrete are left behind. This has to be scrapped off prior to setting them in place. Occasionally one block is poorly formed and needs to be replaced.

All of this adds up to building a retaining wall vs. stacking one up.

The placement of cut blocks too close to a focal point, like a set of stairs, will draw your eye away from the stairs and to the cut block. This causes your wall to not look as good as it could.

Likewise, cut blocks that are too small will cause them to stand out. Instead, we try and cut down several surrounding blocks so the cut block is as large and uniform as possible.

Step downs, step ups, and retaining wall returns are all places where attention to detail, and knowing the best way to finish the wall, are all subtle components that can make a regular retaining wall both stand out as a focal point, and yet not stand out as a concrete structure.

Cleanup

Site cleanup can be simple or full turn key. Extensive excavation usually means that unless equipment worked off of plywood roads there will be lawn damage from tracked machines.

This area can be scrapped clean, new topsoil added, and sodded for a near instant new lawn.

 Plants can be added, and the surrounding areas of the retaining wall can be left in a fully finished ready to enjoy state.

Roadways, driveways, and sidewalks are always power swept, and can be pressure washed as needed.

This full turn key finish means you’re not watching us rush out of your job as fast as possible.

For small projects it may be better to protect the space rather than to fish the lawn afterwards.

Back 40 actually has over 400’ of temporary roadways that will keep your lawn, and concrete completely undamaged. The larger the project the more it usually makes sense to remediate vs protect.

Follow Up

If you have ever tried to get a hold of a contractor for a quote you know how bad most of them communicate. Imagine trying to get a hold of them after you have paid the last payment to them. Good luck!

Because questions come up, and you are our customer hopefully for life, we follow up with you 3 days, 3 weeks, and 3 months after your project is completed.

This is your time to ask or confirm anything you want, or just to let us know how much you are enjoying your new retaining wall.

At the beginning of every season, we check in on our previous years retaining walls to see how they made it through their first winter.

Caps, stairs, and walls are all checked to make sure they are still strong, safe and looking good.

Overhead

Overhead will vary from company to company and contractors will have to charge what they need to, to make a profit. Companies that are profitable will be around to serve you well into the future.

If a company does not truly know their numbers, they may charge less than they need too to stay open. Not charging enough, actually does affect you as the client.

Warranty

Warranty is the last point we will discuss today as we answer the question how can the cost of a retaining wall vary so much between contractors.

In Abbotsford, Mission, Chilliwack and Langley I am not aware of a retaining wall contractor offering a warranty.

The industry standard or best practises warranty is one year on labor, and an Allan Block lifetime warranty on the block.

As of February 2023, we are very close to becoming a Belgard Authorized Contractor. This is an elite recognition by Allan Blocks parent company Belgard. Along with this designation we will be able to offer a 2-year warranty on labor, with a lifetime warranty on the Allan Block used in building your retaining wall.

By now you have hopefully seen how each contractor builds their walls a different way, and why prices for retaining walls vary so much throughout the Fraser Valley. 

To get a wall that not only looks great, but will function year after year, please reach out to us to get a quote!