Retaining Walls: Wood vs. Allan Block
With actual pricing
Actual prices are presented at the conclusion of this article. Please don’t skip ahead!
1/3 of our inquiries from homeowners looking to build or replace a retaining wall ask us what are the main differences between a wood retaining wall vs. a concrete Allan Block retaining wall.
What is the price difference, the longevity, and the pros and cons of wood vs. concrete?
In this article I’ll answer those questions, and provide the thought process behind why I feel this way.
I am an Abbotsford based Landscaper who builds both. I have no stocks or shares in any wood mills, nor do I have any investments in Allan Block, or any other concrete companies.
I simply want my clients to understand the pros and cons of both Wood and Allan Block retaining walls so you can make an educated buying decision for yourself and your family.
Ground rules - Wood vs. Concrete Allan Block comparison
Retaining wall size
Our project today will be for a 75’ long by 3’ (36”) retaining wall.
Building code permits, Engineering, and Geogrid reinforcement will not be required.
On any project without permits and engineering the building practices will vary. The best practices are a higher initial investment, but like all high-quality builds last substantially longer.
Wood is a natural resource commodity and being such does not have a company behind it offering building advice, warranty work, or ANY resources.
What’s even worse is that most pressure treated lumber is not rated for ground contact. Meaning it’s supposed to be off the ground, and never touching soil. It really isn't suitable for a retaining wall.
Typically, you won’t find ground contact lumber at Home Depot, Lowes, Rona or any other local big box store.
On the other hand Allan retaining wall blocks, are a manufactured concrete product where every block is identical, and engineered to last a lifetime or longer. There are hundred of resources available covering all scenarios. Backed by Engineers, scientific research, and evidence.
I am not saying Allan Block is better
Wood is less expensive, and will last approximately 20-30 years.
Allan retaining wall blocks are a larger initial investment and will last a lifetime.
However, both walls will fail much sooner than expected if not properly installed.
Installation in my opinion is the most important ground rule when it comes to building a long-lasting retaining wall in your outdoor space. I’ll keep it as simple as I can.
Both Wood and Concrete Allan Block walls retain soil. This means that decade after decade they hold back hundreds of thousands of pounds of soil. The drier this soil is, the less water pressure and weight they have to hold back.
The majority of retaining walls fail because of a lack of a drainage system behind the wall
A proper drainage system is composed of a minimum of 16” of clear draining gravel directly behind the wall, and a drainpipe at the bottom of the wall moving the water away.
The wall itself whether that’s Wood or Concrete is not what holds the weight back.
Effective retaining wall systems are a combined effort between the wall, and the gravel, and in some cases even further back if Geo Grid or Deadman’s are used. The combination MASS of the wall, gravel and soil is what resists the weight surcharge and prevent the wall from failing and overturning.
Therefore, if a heavier concrete wall like Allan Block is calling for specific amounts of gravel, and drainage to build a structural retaining wall…
A lighter product like wood should also be built to at least the same standards if not better.
Both projects below will be prepared the same way. Temporary plywood roads to protect driveway, lawns, and any other valuable existing landscaping from excavation equipment.
The same size area will be excavated for both the Wood and Concrete retaining walls. Including hauling and disposal of the same amount of soil.
Cubic yard calculation including base and 16” behind wall
42” X 75’ X 16” = 12.96 YD3
Soil expands by 1.3-1.4 times in volume when unearthed so this produces 17 YD3 of soil to excavate, load, haul, and dispose of.
Wood and Concrete retaining walls both need an adequate gravel base and a proper drainage area behind the wall.
Gravel is essential for the base to provide a solid level foundation to build upon. It drains water away to eliminate soil erosion, decomposition of the wall material, and to remove the standing water below the wall to prevent frost heave in the winter time.
Gravel behind the wall
I already covered this above but it’s worth another mention because of how important it is, and how often its not done. Excavating and removing soil behind your wall, and replacing with gravel will add significant costs to the project, but must be done if you expect it to last for longer than 5-10 years.
WELL OVER HALF of all retaining walls are in a state of failure, built unlevel, crooked, have gaps between the Ties/Blocks that open up more every year
Drainage pipe behind the wall
A proper drainage system consists of one 4” pipe behind the base of the wall called the Toe Drain. Larger walls require 2 pipes. On the far end of the wall there should be a clean out cap for future maintenance, and the low end of the pipe should be tied into an appropriate drainage system, such as city storm drain, inground concrete sump, perimeter drain system, or ditch.
Both the Wood and Concrete wall are laser leveled for grade accuracy, and installed with a stringline for straightness.
Both walls will have a 6-12° setback to aid in structural integrity.
The Wood wall will be installed with 5 nails / 10’.
Allan Block Caps glued down with PL Concrete Glue
Summary of the ground rules
- Size 75’ long 3’ high
- Built to best practices based on Allan Block’s Engineering and Scientific Evidence
- Wood RATED FOR GROUND CONTACT – 20-30 yr. durability
- Allan concrete block products – Lifetime durability
- Surface protection – Driveways, lawns, finished landscapes etc.
- Excavation – 17 Cubic yards of soil hauled and disposed of
- Gravel – 13 cubic yards of ¾” clear drain rock for Base and Drainage behind the wall
- 4” Drainage Pipe tied into appropriate discharge area
- Installed level, straight, consistent setback
- Wood - nailed, Concrete - glued
- Site cleaned up, lawn repaired if needed, driveway and street cleaned
Wood retaining wall Vs. Allan Block concrete retaining wall
We are asked which is better between wood and Allan Block very frequently. In most cases it comes down to:
- Recommendation of the company you are hiring
- Suitability for your property
- Design, color, and style options
- Cost of initial investment and maintenance
- New retaining walls tend to be Allan block / Concrete option
- Replacement of wood is 50% wood, 50% Allan Block upgrade
Recommendation of the company you are hiring
For ALMOST every project I would recommend a Concrete Allan Block wall or SIMILIAR concrete, natural stone wall.
Back 40 Landscaping recommends a retaining wall that will last a lifetime
- They look significantly better. Day after day for decades you will look at your wall. While it’s true they do serve a very valuable structural component they also look impressive when built level and straight. They add a tremendous depth to your landscaping, and will be a focal point in your outdoor living space for a lifetime.
- They are available in a huge array of sizes, colors and shapes, sure to meet even the most detailed designers’ dreams.
- Structurally they are stronger and backed by Allan Block Resources
- Less stress – Knowing you will not have to replace or construct another wall can be a huge weight off of your shoulders and your families.
- Our world and lives are too busy as it is. Its more efficient to do your retaining wall project once, and not have to revisit it again.
- Total investment – In the long run a Concrete Allan Block wall will be much more cost effective.
- Higher resale value IF your realtor/home inspector know their true value/cost
When are some times that Back 40 Landscaping would recommend a wooden retaining wall?
- When the client likes the look of a Wooden Retaining Wall
- Existing wooden retaining walls on the property you want to match
- Older home that may be extensively remodeled in the near future
- Initial investment is a concern
- Replacing an existing wooden wall that in your opinion has served you well, and you are comfortable in knowing what you are going to be getting
Suitability for your property
Lean towards an Allan Block Retaining wall when
- Water will be involved – Lakeshore, High water table area, Base of long hill
- Clay soil – Heavy clay soil requires stronger retaining wall
- Vehicle traffic will be frequent, close, and unusually heavy (Commercial vehicle, Contractors trailer, RV, etc.)
- Heavily shaded area – concrete can handle being damp its entire life.
- Limited excavation room behind the wall. Allan blocks can be installed with as little as 12” behind the wall.
Suitability of a Wooden Retaining Wall when
- Moisture in and around the wood can be limited
- Sandy/rocky fast draining soils
- Shorter in height – for a tall wall, Allan Blocks would be the way to go
- Limited finished landscaping around the wall as replacement in the future will cause damage to the landscaping or increase the cost to protect and work around it.
- Large excavation room behind the wall for installing Deadman Anchors (4-10’)
Design, color, and style options
Allan Block walls come in an array of sizes, and colors, and can be cut into curved architectural pleasing designs.
While there are some limitations their installation is almost free of physical restrictions in terms of design, and layout.
Wooden retaining walls are somewhat limited in terms of colors. Yes, you can stain and paint them, but it is not very common due to the in-ground moisture levels. The paint or stain will require more maintenance and upkeep than expected.
Their layout is not easily curved. Rather short sections of wood are cut on an angle to form a sectional bend vs a smooth radius.
Cost of initial investment and maintenance
Actual prices are presented at the conclusion of this article. Please don’t skip ahead!
My goal is to present unbiased information allowing you to read the true physical differences of Wood Vs. Allan Block retaining walls.
Wooden retaining walls have a lower initial cost, and are the smart investment if the retaining wall is not going to be around for ever. Plans to redevelop or change the landscaping in the near future make a wooden retaining wall a smart financial decision.
However, if a wall is likely to be needed for the next few decades it may be foolish to save money upfront and pay double in the long run. These decisions should be weighed heavily if there is no clear winner in the beginning of your buying process.
Your home or new homes value should reflect the investment in the landscaping. More home inspectors are starting to realize that significant oversights to the state of a home’s infrastructure are not tolerable to home buyers.
Allan Block retaining walls have a higher initial cost and win in almost every category, including overall lowest price.
Maintenance of Wood vs. Allan Block retaining wall is a wash, literally. Washing your walls to keep them looking their best is the majority of the required maintenance.
The first few years after construction you will want to keep an eye on the topsoil directly behind the wall. You want the soil a little higher than the wall to allow any rainfall to run over the top and away vs. sit on top and create a weight surcharge at the point it has the greatest leverage to overturn a wall.
The drainage pipe will require virtually no maintenance. There should be a cleanout cap though in case you want to flush the pipe out with a garden hose every 3-5 years to get rid of any sediment buildup.
When both Wood and Allan Block retaining walls are installed the same as already discussed you can expect a lifetime from your concrete retaining wall, and 20-30 yrs. from a wooden one.
What happens with Allan block walls when they are not installed properly?
- Visually distracting if crooked or unlevel
- Will overturn if significant weight surcharge on top of the wall is not part of design
- Will show signs of movement between blocks or will overturn if water is not allowed to drain from behind the wall
Poorly installed wooden retaining walls
- Visually distracting if crooked or unlevel
- Will show signs of separation and localized pushout/blowout if load behind exceed retaining strength
- Water left unaddressed will dramatically shorten a wooden retaining walls life by rotting out the lower sections of the wall
New retaining walls tend to be Allan block / Concrete option
For the most part when a site is having its first retaining wall installed its usually a concrete Allan Block wall. This is in large part because of the benefits already discussed. In most cases the client wants to do it “right” from the start, and wants it to last as long as everything else they are building.
Replacement of wood is 50% wood, 50% Allan Block upgrade
Furthermore, when a wooden retaining wall is due for replacement it seems like half the time they are replaced with a concrete wall, and the other half of the time they choose wooded again.
Anecdotally this suggests 75% of the time Concrete Retaining Walls are the clear choice, and 25% of the time Wooden Retaining Walls are chosen.
100% of the time when wooden retaining walls need replacing, our clients wish the builder installed a Concrete retaining wall right from the start.
Cost of a wooden retaining wall in 2021
The pricing you are about to read is based on everything above. It assumes the site has easy access for excavators, and skid steers, and is within 300’ of truck access located in Abbotsford B.C.
How much does a 75’ x 3’ wood retaining wall cost?
Ground contact rated wood dimensions 3 9/16” X 10’
Expect to pay $23,912.00*
Ground contact rated wood dimensions 4 1/8” X 10’
Expect to pay $23,982.00*
Ground contact rated wood dimensions 6” X 10’
Expect to pay $25,316.00*
How much does a 75’ x 3’ Allan Block retaining wall cost?
Concrete Allan Block with matching concrete capstones:
Expect to pay $27,260.00*
*Tight site access, other site challenges and applicable GST can affect the price.
Surprised an Allan block wall only costs 14% more than a standard 3 9/16” wooden retaining wall?
Wood vs. Allan Block, What is YOUR choice?