Best Materials for a Retaining Wall in Southern Alberta

Concrete interlocking blocks are the best overall material for retaining walls in southern Alberta. They handle freeze-thaw cycles, require no mortar, install without heavy equipment, and come in a range of styles that suit both residential yards and commercial landscapes. For walls under 4 feet, concrete blocks are a straightforward project. For anything taller, engineering and permits may be required.

Choosing the right material depends on the wall's height, purpose, and how much maintenance you want to deal with over the years. Each option has trade-offs, and southern Alberta's climate makes some of those trade-offs more significant than they are in milder regions.

What are the main material options for retaining walls?

Five materials are commonly used for retaining walls in the Lethbridge area. Each has different strengths depending on the project.

Concrete interlocking blocks (also called segmental retaining wall units) are the most popular choice for residential and light commercial projects. They stack without mortar, use a pin or lip system for alignment, and come in various colors and textures. Modern concrete blocks are manufactured to meet ASTM C1372 standards, which include freeze-thaw durability requirements for cold climate use. The National Concrete Masonry Association recommends that in areas of repeated freezing and thawing under saturated conditions, blocks should demonstrate less than 1% weight loss after 100 freeze-thaw cycles in water testing.

Natural stone creates a rustic, high-end look. Limestone, sandstone, and fieldstone are available in southern Alberta. Natural stone is heavy, requires skilled placement, and takes longer to install than block systems. It holds up well in cold climates because stone itself does not absorb much water compared to porous concrete. The trade-off is that natural stone walls are harder to build to precise dimensions and typically need more gravel backfill for drainage.

Poured concrete is the strongest option for tall or load-bearing walls. It requires formwork, reinforcing steel, and professional installation. Poured concrete walls handle high lateral loads and work well where the wall supports a driveway, parking area, or building foundation. In Alberta, walls supporting drained earth must be designed for a pressure equivalent to that exerted by a fluid with a density of not less than 480 kg/m3 at a depth equal to the retained earth.

Pressure-treated timber is the most affordable option for low walls under 3 feet. Timber walls are relatively easy to build but have the shortest lifespan of any material, typically 15 to 20 years before the wood deteriorates. Alberta's building code requires that structural wood elements used in retaining walls be pressure-treated with preservative to resist decay when the wall supports ground critical to building foundations or exceeds 1.2 metres in height.

Boulder walls use large natural rocks, typically 2 to 4 feet in diameter, stacked or placed by machine. They create a natural appearance that blends with Alberta's landscape. Boulder walls rely on their own mass for stability, so they need significant space and a solid base. They are common along driveways and property edges in rural areas around Lethbridge.

How does southern Alberta's climate affect material choice?

Southern Alberta's freeze-thaw cycles are the single biggest factor in retaining wall durability. Lethbridge experiences chinook winds that can swing temperatures from well below freezing to above zero in a matter of hours, creating rapid freeze-thaw cycles that stress all construction materials.

The mechanism behind freeze-thaw damage is straightforward. Water enters the pores of the wall material, freezes and expands by approximately 9%, and creates internal pressure that cracks and breaks down the material over time. Research funded by the Federal Highway Administration found that for this expansion to cause damage, the pores need to be approximately 91.7% saturated, a threshold called critical saturation. Proper drainage behind the wall is the most effective way to keep materials below that saturation level.

Concrete blocks manufactured to current standards have made significant improvements in freeze-thaw resistance over the past 20 years. Products available today perform substantially better than those from 10 to 15 years ago, thanks to improved mix designs, higher cement content, better compaction during manufacturing, and stricter quality control programs. When purchasing concrete blocks for a retaining wall in this region, verify that the product meets ASTM C1372 specifications for freeze-thaw durability.

Deicing chemicals present an additional risk. Calcium chloride and magnesium chloride, commonly used on Alberta roads and driveways, can accelerate concrete degradation when they run off onto retaining wall surfaces. If your wall is near a driveway or parking area where salt is applied, consider redirecting drainage away from the wall face or using a natural stone cap that is less susceptible to chemical attack.

Do you need a building permit for a retaining wall in Alberta?

In Alberta, retaining walls are regulated under the National Building Code, Alberta Edition, and the Safety Codes Act. Whether you need a permit depends on the wall's height and its relationship to nearby structures.

A building permit is required for retaining walls that are critical to the support of building foundations. A retaining wall is considered a structural element of the building if a line drawn from the outer edge of the footing to the bottom of the exposed face of the wall is greater than 45 degrees to the horizontal.

A permit is also required for retaining walls that exceed 1.2 metres (approximately 4 feet) in height when the wall is adjacent to public property, adjacent to an access to a building, or on private property accessible to the public.

For shorter landscape walls under 1.2 metres that are not near a building foundation, a permit is generally not required. However, all construction in Alberta must comply with the Safety Codes Act regardless of whether a permit is issued.

Check with the City of Lethbridge development services before starting any retaining wall project to confirm local requirements. Municipal bylaws may have additional setback or drainage requirements beyond the provincial code.

How important is drainage behind a retaining wall?

Drainage is the most critical factor in retaining wall performance, regardless of the material used. Water pressure behind a wall (called hydrostatic pressure) is the primary cause of retaining wall failure. A wall designed to hold back soil can fail if it has to hold back waterlogged soil, which exerts significantly more lateral pressure.

Every retaining wall needs a drainage layer of free-draining granular gravel fill directly behind the wall face. The NCMA Best Practices Guide specifies compactable, free-draining granular gravel fill behind the wall, with a drain pipe at the base to collect and redirect water away from the structure. A filter fabric between the gravel and the retained soil prevents fine soil particles from migrating into the drainage layer and clogging it over time.

In southern Alberta, where clay soils are common, drainage is even more important. Clay expands when wet and contracts when dry, creating pressure fluctuations against the wall throughout the year. A generous drainage zone of 12 inches or more of gravel behind the wall, combined with proper grading to direct surface water away from the top of the wall, prevents most of the problems that cause retaining walls to lean, crack, or fail.

Surface water management at the top of the wall matters just as much as drainage behind it. Grade the soil behind the wall so water flows away from the wall edge. A low-permeability soil cap over the gravel fill prevents surface water from soaking directly into the backfill zone.

What material lasts the longest?

Lifespan varies significantly by material and by how well the wall was built.

Natural stone lasts the longest, often 50 to 100 years or more with minimal maintenance. Stone does not rot, rust, or degrade from freeze-thaw cycles the way other materials can. The main risk is settlement or shifting if the base was not prepared properly.

Concrete interlocking blocks typically last 30 to 50 years when manufactured to current durability standards and installed with proper drainage. Modern blocks that meet ASTM C1372 freeze-thaw requirements are designed specifically for cold climate exposure.

Poured concrete walls last 40 to 60 years or more, depending on the quality of the mix, the reinforcing steel, and the drainage system. Concrete is vulnerable to cracking from freeze-thaw and from settlement, but well-built walls with adequate drainage have excellent longevity.

Pressure-treated timber has the shortest lifespan at 15 to 20 years. Even with preservative treatment, wood in ground contact eventually decays. Timber walls are best suited for temporary or low-priority applications where the lower upfront investment outweighs the shorter life.

Boulder walls, when properly placed on a prepared base, last decades with virtually no maintenance. Their mass makes them inherently stable, and natural rock does not degrade from weather exposure.

What mistakes should you avoid when building a retaining wall?

Several common errors lead to retaining wall problems, especially in Alberta's climate.

Skipping the gravel drainage layer is the most damaging mistake. Without drainage, hydrostatic pressure builds behind the wall and eventually pushes it forward or causes it to collapse. This happens even with strong materials if water has nowhere to go.

Building too tall without engineering is a safety risk. Walls over 1.2 metres need proper design to resist the lateral earth pressure, and in many cases a building permit and engineering review. A 6-foot wall holds back several tonnes of soil per linear metre. Without geogrid reinforcement or proper structural design, tall walls can fail suddenly.

Using materials not rated for freeze-thaw exposure leads to surface spalling and cracking within a few years. In this climate, any concrete product used in a retaining wall should meet ASTM C1372 or equivalent cold-climate durability standards.

Poor base preparation causes settling and leaning. The base course needs to be level, compacted, and placed on undisturbed soil or a prepared gravel pad. A wall built on loose fill or topsoil will settle unevenly.

Ignoring surface drainage above the wall allows water to flow directly into the backfill zone, saturating the soil behind the wall and increasing lateral pressure. Proper grading and a soil cap over the drainage gravel prevent this.

Frequently asked questions

What is the best material for a retaining wall in Alberta? Concrete interlocking blocks are the best all-around choice for most residential and light commercial retaining walls. They handle freeze-thaw cycles, install without mortar, and come in many styles. For tall or load-bearing walls, poured concrete with reinforcing steel is the strongest option.

Do I need a permit for a retaining wall in Lethbridge? Walls over 1.2 metres (about 4 feet) that are adjacent to public property or building access generally require a building permit. Walls critical to building foundations also require a permit regardless of height. Check with the City of Lethbridge before starting.

How long does a retaining wall last? Natural stone lasts 50 to 100+ years. Concrete blocks last 30 to 50 years. Poured concrete lasts 40 to 60 years. Pressure-treated timber lasts 15 to 20 years. Proper drainage and base preparation are the biggest factors in any material's actual lifespan.

What goes behind a retaining wall for drainage? A 12-inch layer of free-draining granular gravel, a perforated drain pipe at the base, and filter fabric between the gravel and the retained soil. Surface grading above the wall should direct water away from the wall edge.

Can I use landscape timbers for a retaining wall? Yes, for short walls under 3 feet. Alberta's building code requires pressure-treated wood for retaining walls that exceed 1.2 metres or support ground near building foundations. Timber walls have the shortest lifespan of any material, typically 15 to 20 years.

Do chinook winds affect retaining walls? Yes. Chinooks cause rapid temperature swings that accelerate freeze-thaw cycles. This makes material selection and drainage even more important in the Lethbridge area than in regions with more stable winter temperatures. Use materials rated for freeze-thaw exposure and ensure proper drainage to prevent water from saturating the wall or backfill.

Sources

1. Alberta Municipal Affairs, "Standata 19-BCI-007: Retaining Walls," Building Code Interpretation, Government of Alberta, December 2019. https://open.alberta.ca/dataset/b5d1d6b9-6429-4a1a-96af-e7c0500d5017/resource/5baae1df-f2bc-4c32-9505-09e87e8f5208/download/ma-standata-19-bci-007-retaining-walls.pdf

2. National Concrete Masonry Association (NCMA), "Segmental Retaining Walls Best Practices Guide for the Specification, Design, Construction, and Inspection of SRW Systems," NCMA Publication TR308, 2016. https://basalite.ca/wp-content/uploads/2020/06/ncma-srw-best-practices-2016-1.pdf

3. National Concrete Masonry Association (NCMA), "The Durability of Segmental Retaining Wall (SRW) Units," SRW History Article Series, NCMA Education and Research Foundation. https://www.flexmse.com/wp-content/uploads/2015/03/Segmental-Concrete-Freeze-Thaw.pdf