Retaining Walls & Armour Stone: Building Outdoor Structures
An armour stone retaining wall is one of the few outdoor structures that has to be engineered and attractive at the same time. It holds back soil, manages water, and creates usable ground where there was a slope, all while sitting in full view of the property. Get it right and it looks like it grew there. Get it wrong and it leans, bulges, and eventually fails. The crews at Automatic Masonry build these structures throughout Toronto and the GTA, and the difference between a wall that lasts and one that moves is almost always underground.
What Armour Stone Is
Armour stone is large quarried block, typically limestone or dolomite in Ontario, sold in sizes measured by the tonne rather than the unit. Pieces range from something two people can shift to blocks needing an excavator. The faces are left with a natural split texture, and tops and beds are sawn flat so courses sit properly.
An armour stone retaining wall works largely through mass. The weight of the stone resists the pressure of the soil behind it, rather than relying on mortar or reinforcement to do the work. That makes it well suited to Ontario conditions, since a gravity wall built with proper drainage has very little in it that freeze-thaw cycling can damage.
It also reads as permanent in a way manufactured block doesn’t, which is why larger residential properties across the GTA often specify it.
What the Wall Is Actually Doing
Soil behind an armour stone retaining wall doesn’t just sit there. It pushes, and the pressure increases toward the base. Add water and that pressure climbs sharply, since saturated soil is heavier and water itself exerts pressure. Freeze it and the soil expands.
That’s why most armour stone retaining wall failures trace back to water rather than to the stone. A wall that leans forward at the top, bulges in the middle, or shows opening joints between courses is usually a wall with nowhere for water to go.
Managing it comes down to three things: a granular base that doesn’t hold water, free-draining backfill behind the stone, and a drainage pipe at the base that actually leads somewhere. All three are invisible in the finished wall, which is why they’re the first things cut when a price gets squeezed.
How an Armour Stone Retaining Wall Gets Built
The sequence on a properly built armour stone retaining wall runs roughly like this.
Excavation. Dig back far enough to work behind the wall, and down below the frost line for the base. The trench needs to be wider than the stone to leave room for drainage material.
Base preparation. A compacted granular base, levelled carefully. This is the stage that determines whether the courses sit true, and it takes longer than most owners expect.
Setting the first course. The bottom course usually sits partly below finished grade, and it gets set with a slight backward lean so the wall pitches into the slope rather than away from it. Every course above references off this one, so time spent here saves time everywhere else.
Stacking and stepping back. Subsequent courses go up with joints staggered and each course stepped slightly back. Stone is selected and sometimes trimmed to fit, which is where experience shows.
Drainage and backfill. Clear stone behind the wall, a perforated pipe at the base with a defined outlet, geotextile fabric to keep soil from clogging the drainage layer, and native soil only above that.
Cap and finish. A finished top course, then grading behind the wall to move surface water away rather than into it.
Anything over roughly a metre in height, or any wall carrying a surcharge such as a driveway, a pool deck, or a structure above it, should involve an engineer. That isn’t a formality. It changes the base width, the drainage design, and sometimes the wall type entirely.
Armour Stone Compared With Other Options
| Material | Best suited to | Relative cost | Service life | Main limitation |
|---|---|---|---|---|
| Armour stone | Slopes and grade changes on properties with machine access | Premium | Longest of the four when drainage is correct | Needs equipment to place, so tight urban lots are difficult |
| Segmental block | Consistent walls, tiered designs, reinforced heights | Mid | Long, dependent on the reinforcement system | Reads as manufactured rather than natural |
| Natural flagstone, dry-laid | Low garden walls, terracing, steps and patios | Mid to premium | Long at low retained heights | Not suited to significant retained height |
| Poured concrete | Tall walls and tight setbacks where space is limited | Mid to premium | Long, dependent on reinforcement and drainage | Usually needs facing, since bare concrete rarely suits a garden |
Two of these can suit the same slope, and access usually decides. At the low end, our flagstone masonry work covers garden walls, terracing, and steps that often sit alongside an armour stone retaining wall.
Choosing between them is mostly a question of retained height, access, and how the wall should look. The types of stone available in Ontario and how they behave outdoors are covered in our guide to masonry stone types.
What Drives Retaining Wall Cost
We don’t publish figures, because two walls of the same length can differ enormously. What we can set out is what moves the number. Retaining wall cost on any given property is driven by:
- Height and length. Height matters more, since taller walls need wider bases, more drainage, and often engineering.
- Site access. If a machine can reach the work, stone gets placed efficiently. If everything has to come through a side yard, labour climbs.
- Excavation and disposal. Soil has to go somewhere, and rock or fill in the ground changes the work.
- Drainage requirements. Depth to a viable outlet, and whether one exists at all.
- Surcharge. A driveway, pool, or structure above the wall changes the design.
- Stone selection. Size, type, and how much trimming the pieces need.
- Extras. Steps, curves, corners, lighting conduit, and railings all add work.
The pattern worth knowing is that retaining wall cost is dominated by what you can’t see. The visible stone is a smaller share of the project than most people assume, and excavation, base, and drainage often account for more.
Choosing a Retaining Wall Contractor in Toronto
A few questions separate contractors quickly. On any armour stone retaining wall, ask how deep the base goes and what material it uses. Ask what goes behind the wall and where the drainage pipe outlets. Ask whether the height triggers engineering, and if the answer is no, ask why not.
Choosing a retaining wall contractor in Toronto also means checking that they carry the insurance and coverage to have machinery on your property, and that they’ve built walls at the height you need rather than only low garden terracing. Completed examples are the most useful evidence, and ours are on our work portfolio. Structural work of this kind overlaps with our foundation repair and reinforcement project, where soil pressure and drainage drove the whole scope.
Be wary of a proposal that prices stone by the tonne and treats base and drainage as a line item to be trimmed. That’s the wall that moves in five years.
Frequently Asked Questions
How tall can an armour stone retaining wall be without engineering?
Roughly a metre is the usual threshold, though it varies by municipality and site conditions. Any wall carrying a driveway, pool, or structure above it should be engineered regardless of height.
Do I need a permit for a retaining wall in Toronto?
It depends on height, location, and whether the wall is near a property line or a slope. Requirements vary across GTA municipalities, so it’s worth confirming before design is finalized.
What determines retaining wall cost on a residential lot?
Height, access, and drainage depth do most of the work, along with how much excavated soil has to leave the site. Two walls of identical length can differ substantially once those factors are accounted for.
What do you put under an armour stone retaining wall?
A compacted granular base below the frost line, wider than the stone itself, with clear stone and geotextile behind it. The base is what keeps the courses true, so it’s the last place to economize.
What is the cheapest retaining wall option?
Segmental block is normally the lowest installed price at modest heights, and a low dry-laid garden wall costs less again. The cheapest option stops being cheapest once the wall has to be rebuilt, which is usually a drainage decision rather than a material one.
Why is my existing wall leaning forward?
Almost always water. Blocked or missing drainage lets pressure build behind the stone, and the wall rotates outward at the top.
Can armour stone be installed on a tight lot?
Sometimes, but the blocks are heavy enough to need machine placement. Where access is too narrow, segmental block or a mortared stone wall is usually the practical alternative.
How long does an armour stone retaining wall last?
Built on a proper base with working drainage, the stone itself will outlast almost everything else on the property. Failures come from base and drainage shortcuts, not from the material.
Planning an Outdoor Structure in Toronto or the GTA
If you’re planning an armour stone retaining wall, levelling a slope, dealing with a driveway edge that’s slumping, or looking at a wall that has already started to move, the fix starts with what’s happening behind and beneath the stone. We’ll look at the grade, the drainage, and the access, then tell you what the site actually needs. Contact us for a free estimate.
