How to build retaining wall with drainage
Honestly, the number of times we show up to a Calgary yard and the whole mess traces back to a block barrier that was just stacked and called “done”. Looks fine from the street. Then spring melt hits, or you get one of those long rains, and the face starts to bow a bit. You see a hairline crack near the bottom, a gap that was not there last year, and you start wondering if the whole thing is going to slide into the patio. Most of the time, at least, it is not the blocks that were the problem. It is what was (not) put behind them.
A proper setup needs a water path. Not “hope it soaks away” either. You want free-draining gravel behind the structure, a perforated pipe that can carry water out to daylight or a safe outlet, and filter fabric so the fines do not plug everything up. If you skip that, the soil stays heavy and wet, hydrostatic pressure climbs, and the face gets pushed forward a little more each season. I have seen people blame frost first, and yes frost is real here, but trapped water is usually the bigger bully.
On our jobs at Sungreen Landscaping Inc (we have been doing outdoor spaces since 1990), we treat this like the part that decides if you are repairing it in three years or enjoying it for a long time. If the grade is tricky, or the height is getting up there, we slow down and plan the outlet properly, because water has to go somewhere. And once it is in, you still have upkeep. Algae, efflorescence, dirt streaks from overflow. If you are dealing with that already, this guide on how to clean retaining wall blocks is a handy reference before you judge the condition of the masonry.
If you want us to look at your site, we do free consultations and 2D design samples, and we can talk through where the water should run so it does not end up against your foundation or under your paving. For taller soil-holding structures over 4 feet, we pull the City of Calgary permit and work alongside an engineer, because that is not the place to guess. Call (403) 256-7500 if you want it assessed, and yes, every completed project comes with a signed warranty and a warranty rep, which sounds boring until you need it.
Creating a Soil-Holding Structure That Lets Water Escape
Honestly, the number of times we get called out to fix a leaning block face and you can tell right away what happened. Someone stacked the units, tossed native soil behind it, and figured rain and snowmelt would just “go somewhere.” It does, usually straight into the back of the structure, where it sits and pushes. In Calgary you add freeze-thaw and clay pockets and it gets messy fast.
First thing is layout and excavation. You want a trench wide enough for your base material and the first course, and deep enough that the bottom row is partly buried so it does not creep forward over time. I have seen people try to save effort by scraping a shallow groove and setting block on loose dirt. It looks fine for a bit, then the base settles unevenly and the face starts to wave. Use a compacted crushed gravel base, not road crush full of fines, and check level as you go. Slow work, but it pays you back.
Backfill that actually behaves
Behind the block face, think in layers. Right against the back, place a clean clear stone zone so water can move down instead of loading up the soil. Wrap that stone in filter fabric so the fines from the native material do not migrate in and clog it up, because they will, most of the time at least. Then backfill in lifts and compact each lift, especially near the back edge where settlement likes to hide.
For getting the water out, a perforated pipe at the base is the usual move, sloped to daylight if your yard allows it. If there is nowhere to day-light, you are into sump pits or tie-ins that need planning, and that is where a quick site visit helps. At Sungreen Landscaping Inc we include free consultations and 2D design samples, and we have been doing this since 1990, so we have seen the odd lot grading situation around Rocky View County. If you want to talk it through, call (403) 256-7500, or check .
Height, reinforcement, and the stuff people skip
Once the structure gets tall, you usually need geogrid tying courses back into the slope, and the manufacturer has spacing rules that are not suggestions. For anything over 4 feet, we pull the City of Calgary permit and work with an engineer, because guessing is a bad plan when soil pressure is involved. Use the right caps, keep your joints staggered, and do not forget surface grading at the top so runoff is directed away from the backfill, not into it.
Last piece is patience. Let the gravel base and the stone zone do their job, and keep water from dumping in behind the block face from downspouts or a poorly sloped patio. I have watched a perfectly nice yard get wrecked because someone extended an eavestrough right to the back edge and called it “good enough.” It was not. If you do it right, you get a tidy finish, a signed warranty when we do the work, and you are not re-stacking units after the first big summer storm.
Assess the site: soil type, slope, wall height limits, and where water collects
Before you stack a single block, you need to read the yard the way water reads it. I walk it after a rain if I can, boots on, and I look for the dark bands on the soil, the soggy spots that never quite dry, and the little channels where runoff has already made its own plan. A lot of DIY jobs I see in Calgary fail because someone chooses materials first, then discovers the ground is basically mush in spring. If budget is driving the decisions, fine, but do the site check first, then look at options like what is the cheapest retaining wall to build so you are not pricing something that your yard will fight you on.
Soil type tells you what the base will tolerate
Clay holds water and swells, sand drains faster but can wash out, and a lot of newer communities have a mix that changes every few feet because of backfilled lots. I have dug into “nice topsoil” that turned into heavy clay at 200 mm, and that changes what we do for base gravel and filter fabric. If you are trying to figure quantities, measure the length, height, and any curves or corners first, then check a calculator like how to measure a retaining wall h ow many blocks do I need, because soil conditions often push you toward a wider base and that affects footprint and material count.
| Site factor | What to look for on your property | What it changes in the plan |
|---|---|---|
| Clay-heavy soil | Sticky when wet, cracks when dry, puddles linger | More attention to granular backfill and water escape routes |
| Sandy or gravelly soil | Drains quickly, edges slump when dug | Extra separation fabric and careful compaction to stop washout |
| Steep slope | Fast runoff, erosion lines, exposed roots | Terracing or stepping to reduce pressure and slow water |
| Low spots | Greenest grass, moss, sump pump discharge nearby | Outlet location matters, avoid trapping water behind the structure |
Slope and height limits are not “suggestions”
If the grade is steep, breaking it into tiers or tying it into stairs can calm the whole area down, and you can see some good examples here: how to build steps stairs with retaining wall blocks. Height is the big line in the sand: once you get over 4 feet, we pull the City of Calgary permit and bring in an engineer, because you are dealing with real loads and real risk, not just a garden edge. Most of the time, at least, the best first move is a free consult so we can flag the water collection points, check the slope, and save you from putting the whole thing right where spring melt wants to sit. If you want us to take a look, call (403) 256-7500 and we will map out where the water is actually going, not where you hope it goes.
Questions and answers:
Do I really need drainage behind a retaining wall, or can I skip it if the wall is short?
Drainage is usually the part that decides whether a wall stays straight or starts leaning after a few wet seasons. Water builds pressure behind the wall, and that pressure increases fast after heavy rain or snowmelt. Even for a low wall, it’s smart to include at least three things: a free‑draining gravel zone directly behind the wall, a perforated drain pipe at the base, and an outlet path so water can leave. If water has nowhere to go, it will push on the wall and can also wash out fine soil, creating voids that lead to settlement.
Where exactly should the perforated drain pipe go, and which way do the holes face?
Place the perforated pipe at the bottom of the backfill zone, right behind the first course of blocks (or the wall base), so it sits at the lowest point where water collects. The pipe should run the full length of the wall and slope slightly toward its discharge point (a gentle fall is enough). Hole orientation depends on the pipe type: for most flexible corrugated “drain tile,” installers commonly place the perforations down so water enters from the gravel bed and the pipe self-flushes better. For rigid PVC with two rows of holes, many set the holes at about the 4 o’clock and 8 o’clock positions. Whichever pipe you use, the bigger mistake is skipping the gravel envelope and fabric—without those, the pipe clogs with fines.
What gravel should I use behind the wall, and how thick should that layer be?
Use clean, angular crushed stone that drains freely (often sold as drainage gravel). Avoid sand, “road base,” or anything with lots of fines—those hold water and can clog the pipe. A common approach is a gravel zone at least 12 inches (30 cm) thick directly behind the wall, extending from the base up toward the top. For taller walls, increasing that zone (or using a full-height gravel backfill) helps drainage and reduces pressure. Compact the soil behind the gravel in lifts, but don’t over-compact right against the wall face; keep the wall’s drainage stone clean and open.
Should I wrap the gravel and pipe with geotextile fabric, and how do I keep it from clogging?
Yes—fabric is there to separate native soil from the drainage stone so fine particles don’t migrate into the gravel and plug the pipe. Use a nonwoven geotextile rated for drainage (not plastic sheeting). A practical method is to line the excavation on the soil side, place gravel and the pipe, then “fold” the fabric over the top of the gravel like a burrito before adding soil. Keep fabric overlaps generous and avoid tearing it during backfill. Also, don’t mix soil into the drainage stone; a little contamination can reduce flow and shorten the life of the system.
My yard slopes toward the wall. Where can I discharge the drain pipe without making a mess or flooding my neighbor?
Send the outlet to a spot where water can leave safely: a daylight outlet on your property, a gravel dry well sized for your runoff, or a connection to a lawful storm system if local rules allow it. The outlet should be protected with a rodent screen and placed where erosion won’t undercut the slope—often a short riprap apron (larger rock) helps. Avoid discharging right at the end of the wall if that area is soft; concentrated flow can wash out support. If there’s no good gravity path, you may need a different plan (like a dry well) rather than trapping water behind the wall.
