A retaining wall can turn a slope into usable terraces and planting beds. It also holds back a great deal of weight in soil and water, which is why walls that look simple often fail when built without attention to base, drainage and height. This guide explains the basics and when to bring in a professional.
Key takeaways
- Drainage behind the wall is as important as the wall itself.
- Taller walls, walls near structures or driveways, and walls on poor soil usually need professional design.
- Local rules often require permits or engineering above certain heights; check with your building department.
Why walls fail
Soil and water push against a retaining wall. When water cannot drain from behind the wall, pressure builds, especially after heavy rain or freezing. Walls also fail when the base is poorly prepared or the wall is too tall for its design. Leaning, bulging and cracks are common warning signs.
The basics of a sound wall
- A compacted, level base, often of crushed stone, below the first course.
- Drainage: gravel backfill behind the wall and often a perforated drain pipe at the base, leading water away.
- A slight backward lean, called setback, built into many block systems.
- Proper reinforcement for taller walls, such as geogrid layers in the soil behind segmental block walls.
- Weep holes or other outlets in some wall types.
Common wall materials
Segmental concrete blocks are designed for stacking without mortar and are popular for DIY. Natural stone walls can be dry-stacked or mortared. Poured concrete walls are strong but need forms and reinforcement. Timber walls are simpler but eventually rot.
When DIY can work
Short garden walls on firm soil, away from buildings, driveways and slopes above, are common DIY projects. Many block manufacturers publish installation guides with height limits for unreinforced walls. Follow them closely.
When to hire a professional
- Walls above the height your block system or local rules allow without engineering.
- Walls holding back driveways, buildings or steep slopes.
- Sites with poor drainage, clay soil or springs.
- Replacing a failing wall.
Permits and rules
Many localities require a permit, and sometimes an engineer’s design, for retaining walls above a certain height or near property lines. The threshold varies, so check with your local building department before designing. A reputable contractor should know the rules and pull required permits; the FTC lists contractors asking homeowners to get permits as a warning sign.
DIY or pro at a glance
| Situation | Usually |
|---|---|
| Low garden wall, firm soil | DIY with manufacturer guide |
| Wall near driveway or house | Professional |
| Tall wall | Professional, often engineered |
| Poor drainage or clay | Professional |
| Failing existing wall | Professional assessment |
Tip: Before building, watch the slope during heavy rain. If water runs down the hill toward where the wall will be, plan drainage for that flow, not just for rain that falls behind the wall.
Before digging
Walls need excavation for the base and drainage. Contact 811 before digging, and check for private lines such as irrigation and lighting cables; see Call 811 Before You Dig: What Homeowners Need to Know.
Planting on and around walls
Terraces created by walls are good planting beds. Choose plants suited to the drainage, which is often better at the top of a wall, and avoid planting large trees whose roots may push against the wall.
Which approach fits
For a short decorative wall, a careful DIY build following the manufacturer’s guide is reasonable. For anything structural, tall or close to buildings and driveways, get a professional design and installation. See How to Hire a Landscaper: A Step-by-Step Guide for Homeowners.
Warning signs in an existing wall
- Leaning or bulging sections.
- Cracks that widen over time.
- Blocks or stones that have shifted or fallen.
- Water seeping through the face or pooling at the base after rain.
- Soil washing out from beneath the wall.
If you see these, have the wall assessed before adding weight above it, such as new plantings, a patio or parked vehicles. Failing walls can collapse suddenly, especially after heavy rain.
Maintenance
Keep drain outlets clear, direct downspouts away from the area behind the wall and avoid piling soil or mulch higher than the wall was designed for.
Common mistakes
- Skipping gravel backfill and drain pipe.
- Building on unprepared soil.
- Exceeding the height limit of the block system.
- Ignoring local permit rules.
Frequently asked questions
How tall can a DIY retaining wall be?
It depends on the block system, soil and local rules. Follow the manufacturer’s limits and check with your building department.
Why is my retaining wall leaning?
Often because of water pressure behind it or a weak base. Have it assessed.
Do I need a permit?
Many localities require one above a certain height. Check locally.
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