Building a retaining wall: handling level differences and water

Why you need a retaining wall

Your garden has a slope. Without a wall, the upper soil gradually erodes downward, leaving mud against your lower plantings. A retaining wall holds the soil at height and creates clean lines.

Retaining walls are not just practical - they add structure to your garden. A well-built brick wall can stand for decades with minimal maintenance.

How tall can your retaining wall be?

This determines how solid your foundation must be.

Up to 60 cm tall: Small wall. You can usually build this yourself with stacked blocks or bricks. No engineer needed.

60-120 cm: Medium wall. Now structure matters. You need good drainage and a solid base.

Over 120 cm: Large retaining work. Have a professional check this. Soil pressure increases dramatically.

For this guide we focus on small to medium walls (up to 100 cm) with bricks or stacked blocks.

Choosing materials

Bricks: Classic, beautiful, good for curved lines. You stack them in mortar (one mortar bed per layer). Durable but labour-intensive.

Stacked blocks (concrete blocks): Modern, quick to place, no mortar needed. Dry stacking. Efficient but less flexible in shape.

Wooden posts: Rustic, cheap, lasts 10-15 years. Not suitable for wet zones (rot).

For this guide we use bricks - the most durable and popular choice.

Drainage is critical

This is what many people forget - then their wall fails. Water pushes against your wall from behind, creates pressure, and your wall bows forward and cracks.

You need two drainage layers:

1. Behind your wall: A drainage layer of gravel or geotextile so water can pass through and does not build up.

2. Below your wall: Drainage pipes (perforated tubes) that carry water away from your foundation.

Step-by-step

Step 1: Preparation and marking

Determine where your wall runs. Chalk a line on the ground. Check the height on both sides with a level. Your wall must run level - no tilting.

If you want a curve, make it gentle - sharp angles break.

Step 2: Excavate and create foundation

Dig a trench where your wall will sit. The trench must be 30-40 cm deep and at least 60 cm wide (your wall thickness). Make the bottom flat.

Fill 10-15 cm of compacted coarse gravel into the trench. This is your foundation (bearing layer). Compact well. This prevents your wall from settling.

Step 3: Place drainage pipe

Lay a perforated drainage pipe (perforated on top) on the gravel foundation. This pipe carries water away from your wall. Connect it to your garden drainage or let it run downslope.

Step 4: Start first course of bricks

Stack your first layer of bricks in mortar. Ensure they are level (check with level). Apply mortar well between each brick.

Ensure good adhesion - full mortar joints, no gaps.

Step 5: Build up course by course

Build your wall, one course at a time. Stagger your bricks (running bond) so no vertical cracks form. Each course roughly 15 cm tall.

As you build, fill the back (soil side) with geotextile and gravel. This prevents water pressure.

Step 6: Fill the back (drainage layer)

As your wall rises, fill behind it at the same pace:

• Gravel or drainage gravel (5-10 cm thick)
• Geotextile cloth (prevents sand from washing into gravel)
• Garden soil (planting substrate) all the way to top

Do this layer-by-layer, not all at once.

Step 7: Top finish

When your wall is done, cap the top (decorative stones or capping blocks). This keeps water out of your mortar and looks finished.

Maintaining your retaining wall

A well-built brick wall lasts 50+ years. That said:

• Check drainage regularly - no pooling water
• Inspect in spring for cracks (frost damage)
• Prevent plant roots from growing under your wall

Frequently asked questions

How thick should my wall be?

Normally 30-40 cm thick (one brick deep plus mortar). For tall walls (70+ cm), make them thicker (45-50 cm).

Can I make curved corners?

Yes, brick walls can have curved lines. Just ensure your curves are gentle (sharp angles break). Radii larger than 1 metre are safe.

My garden is steep - can I still build a retaining wall?

Yes, but the steeper it is, the more solid your foundation must be. Steep slopes (more than 1:2 gradients) require better drainage and possibly thicker walls. Have a professional review.

What do bricks cost?

For a small wall 5m x 50cm tall: roughly 500-1000 euros (stones plus mortar plus labour).

My wall weeps water - what now?

Check drainage behind your wall. Water may be backing up (blocked drainage pipe or poor geotextile). Open the drainage again.

A well-built retaining wall stands firm

A retaining wall looks simple but carries enormous weight. Your preparation determines whether your wall stands for decades or fails within ten years.

On gardenworld.app you can upload your sloped garden and see how walls structure it. Design first, build later.

For garden slope work and retaining walls, gardenworld.app offers planning and visualisation.