How to Build a Concrete Raised Garden Bed (Step-by-Step Guide)

Modern curved concrete garden bed filled with soil and plants.

A concrete raised garden bed is a long-term structural investment in your garden. Unlike timber beds, concrete does not rot, shift, or deteriorate due to moisture exposure. When built over a compacted base and aligned correctly from the first course, it can remain stable for decades.

This guide walks through the entire process, from ground preparation to final soil filling, using practical measurements and professional construction principles.

Recommended Size Before You Begin

Before construction starts, choose dimensions that balance usability and structural strength.

Ideal Dimensions

Width

90–120cm for access from both sides
75cm if access is from one side only

Height

40–50cm for general planting
60–75cm for reduced bending strain

Soil Depth
Many gardeners ask about the depth of a raised bed for healthy root development. In most cases, a minimum of 30–40cm of quality planting soil is sufficient for vegetables and flowering plants, while deeper beds (50cm or more) provide better root expansion and moisture retention.

Choosing the right size reduces soil pressure against the wall and ensures long-term stability.

Step 1: Clear and Level the Site

Person using a garden rake to level rocky soil.

A stable foundation begins with proper ground preparation. Skipping this stage leads to settlement, cracking, and misalignment later.

Ground Preparation

Remove turf and all organic material
Excavate to firm subsoil
Clear roots completely
Extend excavation slightly beyond bed footprint

Base Installation

Spread hardcore evenly to 8cm depth
Rake level before compaction
Hardcore distributes weight evenly and improves drainage beneath the structure.

Tip: Never build over soft or organic soil. Decomposition underneath causes uneven settlement.

Step 2: Compact the Hardcore Base

Blue vibrating plate compactor leveling a gravel base.

Compaction transforms loose aggregate into a stable load-bearing platform. This is one of the most critical structural stages.

Compaction Process

Use a plate compactor in overlapping passes
Compact until surface feels firm underfoot
Recheck level with a long spirit level
Correct low spots before final pass
Proper compaction prevents shifting during freeze-thaw cycles and soil loading.

Tip: Lightly dampen the hardcore before compacting to improve density.

Step 3: Mark Out the Raised Bed Shape

Using neon green spray paint to mark garden layout.

Precise layout ensures straight walls, square corners, and a professional appearance.

For Straight Beds

Insert stakes at each corner
Stretch string tightly between them
Confirm right angles using the 3-4-5 method
Mark the footprint clearly

For Curved Beds

Install a central pivot stake
Attach string to create consistent radius
Mark arc carefully along string line
Accurate marking ensures blocks align correctly without correction later.

Tip: Double-check measurements before mixing mortar. Adjusting layout after block placement is difficult.

Step 4: Lay the First Course of Blocks

Worker placing a concrete block onto a mortar bed.

The first course determines the accuracy of the entire structure. Even small errors here multiply as you build upward.

Mortar Preparation

Mix 5 parts sand to 1 part cement
Achieve firm, workable consistency
Avoid overly wet mixture

Block Placement

Spread 5cm mortar bed
Set corner blocks first
Stretch string line between corners
Place intermediate blocks along line

After Placing Each Block

Check level front-to-back
Check level side-to-side
Confirm alignment to string
Strike mortar joints while workable
Proper joint compression strengthens the bond and improves weather resistance.

Tip: Spend extra time leveling the first course. It determines the quality of everything above.

Step 5: Build the Second Course with Staggered Joints

Worker placing a rope onto concrete block for alignment

Stacking blocks in a running bond pattern improves strength and distributes structural load.

Staggered Joint Installation

Offset vertical joints from the course below
Maintain 10–15mm mortar joints
Cut half blocks where required
Recheck level across entire wall
Staggered joints resist lateral soil pressure and reduce cracking risk.

Tip: Do not widen joints to fix alignment errors. Reset the block instead.

Step 6: Form Curved Sections (If Required)

Building a curved garden wall using grey concrete blocks.

Curved designs require careful alignment to maintain consistent wall height and appearance.

Curved Wall Technique

Place blocks short-end down
Build from both ends toward centre
Trim final block precisely
Confirm consistent height across arc
Consistent mortar thickness improves visual smoothness in curved sections.

Tip: Slightly thinner joints allow smoother curvature and tighter fit.

Step 7: Apply the First Render Coat (Scratch Coat)

Man using a trowel to apply textured scratch coat.

Rendering protects the blocks from weather and gives a cohesive finished appearance.

Render Mix

5 parts sand
1 part cement
Small amount of washing-up liquid

Application Method

Apply 5–10mm thick layer
Cover entire exterior surface
Render halfway down interior wall
Score diagonal scratch lines before setting
The scratch pattern creates mechanical bonding for the finishing coat.

Tip: Lightly dampen the wall before rendering to improve adhesion.

Step 8: Apply the Second Render Coat

Smoothing wet concrete render with a blue sponge float.

The second coat refines appearance and seals the surface.

Finishing Process

Apply thinner smoothing coat
Use damp sponge to level imperfections
Avoid overworking surface
Allow gradual curing
Rapid drying increases cracking risk.

Tip: In warm conditions, mist lightly during curing to prevent hairline cracks.

Step 9: Cure, Paint, and Fill the Raised Bed

Hand painting the top of a concrete wall white.

Curing strengthens mortar and render. Rushing this stage weakens durability.

Curing and Painting

Allow minimum 7 days curing
Ensure surface is dry before painting
Use breathable masonry paint

Step 10: The Raised Bed Is Now Ready for Planting

At this stage, the structure is fully cured, filled correctly, and structurally stable. The foundation has been compacted, the mortar joints have strengthened, the render has cured, and you’re ready to plant in raised bed conditions that promote strong root growth and consistent drainage.

Soil Filling Structure

Bottom layer: One of the most common questions is what to put on the bottom of raised bed structures for proper drainage. Add coarse rubble, broken brick, or larger stones to a depth of approximately 8–10cm.
Middle layer: Add subsoil or excavated soil to build bulk and structural support. Lightly firm this layer in stages to minimize future settling.
Top layer: Finish with compost-rich loam or high-quality garden topsoil. Blend compost thoroughly to improve nutrient content, soil structure, and moisture retention for healthy root development.
Maintain at least 30–40cm (12–16 inches) of quality planting soil for most vegetables and flowering plants.

Tip: Properly layering a raised bed is essential for drainage, stability, and long-term plant health.

Common Mistakes to Avoid

Skipping compaction results in settlement cracks and uneven structural movement over time.
The uneven first course creates crooked walls that worsen with each additional layer added.
Mortar that is too wet weakens structural joints and reduces overall wall strength.
Insufficient curing time leads to cracking, flaking render, and compromised durability.
Shallow soil depth restricts root development and limits healthy plant growth

From Base to Bloom

A properly constructed concrete Raised Garden Bed becomes a permanent feature in your landscape. With careful leveling, proper compaction, consistent mortar joints, and correct curing time, it will withstand soil pressure and seasonal weather changes for many years.

FAQs

1. How do I fix settling or cracking in my concrete raised bed?

Ensure proper compaction of the base before construction to avoid uneven settling. If cracks appear later, carefully inspect for weak mortar joints or a shifting base. Use a masonry repair product to seal small cracks, but address underlying issues before further planting.

2. Can I build a concrete raised garden bed on uneven ground?

It’s crucial to level the site before construction. Uneven ground can lead to improper compaction and settling. If your site isn’t perfectly level, consider using a leveling compound or re-excavating to create a stable foundation.

3. How do I prevent water drainage issues in my concrete raised bed?

Ensure proper drainage by layering coarse rubble or broken brick at the bottom of the bed. Adding subsoil and using a mix of compost-rich loam for the top ensures adequate moisture retention without waterlogging. Check regularly for clogs in drainage areas.