How to Build a Retaining Wall — Step-by-Step UK Guide

What You'll Need

Before You Start

• Check for underground services — water, gas, electric, drainage. Use a CAT scanner and contact utility providers.
• Determine the retained height and any surcharge loads (vehicles, buildings, stored materials above the wall).
• If the wall will be over 1m high, or supports a driveway or building, engage a structural engineer.
• Check planning permission requirements with your local authority.
• Assess the soil type — clay soils expand when wet and shrink when dry, requiring deeper foundations.
• Use our Retaining Wall Calculator to estimate materials and our Drainage Fall Calculator for the drainage pipe gradient.

Step-by-Step Instructions

Step 1: Excavate the Foundation Trench

Dig a trench at least 450mm deep and wide enough for the foundation (minimum twice the wall width plus 300mm extra behind for drainage). The trench must sit on firm, undisturbed ground. On clay soils, you may need to go deeper (750mm-1m) to reach stable ground below the zone of seasonal movement.

Step 2: Pour the Foundation

Pour a concrete foundation at least 200mm thick using C25 concrete (1:1.5:3 ratio). The foundation must extend at least 150mm beyond each face of the wall. Level the surface and allow to cure for at least 24 hours (48 hours in cold weather) before building on it.

Step 3: Lay the First Course

Set out the first course of blocks or bricks on a mortar bed (3:1 sharp sand to cement for below-ground work). Check every block is level in both directions and that the course is straight using a string line. The first course is critical — every subsequent course follows it.

Step 4: Build Up in Courses

Continue building in courses, staggering the joints (stretcher bond) for strength. Check level, plumb and line with every course. Apply mortar to both the bed joint (horizontal) and perpend joints (vertical). For reinforced walls, fill the block cores with concrete and insert vertical steel reinforcement bars as specified.

Step 5: Install Weep Holes

Insert 50mm PVC pipes through the wall at the base, approximately 1m apart along the length. These weep holes allow water to escape from behind the wall. Angle them slightly downward toward the front face. The first row of weep holes should be at or just above ground level on the front face.

Step 6: Apply Waterproofing

Once the mortar has cured (at least 24 hours), apply a waterproofing membrane or bituminous coating to the entire back face of the wall. This prevents water from penetrating through the blockwork and causing frost damage or efflorescence on the front face.

Step 7: Install Drainage Behind the Wall

This is the most critical step. Lay geotextile filter fabric against the back of the wall and up the sides of the excavation. Place a 100mm perforated land drain pipe at the base, bedded on 100mm of 20mm clean drainage gravel. Fill behind the wall with drainage gravel to at least 300mm depth. The filter fabric prevents soil from migrating into the gravel and blocking the drainage.

Step 8: Backfill

Backfill behind the drainage gravel with the excavated soil in layers of 150-200mm, compacting each layer. Do not use heavy machinery close to the wall until the mortar has fully cured (minimum 7 days). Leave the top 150-200mm for topsoil if planting above.

Step 9: Cap the Wall

Fit a coping or capping to the top of the wall to shed water. A concrete coping with a drip groove or a natural stone cap works well. Overhang the front face by at least 25mm to prevent water running down the wall face.

Common Mistakes to Avoid

• No drainage: The number one cause of retaining wall failure. Water pressure behind the wall can be two to three times the soil pressure alone. Always install drainage gravel, filter fabric and weep holes or a land drain.
• Shallow foundations: An undersized foundation allows the wall to slide or tip. Minimum 450mm deep, minimum twice the wall width.
• Building too high without engineering: Above 1m, the forces are significant. Even well-built gravity walls fail above this height without proper design.
• Using standard bricks: Standard facing bricks absorb water and break down below ground. Use engineering bricks (Class A or B) or dense concrete blocks for retaining walls.
• Backfilling with clay: Clay holds water and creates hydrostatic pressure. Always use free-draining gravel immediately behind the wall.
• Forgetting filter fabric: Without filter fabric, fine soil particles migrate into the drainage gravel and block it within a few years.

Cost Estimate (2026 UK Prices)

Use our Retaining Wall Calculator and Drainage Fall Calculator for exact quantities for your project.