Retainer walls are used to hold back a bank of earth or other material, and so may be used to contain the soil in a raised bed or to terrace a garden. They can be made from a range of materials, such as dry stones, concrete walling blocks or bricks.

As retaining walls must be built to withstand a great deal of pressure from the soil and water, any such walls greater than 75 cm (2 1/2 ft) high or that must be built on a steep slope should be designed by a structural or civil engineer who is familiar with site and ground conditions. Rather than requiring professional advice to build one high wall, it may be simpler and cheaper to terrace the garden in a series of shallow steps.

• A retaining wall should contain a number of features:
• A waterproof membrane or tanking on the rear of the retainer wall (i.e. the side facing the slope)
• A layer of gravel approximately 20 cm thick, running up the rear of the wall, optionally contained by a filter membrane
• Weep holes set at 1 metre intervals, 8 cm (or one course of bricks) above ground level
• A good foundation, at least 45 cm deep - deeper for clay soils
• A land drain running horizontally along the back of the wall

As noted above, weep holes should be added at the base of any retaining wall to allow water to escape. These can be formed from clay, copper or plastic pipes and should be set approximately 1 metre apart and 8 cm above the lower ground level. They should be positioned at a slight downward angle and finished flush with the face of the wall. A drain should also be inserted horizontally along the back of the wall - below the weep hole level.

To allow water to drain to the bottom and escape more easily, the wall should be backed a sheet of polythene (or other waterproof material) down to the weep hole level. It should then be followed with a layer of hardcore, gravel or similar porous material - this will help prevent water seeping beneath the wall. The back of the wall should also be treated with a waterproof sealant.

Any retaining wall used to hold back soil on a slope must be extremely strong, and the higher the wall, the stronger it will need to be. You can make a wall stronger by using larger bricks or blocks.