Why a Retaining Wall Earns Its Keep

If your garden drops away sharply behind the house, you already know the problem. Rain runs down the slope, soil creeps downhill, and the flat patch of lawn you wanted keeps shrinking. A retaining wall does one simple job: it holds soil back at a higher level on one side than the other, so the ground above stays put and the ground below stays usable.

Before you buy a single stone, work out how much height you are dealing with. Anything up to about 600mm can usually be tackled by a competent DIYer with a decent weekend and a strong back. Between 600mm and 1.2m you are in serious territory and want an engineer to check the design. Above 1.2m, or any wall close to a boundary, a highway or a building, get proper structural advice — retaining walls fail suddenly, not gradually, and a collapsed wall can bring down a fence, a shed or worse.

Two more practicalities. First, on clay soils, expect movement as the ground shrinks and swells through the seasons; walls need to be designed for that. Second, check whether you need permission. A wall over 1m next to a highway, or over 2m elsewhere, often needs planning consent, and the Party Wall Act may apply if you are excavating near a neighbour's structure.

Choosing Between Stone, Concrete and Timber

Natural stone is the material most people picture, and for good reason — it ages beautifully and suits everything from a Cotswold cottage to a modern extension. Expect to pay more than for blockwork, and expect to spend far longer laying it. In return you get a wall that will outlive you.

• Natural stone — sandstone, limestone, granite, slate or reclaimed brick. Buy more than you need, ideally 10–15% extra for cutting and breakages, and order it all at once so the colour and thickness stay consistent.
• Concrete blocks — cheaper and quicker, especially for taller walls. Face them with stone slips or render if looks matter.
• Timber sleepers — good for low walls and raised beds, but they rot. Use new oak or tanalised softwood on a gravel bed and accept that you will replace them eventually.
• Gabion baskets — forgiving on uneven ground and excellent for drainage, though they need a lot of stone to fill.

A quick word on mortar. Dry stone walling looks superb but needs skill and a wide base. For most garden walls, a lime or weak cement mortar (roughly 1 part cement to 4–5 parts sand) is more forgiving and lets the wall flex slightly rather than crack.

Getting the Foundation Right

Nine out of ten failed retaining walls were badly founded. Dig a trench that is at least 150mm deep for a low wall, and deeper where the ground is soft or the wall is tall — a common rule of thumb is a foundation depth equal to roughly a tenth of the wall height, plus a margin for frost. On heavy clay, go deeper still.

Make the trench about three times the width of the wall you plan to build, and fill it with concrete to a minimum of 150mm thick. Level it carefully, then leave it to cure for at least two days before laying a single stone. Build the first course below the lower ground level, not at it, so the base is buried and protected. A wall sitting on bare soil, or on a few loose bricks, will lean within two winters.

Drainage: The Part Everyone Forgets

Water is what destroys retaining walls. Trapped behind the stone, it freezes, expands, and pushes the wall forward until it topples. You need two things working together.

• Backfill with clean gravel. Leave a 300mm gap behind the wall and fill it with 20mm clean stone or gravel right up the height of the wall. This lets water drain freely instead of pressing on the stonework.
• Line the back with geotextile. A permeable membrane stops soil washing into the gravel and clogging it.
• Add a perforated pipe. A 100mm perforated drainage pipe at the base, laid in the gravel with a slight fall, carries water away to a soakaway, drain or lower ground.
• Include weep holes. Small gaps every metre or so near the base give any remaining water an escape route.

If your garden slopes towards the house, that pipe needs somewhere sensible to go — never discharge it at your neighbour's boundary or into a conservatory base.

Laying the Stone and Finishing Well

Work from the lowest point and build in courses, keeping each stone level. Lean the wall back slightly — a batter of about 1 in 20 towards the retained soil — so gravity helps rather than fights you. Pack the stones tightly, use smaller stones as chocks, and keep the joints consistent. Cap the top with a soldier course or a row of flat coping stones set on mortar to shed water away from the face.

Leave a gap between the wall and any fence or planting of at least the wall height, and never plant trees with vigorous roots within a few metres. Then step back and watch the space above become a proper terrace, and the space below become a garden again.