A slope is not a fault. It gives a garden height, views and a sense of drama that a flat plot simply cannot offer. What it does not give you is somewhere to put a table and chairs without chasing them down the lawn. Terracing solves that by cutting the gradient into a series of level shelves, each held back by a wall, each with its own soil, drainage and planting conditions. Done well, the awkward bank at the bottom of the garden becomes the most interesting part of it.
The trick is doing the jobs in the right order. Walls first and drainage later is the classic mistake, and it usually means rebuilding them within a few seasons. Read the ground, plan the levels, sort the water, then build.
Read the slope before you lift a spade
Before you spend anything, spend an afternoon measuring. Stretch a line level between two pegs a few metres apart and note the drop over that distance. Do it in several places across the plot, not just one. A garden that falls 600mm over ten metres is gently sloping and relatively easy to work with; one that drops 1.5 metres over the same distance is a different project altogether and will shape everything you do.
Use a few test pits to find out what you are dealing with. Push a spade down a spit deep in three or four spots and look at the soil. Heavy clay will hold water and needs generous drainage behind every wall. Sandy or gravelly ground drains fast but may not hold a wall's footing well. Then watch the garden during heavy rain. Where does water appear, where does it slow down, and where does it come from? If a neighbour's runoff or a natural spring crosses the boundary, terraces will not fix it on their own.
Note the aspect too. A south-facing slope dries out quickly and bakes in summer, while a north-facing one stays damp and grows moss on steps and paving. Both are workable, but they call for different planting and different surface finishes.
How many terraces, and how high?
A single tall wall is the option most people picture, and it is usually the worst one. Two or three lower walls look better, cost less per square metre, and are far easier to build safely. As a rule of thumb, keep individual level changes somewhere between 600mm and a metre. Anything above about a metre is a serious structure that needs proper design.
Width matters as much as height. A terrace needs to be genuinely usable, so think in terms of purpose rather than numbers alone:
- Seating and dining: allow at least 3 metres of depth so chairs can be pushed back.
- Planting beds: 1.5 to 2 metres gives room for shrubs and perennials to fill out.
- Paths and access: 1.2 metres is comfortable for a wheelbarrow; 900mm is a squeeze.
- Lawn or play space: 4 metres or more, otherwise it becomes a corridor.
Sketch the levels on paper before you dig, working from the bottom of the slope upwards. Each terrace should sit level front to back, with a very slight fall away from the wall on the surface above it.
Retaining walls that will last
Timber sleepers are the most popular choice for a first project. They are straightforward to handle, suit informal planting and can be built in gentle curves. Anchor them properly: drive steel rods or timber stakes through the sleepers into the ground behind, and set the bottom course on a bed of compacted hardcore rather than bare soil. Avoid old creosote-treated railway sleepers in beds where you plan to grow food.
Brick, block and natural stone walls need a concrete footing dug below the frost line, and blockwork usually needs a reinforcing mesh within the mortar. Whatever material you choose, lean the wall very slightly back into the slope — a batter of about 1 in 10 to 1 in 12 is enough. A wall that leans into the soil it holds is working with gravity; a perfectly vertical one is fighting it.
Two details separate a wall that lasts twenty years from one that bulges in three:
- Weep holes at the base, spaced along the wall, to let trapped water escape.
- Gravel backfill wrapped in a geotextile membrane behind the wall, so water passes through but soil does not.
If a wall is going to be over about a metre high, holding back a driveway, or supporting ground above a neighbour's boundary, get a structural engineer involved and check whether building regulations approval is needed. It is money well spent compared with a collapse.
Drainage decides whether the wall survives
Most retaining wall failures are water failures. Saturated soil weighs far more than dry soil and pushes outwards with real force, so every terrace needs somewhere for the water to go.
The standard solution is a French drain running along the back of the wall: a perforated pipe laid in a gravel-filled trench, wrapped in geotextile, with a gentle fall towards a soakaway, a drainage channel or a suitable connection. On shallow terraces with modest walls, weep holes and gravel backfill may be enough. On steeper plots, plan the whole system before you build the first wall, because retrofitting a drain means digging out the terrace you have just finished.
On the surface, keep paving and soil falling away from the wall by roughly 1 in 60, and use channels or gullies where water naturally gathers. Never redirect water onto a neighbour's land, and check local rules before installing a soakaway — some areas restrict them.
Steps, paths and getting around
Steps connect the terraces, and badly proportioned ones are a daily annoyance. A rise of 150 to 180mm with a going of 300 to 400mm feels natural underfoot. Add a landing every eight to ten steps, and make sure the landing drains.
Where you need wheelbarrow or mower access, use gentle ramps rather than steps, with a gradient no steeper than about 1 in 12 and a slight crossfall so water runs off rather than pooling. Wide, shallow steps with a paved tread are easier to maintain than narrow ones, and they handle a barrow surprisingly well.
Planting a terraced garden
Each terrace has its own microclimate, which is one of the joys of a sloped plot. Upper terraces are usually drier, windier and sunnier; lower ones hold moisture and stay cooler. Plant accordingly. Lavender, rosemary, thyme, sedum and ornamental grasses will thrive on a hot, free-draining top terrace, while ferns, hostas, astilbe and hydrangeas prefer the damper levels lower down.
Use the wall faces themselves. Alpines, aubrieta, campanula and erigeron all enjoy the sharp drainage of a stone or brick face, and crevices in a dry stone wall will colonise themselves within a season or two. Avoid large trees on the upper terraces — roots and wind loading will test any wall — and set drip irrigation lines on each level, since water does not travel upwards.
A sensible order of work
Build from the bottom of the slope upwards. Each wall then sits on ground that is already stable, and you are never working above an open excavation.
- Survey the levels and note where water moves during rain.
- Decide the number, height and width of terraces, and mark them out with pegs and string.
- Install the drainage behind the lowest wall before building it.
- Excavate, lay footings, and build the wall with its gravel backfill and weep holes.
- Backfill and level the terrace behind it, keeping a slight fall away from the wall.
- Repeat up the slope, then add steps, paths and surfaces.
- Plant last, once the ground has settled and the irrigation is in.
Tackle one terrace per season rather than the whole garden at once. It keeps the work realistic, spreads the cost, and gives you time to live with each level before committing to the next. If the slope is steep, the ground is unstable, or you are unsure about a wall or a drain, bring in a landscaper or engineer for advice early — it is far cheaper than fixing a wall that has moved.
Photo: Namden / Pixabay
