How to Build Strip Foundations for Garden Walls

Start With the Wall, Not the Trench

The foundation only needs to be as beefy as the wall above it. Get the proportions roughly right and a garden wall will sit happily for decades; guess badly and you'll be watching it lean within a couple of winters. As a working rule, a strip foundation should be about three times the width of the wall it carries, and deep enough to sit below the frost line.

For most domestic garden walls, these figures will serve you well:

• Up to 600mm high: 300mm wide, 150mm thick, 450mm deep
• 600mm to 1m high: 400mm wide, 150mm–200mm thick, 450–600mm deep
• Over 1m high: 450mm wide, 225mm thick, 600mm deep, and worth a chat with a structural engineer

Depth is measured from ground level to the underside of the concrete. In most of the UK, 450mm gets you below the level where frost heave does its worst, but shrinkable clay, mature trees, filled ground or a sloping site all push that figure deeper.

Setting Out and Digging the Trench

Mark the line with pegs and a taut string, offset from the trench edge so the string doesn't end up in the spoil. Check your corners with the 3-4-5 method — 3m along one side, 4m along the other, 5m across the diagonal — before you cut anything. If you're anywhere near a boundary, be certain where the line sits; a foundation that creeps onto a neighbour's land is a conversation nobody enjoys.

Dig with a spade to roughly the trench width, keeping the sides as vertical as the ground allows and piling spoil well back from the edge. Dig to your finished depth plus the thickness of any hardcore you're using. Stop when you hit firm, undisturbed subsoil. Loose pockets, old roots and rubble all need clearing out and replacing with compacted hardcore — concrete will bridge a soft spot, but it won't thank you for it.

If the trench fills with water, bail it out and work out why before you pour. Standing water means a high water table or a flow you need to divert, and pouring into it weakens the mix.

Compacting the Base

A 50mm–75mm layer of compacted MOT Type 1, or 100mm of clean hardcore, gives you a flat working base and stops the concrete mixing with soft soil. Tip it in, spread it to a uniform thickness, and run a whacker plate over it in overlapping passes until it stops moving. Check the level with a straight edge and a spirit level as you go.

On very firm, dry clay or chalk you can sometimes skip the hardcore, but the trench bottom still needs to be flat and firm. If you've dug a bit deep in places, fill those dips with compacted hardcore rather than extra concrete — it's cheaper and it doesn't shrink.

Mixing and Pouring the Concrete

A C20/25 mix (sold as GEN3) is the right choice for garden wall foundations. Work out your volume first: length × width × thickness, in metres. A 10m wall on a 400mm × 150mm foundation needs 0.6m³, so ready-mix is worth ordering once you're past about 1m³. For smaller jobs, a 1:2:4 mix by volume — one part cement, two sharp sand, four 20mm aggregate — will do nicely.

Pour in one continuous run if you can. If you must stop, stop square and clean, and don't let the first pour go off before the second arrives. Minimum concrete thickness for a garden wall is 150mm; 100mm is only acceptable under a very low wall on genuinely firm ground.

Don't pour onto frozen ground or into standing water, and if the forecast is below 5°C and falling, wait.

Levelling, Finishing and Curing

Tamp the concrete down with a board to remove air pockets, then strike it off level using a straight edge spanning the trench. Check with a spirit level on the straight edge, and keep checking — a foundation 20mm out over 5m will show up in every course above it. Leave the surface slightly rough; mortar grips it better than a polished finish.

Then leave it alone. Cover with polythene in hot or wet weather, keep it damp for the first day or two, and give it at least 48 hours before you walk on it and three to seven days before you start laying. Cold weather stretches that, so allow a week if temperatures are hovering in single figures.

Building Off the Foundation

Once it's cured, set out the first course dry, then lay it on a full mortar bed. A garden wall should have a damp proof course about 150mm above ground level, with the courses below it in a frost-resistant brick or block. On any run over about 6m, build in movement joints every 6m to 9m — a simple 10mm gap with a compressible filler — or the wall will crack on its own terms.

Finally, backfill carefully with soil or gravel, sloping the ground away from the wall so water runs off. Whatever you do, don't batter wet clay back against it; that's how a perfectly good foundation ends up carrying a wall that leans.