How do I build a low garden wall?

A low garden wall is one of the more satisfying DIY jobs and one of the easier ones to get structurally wrong. Almost all failures come from a foundation that was too shallow, a mortar that was too strong, or the absence of a coping to keep water out of the top.

Safety

What you'll need

• Bricks or blocks, plus 5% for cuts and breakages
• Concrete for the foundation — C20 mix or ready-mixed
• Building sand and cement, plus plasticiser or lime
• Coping stones or bricks on edge
• A brick trowel, spirit level, line and pins, and a gauge rod

Step by step

1. Check what the wall must do and how high it will be.Free-standing, on level ground, under 600mm and away from a highway is straightforward DIY. Anything retaining soil, taller than about 1m, or where a collapse could hit a person or a road needs designing.
2. Dig the foundation trench.At least twice the wall width, and at least 300mm deep for a low wall — deeper on clay or where the ground is soft, down to firm undisturbed subsoil. Below the frost line, which in the UK is normally taken as 450mm in exposed areas.
3. Cast the foundation level.A C20 concrete mix, at least 150mm thick, tamped and levelled. Get it flat — every millimetre of error in the foundation has to be made up in the first mortar bed.
4. Let it cure and set out the first course dry.24-48 hours before building on it. Lay the first course out dry with the joints spaced to check the wall length works in whole bricks before mixing any mortar.
5. Mix the mortar to the right strength.For a garden wall in normal exposure, 1 part cement to 5 or 6 parts building sand with a plasticiser, or a lime-based mix on an older or softer brick. A stronger mix is not a better mix — it cracks the bricks instead of the joints.
6. Lay to a line and check every course.Build the ends up first, run a line between them, and lay to it. Check level along and across, and plumb at the ends, on every course. A gauge rod marked with the course heights keeps everything consistent.
7. Use a proper bond.Stretcher bond for a half-brick wall, English or Flemish garden wall bond for a one-brick wall. Perpends must be staggered — vertical joints lining up is a straight structural weakness.
8. Include a damp-proof course and movement joints.A DPC at the base, at least 150mm above ground, stops rising damp making the wall spall in frost. On runs over about 6m, include a vertical movement joint.
9. Point the joints as you go.When the mortar is thumbprint-hard, strike the joints — bucket-handle or weather-struck sheds water best on an exposed wall. Recessed joints look good and hold water, which is a problem outdoors.
10. Fit a coping with a drip.Coping stones or bricks on edge, bedded in mortar, oversailing both faces by at least 25mm with a drip groove underneath. Water getting into the top of a wall is what destroys garden walls.

Tips

• Build in cool damp weather rather than hot sun. Mortar that dries before it cures has almost no strength.
• Cover new work with plastic overnight if frost is forecast. Frost on green mortar ruins it and it will not recover.
• Keep the bricks under cover before laying. Saturated bricks float on the mortar and will not stay where you put them.

Common mistakes

• Using a strong 1:3 mortar mix — Mortar should be weaker than the brick, so that movement cracks the joint rather than the brick. A too-strong mix transfers stress into the bricks and spalls their faces.
• Leaving the top of the wall uncapped — Water enters the open top, saturates the wall and freezes. Uncapped garden walls fail from the top down within a few winters. A coping with a drip is not decoration.

If it doesn't work

Questions people ask