Bonding Wall Corners

A garden wall has no floor or roof holding it together. Along a straight run it moves more easily with wind, frost and settling ground. A corner acts as a brace: two walls meet at a right angle and support each other. That only works if they are properly bonded.

If the second wall is simply butted up against the first, you get a continuous vertical joint from the foundation to the top. Each part then moves on its own. In practice a crack often appears in such a corner after one or two winters, and one length of wall starts to lean slightly. A bonded corner prevents this because the load spreads in both directions.

The corner is also the point the whole wall is set out from. If it is not square or not plumb, the error carries into every course and later into fence panels and the gate.

The method varies a little with the material, but the principle stays the same: each course at the corner is laid the opposite way to the one below it.

1. Prepare a strip foundation that runs through the corner without a break. Its depth follows the design and must reach below the local frost line, which differs from region to region.
2. With shuttering blocks, set vertical reinforcement bars into the foundation at the corner and at regular intervals along the wall.
3. Set out the corner with string lines and check it is square with a builder's square or the 3:4:5 rule (a triangle with sides in that ratio has a right angle).
4. Lay the first course perfectly level, always starting with the corner unit.
5. In the second course, turn the corner unit through 90 degrees so it covers the joint below. The other units are usually offset by half their length, and bricks by at least a quarter.
6. Cut corner units as needed, or use the manufacturer's corner and half blocks.
7. With shuttering blocks, add horizontal reinforcement and bend it round the corner so it runs from one wall into the other.
8. After every course, check the corner for plumb with a spirit level and check the line with a string stretched between corners.

Bricklayers usually build the corner several courses higher than the rest of the wall first, stepping it back as they go ("racking back"). The wall between the corners is then laid to a line, so the corner guides both height and alignment.

Ordinary bricks are bonded at the corner with stretchers and headers. A stretcher lies along the wall, a header lies across it. At the corner they alternate so that vertical joints never line up from one course to the next. This takes accurate cutting and an experienced hand, but it gives a neat and strong corner.

Shuttering blocks are hollow concrete blocks that act as permanent formwork and are filled with concrete as the wall rises. Here the bond comes from alternating the blocks combined with reinforcement. The corner void should be filled with concrete for its full height so it works as a hidden column. Depending on the system, the blocks are laid dry or with mortar, as the manufacturer specifies.

Decorative and split-face wall blocks often come with their own corner or pier units. These make the job much easier, but you still need to alternate the courses and fill the cores if the manufacturer requires it.

For low dwarf walls a standard bond is enough. For taller walls, walls on a slope or retaining walls holding back soil, a structural engineer should decide on the reinforcement and bond.

A level, accurately built foundation, ready-made corner units and a true right angle all make the work easier. Corners also go faster with dimensionally accurate blocks that need little cutting.

Corners at angles other than 90 degrees take longer because every unit has to be cut. A taller wall, a sloping site where the wall steps down, or joining onto an existing old wall all add work. Poor access slows things down too, for example a narrow strip between a neighbour's fence and your garden. If an old, unbonded corner needs repairing, it often has to be taken down and rebuilt.

• Butt-jointed corner without a bond. The two walls just stand side by side and sooner or later separate with a crack.
• Broken foundation. If the foundations of the two walls only touch, each settles differently and the corner cracks.
• No reinforcement at the corner. With shuttering blocks the corner column carries the load, and without steel it loses its purpose.
• Poorly filled cores. Concrete poured too high in one go or mixed too stiff leaves voids at the corner.
• Movement joint at the corner. Long walls need movement joints, but not right at the corner.
• Checking only at the end. A few millimetres out on each course becomes a visibly crooked corner at the top.

You can build a low dwarf wall or plinth up to about knee height yourself if you take your time. For taller walls, retaining walls and walls next to a road, a bricklayer is the safer choice. Before you start, ask your local building authority whether the wall needs any approval. Rules usually depend on height, position and whether the wall is next to a road or on the boundary with a neighbour.

1. Confirm the boundary and set out the corner with two string lines.
2. Check the corner is square using the 3:4:5 rule.
3. Clean the top of the foundation and dampen it if the mortar requires it.
4. Bed the corner unit of the first course in mortar and level it in both directions.
5. Lay the rest of the first course in both directions to the line.
6. Turn the corner unit through 90 degrees in the second course and continue with staggered joints.
7. Check the corner for plumb and both walls for line after every course.
8. With hollow blocks, fill the cores with concrete every few courses and compact it with a rod.
9. Strike off excess mortar before it sets so it does not stain the face.
10. Protect the finished corner from rain, frost and strong sun until the mortar and concrete have hardened.

Before you accept the work, look at the corner up close and from a distance. Focus on these points:

• Vertical joints do not line up in neighbouring courses
• Corner units alternate from one side to the other
• The corner is plumb for its full height, checked with a spirit level or plumb line
• The angle between the walls matches the drawing, usually a right angle
• Joints are roughly even in thickness and fully filled
• With shuttering blocks, the corner is reinforced and the cores are filled
• There are no cracks or loose units at the corner
• The top of the wall is level at the corner and runs smoothly into both walls