Arches have been used to bridge openings in walls for thousands of years, and medieval bricklayers and stonemasons developed a wide range of variations on the basic semi-circular arch that are still in use today. In the garden, an arch can frame a gateway from an attractive divider between different parts of the garden and can even provide viewing ports in boundary walls. The construction technique is much the same in every case.
Planning the arch
While it’s possible to form an arch in a wall, one block thick (a wall built in stretcher bond), such a construction is not strong enough for a full-height gateway. You should build any wall of this height in 230mm (9 in) thick masonry.
Freestanding arches
If you plan to finish off an opening in a garden wall with an arch, the first step is to build up the wall in blockwork to the level at which the arch will start – known as the springing point. This is usually at between 1.5 and 1.8 m (5 to 6 ft) above ground level, to allow ample headroom beneath the centre of the arch. Make sure that the coursing at each side of the opening is level, otherwise the arch will start its life lopsided and will always look unsymmetrical.
Making an arch former
The first step in constructing the arch itself is to make up a former to support the arch blockwork while you lay it. Measure the pier separation carefully, divide the figure by 2 to get the radius of the curve and draw out two semi-circles to this radius in plywood or chipboard (particle board). The simplest method is to use a makeshift compass – a pencil, a piece of string and a nail.
Building the arch
You can now lay the first block at each side of the arch, setting it on a wedge-shaped mortar bed so that the stretcher face (which will form the underside of the arch) is firmly pressed against the former. Mark a true vertical line on the face of the former, passing through the center of the semi-circle, and position a block (without mortar at this stage) on the centre point at the top of the former. This will form the keystone of the arch.
Built-in arches
If you decide to continue the wall upwards so that the arch will be completely surrounded by masonry, simply continue laying courses blocks on top of the existing wall at each side. In each course, cut the block that will match the angle of the ring at that point, allowing for a 10mm (1/2 in) thick mortar joint all round the curve. For the neatest possible effect, use a small angle grinder to cut these blocks to the precise angle required.
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