An arch is any curved structure which spans an opening. The most common dry stone forms are semicircular (Norman) arches, with pointed (Gothic), and flattened (elliptical) arches also found. Other more ornate forms require ornamental mouldings or tracery, and are normally only found in buildings.
In the construction of arches, extra attention must be given to safety considerations, both during building and in ensuring the stability of the final structure.
The various parts of an arch have specific names as shown.
In walling terms, the pier may be a wall head, square pillar, the side of a smoot, or even the main body of the wall itself, in the case of arched niches. The impost is the upper course of the pier, and does not necessarily project. The intrados, also called the archivolt, is the inside curve of the arch. The extrados is the outside curve.
Arches can be used in place of lintels either for ornamental purposes, or where lintels of a sufficient size cannot be found, for example where a wall crosses a stream.
Stone arches are also used to build packhorse or footpath bridges across streams, although modern versions usually incorporate mortar.
Gothic arches tend to be used for more decorative purposes, or to make better use of flatter voussoirs. The example below is a reconstructed bread oven at Hutton Roof, near Penrith, Cumbria.
The structural theory relating to arches is quite complex, but put simply, ‘the arch creates an opening, while transferring to either side the weight of thrust that tries to fill that opening’ (McRaven, 1989). Semi-circular arches are the most efficient at doing this, with Gothic and elliptical arches less so.
Arches can be built out of a wide variety of stone types, although stone that is workable to some degree is preferable. A good supply of slightly tapering stones is helpful.
Formers
All arches need temporary support below the intrados to support the stones during construction, which is provided by a ‘former’ or ‘form’. For smaller elliptical arches the former can simply be a thin sheet of plywood, wedged between the foundation stones as shown. The plywood should be the width of the wall at ground level.
Raised arches, or arches of a specific shape such as semicircular or Gothic arches generally require a more accurate former. Cut two templates of the required shape from quarter-inch (6mm) plywood. Battens, 1″ (25mm) thick are then nailed between the two templates. Start with the battens at either end and at the apex, and then space the other battens evenly around the arch, making sure the gaps are smaller than the thinnest voussoir. If you are using very thin stone, it’s best to use battens only at the edges and apex, and use plywood elsewhere to make a solid former. Mark the apex on the former, as this will help with accurately positioning the keystone.
Rest the former on thin wooden wedges when setting it in position, to make it easier to remove the former on completion of the arch. If working on soil, first place flat stones under the wedges to stop them being forced into the soil.
For raised arches, the former should be raised on 4 x 4″ (100 x 100mm) timber legs, braced and supported as shown.
Alternatives to these simple wooden formers for small arches have been known to include tractor tyres and whisky barrels. However, safety is an important issue with larger arches, and advice should be sought for any structure requiring scaffolding.
Construction
Piers
Generally the piers are either pillars, wall heads or the sides of cripple holes. If the pier is set on a concrete foundation, it must be strong enough to also support the weight transferred onto it by the arch. If set on stone footings, you must choose the largest, most regular stones for the footings, as they will support more weight than normal.
Where the arch forms the roof of a niche, set the piers on the slab which forms the base of the niche.
Basic procedure
The basic procedure is similar for all shapes of arch.
- Place the springers at either end so that they have level bases and their ends touch the former.
- Place the first voussoir against each springer.
- Continue placing voussoirs, alternately one side and then the other.
- Jam the keystone into place at the apex.
A shallow arch may be spanned with one or two rows of voussoirs. With deeper arches, three or more rows of voussoirs will be needed (see below).
Stone placement and alignment
Selection of good springers is important. They must have a flat base to sit on the pier, with an angled top surface which points towards the centre of the base line of the former. Getting this angle correct sets up the subsequent placement of the voussoirs. In semicircular arches each voussoir should point along a radial line as shown. Mark the centre point clearly on the former.
Other arch shapes are not so simple. The voussoirs should be set with their middle line at 90˚ to the tangent at the point where they touch the intrados. This principle holds true for each voussoir on any curve, whatever its shape. The angle can be checked using a square, or by eye.
When building out of uncut stone, some wedges may be needed on the extrados (outer edge), but keep use to a minimum as stone contact between the voussoirs is reduced.
Maintaining alignment and contact can be a problem with flat, level bedded stone. Take care not to reduce the contact between voussoirs to a single point, or the arch is likely to collapse. This type of stone is best suited to larger radius semicircular arches, or to pointed arches, as shown.
Slightly rounded stone can be used for a semicircular arch. Although contact is limited, even if a stone slips slightly it should not fall out, and although the arch may become misshapen, it shouldn’t collapse.
Whatever the shape of the voussoirs, you must always place each one with the thickest end at the extrados. If you place any the other way round, they will be forced out as the arch settles.
Keystones
The shape of the keystone must accurately match the remaining gap. It can be a few millimetres wider, but must not be narrower or it will slip through. If it is too wide it will be impossible to hammer in, or may move some of the voussoirs out of line.
The last two voussoirs on a pointed arch should just touch each other at the apex. The wedge-shaped keystone should be placed rather than hammered in. The keystone should be at least the same height as the voussoirs, or protrude above them, so that the weight of the wall is transferred down through the voussoirs. As the keystone cannot slip through the apex of a pointed arch, it’s permissible to use small wedges if necessary between the keystone and adjacent voussoirs, to get a tight fit.
Depth of arch
An arch in a narrow wall may be constructed of voussoirs which span the wall. This is the simplest type of structure. Normally, two sets of voussoirs will be needed, making two matching arches which are tied together. A deeper arch will need a third set of voussoirs in the middle.
Try to interlock the voussoirs where possible, eliminating voids from the centre of the wall. This is easier to achieve if the arches are constructed at the same time. When building three arches you must work across the wall, rather than building each arch independently.
Ideally the voussoirs should interlock as shown, but in practice a few voids are likely. This is not a problem provided there is good contact between adjacent voussoirs in any one arch. Once each arch has been tested for strength, these voids should be wedged from the top.
Stones of a more triangular profile can be used, provided that good contact is maintained with the adjacent voussoirs in the same arch. A temporary wedge can be used, which will fall out once the former is removed.
Note in the diagram that it will often be necessary to trim the faces of the voussoirs to match the batter of the wall. If it’s not possible to trim the whole face, then round off the top edge so that the voussoir blends better with the wall.
Gaps at the top of the voussoirs should be wedged when the arch has been completed, with small gaps wedged to help lock the arch. Each wedge should be only marginally thicker than the gap, to squeeze the voussoirs together, rather than forcing their tops apart.
Finishing
When both sides of the arch are completed you can test it by sitting on it at the apex. It should not move, although a small amount of settlement against the former shouldn’t be a problem. Check each voussoir by trying to rock it, and remedy any slight movement with wedges. If any voussoir can be easily displaced you will need to dismantle the arch to this point, possibly substituting some different stones or the keystone, and rebuild it.
The weight on the arch locks it together and strengthens it.
A long traced stone placed on top of the keystone helps transfer the weight through the keystone to the voussoirs.
Where the arch forms a simple top to a gateway, with or without topstones above, the lack of weight means that the arch must be mortared to hold it together.
The matter of weight dispersal leads to some debate over the best time to remove the former. Some wallers prefer to leave it in place until the arch is complete and under full stress. Others prefer to remove the former once the arch is complete, or after the first layer above is completed. If the arch is solid there should be no problems, and any small movement will only lead to minor distortions. If there is a major disaster you have less to rebuild than if the arch had been completed!
It’s best to use voussoirs of similar height, dressing them as necessary, as it is then easier to fit the face stones above the arch, and they won’t interfere with the transfer of weight through the arch. If occasional longer voussoirs are used, they look better if they are placed symmetrically.
