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Building ARCHES
Tips and Techniques to building arches by Dr. Gerard C.J. Lynch, LCG, Cert. Ed. M.A. (Dist.), Ph.D.
The types of bricks normally used for building arches are as follows:
Standard uncut bricks
These may be common or facing-brick quality bricks that have not been cut to the established arch template for the wedge-shaped voussoir. Most frequently they are laid on edge with “v” joints in half-brick rings in a rough arch.
Standard bricks cut to a wedge-shaped voussoir
Handmade or machine-made standard bricks that have been axed, or cut, to the shape of the established voussoir template, either by hammer and bolster and dressed with the scotch or using a bench-mounted disc-cutter. The axed voussoirs can then be laid with uniform, parallel joints.
Rubbing bricks or rubbers
These are a premier class of handmade, low-fired and soft-textured bricks that are capable of being cut and abraded to the precise shape of the voussoir template, or any other profile, to suit the design of the arch. They are used only in first-class work where neat, thin, joints are specified, as in gauged arches.
Purpose-made bricks
As their name suggests, these are bricks purposely made to the required voussoir template to suit the specified shape of the arch.
Temporary supports for building arches
When building, or “turning,” a rough, axed or gauged arch, it is essential to provide a temporary structure to support the voussoirs during construction. The upper surface of any support conforms to the underside, or “soffit,” of the arch design and is usually made about 1/8 inch smaller in overall length than the actual width of the opening so that it can be placed and removed without pinching on the jambs. There are two basic types of temporary arch support:
Turning pieces
Turning pieces are cut to size and shape from one single unit of timber and if level on top are intended for the flat, or “jack,” arch. Given a specified curved surface, the shallowest piece will be for a camber arch and beyond that for low-rise segmental arches. (Figure 1)
Centers
The carpenter traditionally makes centers from several sections of timber cut to the necessary sizes and shapes and assembles them into “ribs.” Each rib is then spaced and fixed apart, at a specified distance, to suit the particular arch. Timber battens, termed “laggings,” are fixed onto the upper surface providing the outline and soffit. “Open-lagged” centers have predetermined spaces between each of the laggings and are used for rough arches, whereas “close-lagged” centers are used for axed and gauged arches where greater accuracy is required. (Figure 2)
Modern arch centers
In most cases today, timber arch centers are made with a main framing of quality shuttering ply-board, with additional timber strutting and supports placed where required, overlaid with thin-sheet ply-board cut to fit that has largely replaced laggings. Today, proprietary purpose-made permanent and temporary metal or reusable polystyrene arch supports are often used instead of traditional timber centering on site.
The design and sophistication of the temporary support is dependent on the geometrical outline of the span, or “intrados,” and the weight that is to be borne. The support must be accurately set out by geometry, cut to shape and assembled. It must be rigid in its construction so that it doesn’t change shape under loading as the arch is being turned. It is vital that the upper surface be accurate in outline, flat and at 90 degrees across its width to the face so that the voussoirs can be set out and laid accurately over it.
In terms of loading and strength, some arch shapes create less loading than others. As a semicircular arch is being turned equally from either side, its weight initially compresses the center at the “haunches,” but this is reversed and stability returns as the arch nears completion. Also, with arches, like the semicircular, equilateral Gothic or similar profiles, the weight of the lowest voussoirs do not actually gain full support of the center. This only occurs with the higher voussoirs when the joint planes begin to travel beyond their “angle of repose,” which is approximately 30 degrees. Finally, one must always factor into the design of any turning piece or center the extra incumbent weight from voussoirs that have been dampened to assist setting and are thus much heavier than their dry weight, as well as the imposed loading of any standard face brickwork built over the arch before it is removed, or “struck.”
Positioning the turning piece or center
Before positioning, the “springing line” should always be drawn across the turning piece or center with a pencil, and by measuring half the overall length of the temporary support a center-line should then be drawn onto its face and across the width of the soffit using the try square to ensure accuracy. The support is then placed on vertical timber props, positioned centrally within the opening, about 1/4 inch from the face line. It must rest upon “folding wedges” that facilitate the fine adjustments necessary to ensure it sits plumb and level and that it has the correct height, with its drawn springing line running true to that of the abutments. Folding wedges also play a key function in easing the arch support and aiding its safe withdrawal once the arch has sufficiently set.
When building a segmental arch, where the striking point cannot be positioned on the center, this writer has always found it beneficial to establish a carefully positioned diagonal timber cross-brace across the opening, between the upright props, to aid stability and also provide the position upon which to plumb down the center-line upon which to locate the “striking point.” Upon this center-line the striking point can be precisely measured and established using a nail, or screw, to secure either a line or trammel to check the radial alignment of the voussoirs. Constructing a two-ringed semicircular rough arch
Of the three different classes of arches, rough, axed and gauged, the former is often incorrectly thought to be relatively easy — this is mistaken, as it still requires knowledge and skill to build one correctly. A rough arch is normally set out on the center rather than a full-size drawing used for axed and gauged arches.
The procedure for turning a two-ringed rough arch is: Measure the gauge of the brick voussoirs, mark the width of the key brick in pencil upon the face of the center and draw the lines across the full width of the support using the try square. Measure down the curved surface of the center from the established key brick to the springing line and subdivide this length into a number of equal spaces that will facilitate the brick gauge and joint allowance and set the dividers to that measurement.
The size of the joint on the intrados, or soffit, should be as small as possible — typically 1/4 inch — to prevent it from being aesthetically excessive at the extrados of the first ring. Once this voussoir spacing has been agreed upon, the positions are marked on either side of the face of the center, working from the key brick to the springing. These marks are then carefully drawn across the top of the center using the try square. This is frequently neglected, but it is vital to ensure that as the voussoirs are being laid, they remain parallel across their full width and fit precisely at the key, presenting a neat, uniform, aesthetic finish to the soffit.
To check radial alignment, either lines or a length of batten is fixed at the radial center, or striking point, established on the suitably positioned cross-brace. As the bricks are not wedge-shaped, the line cannot check them, only the “v” joints between. An alternative and excellent method is to cut a length of ply-board slightly longer than the measurement from the striking point to the completed arch extrados, and parallel in width to the brick voussoir gauge. Establish a center-line along its length and screw one end, through the center-line, to the striking point, and it can then be used to check each parallel voussoir brick into radial position. Marked with the width of the collar joint and line of the outer ring, it not only guides the second ring of voussoirs, but also serves to check the face height and curve of the arch extrados.
If for some reason the surrounding brickwork of the abutments is not built up to the overall height of the arch, then either timber profiles or “dead men” of bricks are accurately positioned for plumb so that a line can be strained across the opening to check that the arch is built to the overall wall, or face, line.
The first voussoirs, or “springers,” are laid on either side of the springing line. Sometimes, to spread the load in a two-ringed rough arch, springing bricks are laid as stretchers on face, particularly on a segmental arch where the full soffit is not visible. Each voussoir — suitably dampened if porous — is then “buttered” with a full bed of mortar slightly thickened towards the extrados in order to form the necessary wedge-shaped joint. The brick is then bedded onto the brickwork just up to the face line and positioned so its soffit follows the pencil guideline on the support. Once placed, it is checked with the line or trammel for correct radial alignment.
Raising the face line as necessary, continue to butter and bed the remaining voussoirs, building up both sides alternatively so the support remains evenly loaded. Set each voussoir accurately and square across the soffit to its mark and check that it answers to all the respective guides.
It is essential to keep the surface of the support free of mortar, as mortar can get under the brick, causing it to ride up out of line, which will spoil the aesthetics of the arch, and cement-based mortar can be difficult to remove without damage to the bricks. The normal method to avoid this is for the brick mason to use a trowel to neatly cut away about 1/8 inch of mortar from along the soffit of the brick after it is buttered, so the joint does not squeeze out onto the center as it is bedded. An alternative method is to place appropriately sized wood or foam strips against the previously laid voussoir at the soffit that are then withdrawn by sliding out, leaving a joint space for pointing later.
The final brick of the first ring is the key brick that must be bedded upright, sitting centrally upon the center-line at the crown of the arch with full and even mortar joints on either side.
The second ring is then ready to be set out, with the same sized joints if possible, in the same manner as the first ring, by indicating the positions of each voussoir upon a narrow bead of setting-out mortar, acting as the front of the true collar joint, placed to the front edge of the first ring. As the distance to be travelled across the curve of the arch by the second ring is greater, there are more brick voussoirs in the second ring. Sometimes the second ring bonds out to a key brick, but often it will work out to a joint between the two final voussoirs directly over the center of the key brick in the first ring.
The actual mortar collar joint is now spread over the first ring on either side for the first three voussoirs, and each brick is buttered and bedded into position, being checked by eye for square by ensuring the face of the voussoir runs perfectly parallel to the face line. All the respective guides should also be used to check the voussoir for accuracy, with particular attention paid to the extrados mark drawn on the trammel that controls the radial alignment, curve and face height, which in turn dictates the collar joint thickness.
After the final key brick is laid and firmly jointed, a long-standing tradition is to very slightly ease the completed arch by about 1/16 inch so that it can take up its bearing whilst the mortar is still green and pliable. This, it is contended, helps to prevent this occurring when the arch support is finally struck. Ideally the arch center should not be struck until the surrounding standard brickwork is laid up to the level of the “haunch” of the arch.
If the arch is to have a pointed finish, then once the mortar has sufficiently stiffened, the face is carefully raked out to the specified depth to provide a squared recess and is dusted clean of all debris. Once the center is struck the arch face is pointed along with its soffit to the specified profile. If, however, the arch is to have a specified jointed finish, then this is executed once the mortar has stiffened sufficiently to be neatly profiled. Once the center has been struck then the soffit is pointed in with a stiff matching mortar and jointed to compliment the arch face.
Dr. Gerard C.J. Lynch, LCG, Cert. Ed. M.A. (Dist.), Ph.D., is a historic brickwork consultant, master bricklayer, educator and author. Copyright Dr. Gerard C.J. Lynch, with some rights held by Traditional Masonry.
