Turning a Brick Arch Over an Opening

Thrust Goes Wherever the Rise Isn't

An arch works by turning vertical load into an inclined force running down through the ring to the springing. Only part of that force is vertical; the rest pushes outwards, and the flatter the arch the larger that outward share becomes. The relationship is easiest to see in the classical result for a uniformly loaded parabolic arch, where the horizontal thrust varies as load times span squared divided by eight times the rise: halve the rise and you double the sideways push for the same load. A jack arch is that relationship taken to its limit, which is why a nearly flat arch is the most demanding thing on this page and not, as it looks, the simplest.

So the arch is never the whole job. What stands beside it matters as much as what spans the opening. A pier between two windows carrying an arch on each side has the two thrusts pushing against one another and largely cancelling; the arch at the end of a run has nothing on the far side and drives its thrust into a jamb that may be a single brick wide and stop at a corner a metre away. Openings near a return, a movement joint or the end of a wall are the ones that come back. Masonry arch analysis puts it as a line of thrust: the arch stands as long as a thrust line can be traced that lies entirely inside the depth of the ring, a position set out in Jacques Heyman's The Stone Skeleton, and keeping that line inside the middle third of the ring depth is what avoids tension at the joints.

Because of that, arches fail by hinging rather than by crushing. Four hinges anywhere along the ring convert it from a structure into a mechanism, and the movement needed to start the process is very small — a few millimetres of outward spread at an abutment will do it. That is the classic picture on an old elevation: a hairline running out of one haunch, the key sitting proud or low, and a crack through the spandrel above. Nothing crushed; the geometry simply stopped being able to hold a thrust line.

Where the arch has to carry a floor, a roof, or brickwork much above its own height, this stops being a bricklayer's decision and becomes an engineer's. TMS 402/602 Building Code Requirements and Specification for Masonry Structures governs the design in the American system and BS EN 1996-1-1 Eurocode 6 in the European one, under whatever general masonry provisions the local authority has adopted. Take the ring depth, the number of rings and the abutment check from whichever applies, not from what the last arch happened to be.

What Comes Apart, and What Was Never Meant to Stay

Five things sit over that opening while the work is going on, and only three of them are still there at the end. The ring of voussoirs and the brickwork it carries are permanent. The centre, the bearers under it and the folding wedges under those are temporary works — they exist to hold the geometry until the mortar can, and in the United Kingdom their design and their removal fall under BS 5975 Code of Practice for Temporary Works Procedures and the Permissible Stress Design of Falsework rather than under anybody's judgement on the day.

That split is also the split in the take-off. The permanent items are counted in bricks and in mortar; the temporary items are counted in sheet material, timber and hire, and they are the ones routinely left off a price because they do not show up in the finished elevation.

Cutting the Wedges

One fact makes this much easier than it looks: on a circular arch every voussoir is the same wedge. The taper does not vary from springing to crown, so a single template serves the whole ring, and the only bricks that differ are the two at the springings, which are cut to sit square on the skewbacks. Elliptical, three-centred and Tudor arches do not share that property — their radius changes along the arc — and a template made at the crown of one of those will be wrong by the time it reaches the haunch. Establish which kind you have before making a template, because it is the difference between one pattern and a dozen.

Cut wet on a bench saw with the template clamped or scribed, keep the cut square through the depth of the brick so the bed is not wound, and lay the ring out dry on the boards in the order it will be built. Traditional axed work does the same job with a bolster and a scutch and accepts a coarser fit; gauged work is a different trade again, using soft rubbers rubbed to shape on a stone and bedded in lime putty joints of a millimetre or two, and it is worth knowing that a gauged arch is repairable in a way that a hard-mortar cut arch is not. Historic England's Practical Building Conservation: Earth, Brick and Terracotta is the reference for that end of the work and for matching existing arches on repair.

Purpose-made arch bricks are the third route and the one that has to be decided earliest. In the British system BS 4729 Clay and Calcium Silicate Bricks of Special Shapes and Sizes — Specification covers the standard specials, with anything outside it made to drawing; either way the lead time comes off the manufacturer's own production schedule and is measured in weeks, not days. Order the ring bricks separately from the general facings whichever route you take, and add breakage on top of the count — a cut wedge that chips at the soffit arris is scrap, because that arris is the visible edge on the finished soffit.

Turning It

The ring goes up from both springings at once, alternately, never one side and then the other. Build the skewbacks first — the inclined beds cut into the jambs that the first voussoir sits on — and check that their faces are square to the ring, because a skewback out of angle puts the first brick out and every brick after it inherits the error with interest. Bed each voussoir on a full joint worked from the back so it is solid to the soffit; mortar pushed in from the face afterwards leaves a void behind a neat-looking line, and a void in an arch ring is the place a hinge will find first.

Keep the face plane as ruthlessly as the curve. A voussoir tapped in with the trowel handle stays where it is put; one persuaded with the blade rocks on its bed and drifts. Check the face against a straightedge across the ring every few bricks and check the soffit against the centre with a light behind it — a brick standing proud of the intrados by two millimetres is visible on a rendered reveal and impossible to correct once the key is in.

1. Cut and check the skewbacks, then set the centre and wedge it dead to the springing line.
2. Mark the voussoir divisions on both faces of the centre and dry-lay the ring to confirm the count closes on a key.
3. Build alternately from both springings toward the crown, bedding full joints worked from the back.
4. Fit the key last, driven home only enough to bed it — a key hammered tight lifts the haunches off the centre.
5. Build the spandrel and the brickwork above the arch while the centre is still carrying it.
6. Leave the centre in place until the mortar has the strength the specification requires, then ease the wedges.

Easing the Wedges

Striking the centre is the moment the arch becomes a structure, and it is a scheduled operation rather than a tidying-up job at the end of the week. The arch has to take its own weight and the weight of whatever stands above it in one movement, and mortar that has not developed enough strength will simply squeeze at the soffit joints and let the ring drop. The timing comes from the mortar and the specification — ASTM C270 Standard Specification for Mortar for Unit Masonry describes the material but not the striking time, and lime mortars need very much longer than a cement-lime mix. Nothing about it is judged by looking at the joint.

Ease, do not knock. Folding wedges exist so the centre can be lowered a millimetre at a time, from both ends alternately, keeping it level as it comes away. Dropping one end first loads half the ring, which is the same four-hinge geometry as an abutment spreading, only faster. Some settlement is expected and correct: the joints close up slightly, the ring beds down onto its skewbacks and the arch finds its thrust line. What you are watching for is anything asymmetric — one haunch closing and the other opening, or the key moving relative to its neighbours.

Then leave it alone. The centre comes out sideways once the wedges are slack, and the opening should not be loaded, propped against or used to carry a scaffold tie for as long as the specification says. Cover the top of unfinished work over the arch when you leave: rain running down into an open ring washes fines out of joints that are still green, and on a soft facing the staining that follows is on the most conspicuous course of the elevation.

The Arch That Is Carrying Nothing

A large share of the arches turned today are not structural at all. In veneer construction the opening is spanned by a steel angle and the ring is built off it, so the bricks are cut to a taper for appearance while the angle does the carrying. That does not make the geometry optional — the ring still has to close on a key and still has to be struck from a real radius — but it moves the failure mode. What fails on a supported arch is the support.

Steel carrying masonry is held to much tighter deflection limits than general framing, because brickwork cracks long before a beam is uncomfortable to walk on; the limits usually applied are a fraction of span in the region of one six-hundredth together with a small absolute cap, and Brick Industry Association Technical Notes on Brick Construction 31B: Structural Steel Lintels is where to take them from rather than from a joist table. Bearing length at each end matters as much as the section, and the angle's horizontal leg has to support the ring without its toe showing under the soffit.

The other veneer detail that gets missed is water. Flashing runs over the arch rather than through it, stepping up and across the ring and turning up behind the drainage plane, with weeps immediately above it at the spacing the adopted code sets. Skip it and every drop that gets through the ring — and a ring with wide back joints passes a good deal — collects on the steel, which then rusts, expands and lifts the arch off its bearing. Allow for movement too: clay brickwork grows after it leaves the kiln while steel does not, so the joint arrangement around the opening has to let the arch move relative to what carries it.

Reading One That Has Already Gone Wrong

Arches record their own history, which is a gift on a repair survey. A key sitting low with fine cracks radiating from the haunches is a spread abutment — look at what is beside the opening and what changed there, not at the arch. Wide open joints at the back of the ring with tight ones at the soffit mean the ring was turned as a rough arch beyond the taper a joint can carry. A soffit that dips in the middle while the joints stay closed is a centre that deflected during the turn and was never going to recover.

Two more are worth naming because both get misdiagnosed as failure. Staining and efflorescence concentrated on the arch alone usually mean water standing on the back of the ring, which is a flashing problem above rather than a masonry problem in front of it. And a jack arch that appears to sag when a straightedge says it is dead level is doing exactly what it should — jack arches are built with a slight upward camber for that reason, proportional to the span, taken from the arch supplier's shop drawing or from Technical Notes 31 rather than from the bricklayer's eye.