Summary

A brick arch is one of the more skilled pieces of bricklaying a tradesperson will be asked to price, and one where the client's expectations (a period feature) and the structural reality (a load-bearing element over an opening) need to be reconciled clearly before work starts. There are two fundamentally different ways to build what looks like a brick arch, and confusing them — or letting the customer assume one when you're building the other — is where arch jobs go wrong.

A true (structural) arch relies entirely on the geometry of wedge-shaped bricks (voussoirs) in compression: each brick pushes against its neighbours, and the whole assembly transfers the load above the opening outward and down into the abutments either side. This is genuine historic masonry technology, still built today, particularly on conservation and heritage work, but it demands accurate setting-out, correctly cut or purpose-made radial bricks, and a temporary support (centring) that stays in place until the mortar has developed sufficient strength.

A decorative arch on a concealed lintel looks identical from the front but is not structural in its own right — a standard steel cavity lintel (the same product used over any window or door, see lintel types) carries the actual load, and the arch-shaped brickwork below it is a facing detail, often using specially shaped voussoir bricks or simply tapered mortar joints to create the arch appearance. This is now the standard approach for most new-build and extension work with an "arch" feature, because it removes the risk of arch failure entirely — the steel lintel is doing the structural job, and the brickwork is decorative. Many bricklayers build this without disclosing the distinction to the client, which is poor practice: know which one you are quoting and building, and be able to explain it.

This article covers both methods, with the setting-out geometry, centring construction, and mortar and curing requirements that apply to true structural arches, plus guidance on when the concealed-lintel approach is the right call.

Key Facts

Quick Reference Table

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Arch Type Description Typical Span Structural Method
Flat (soldier) arch Bricks vertical, slightly tapered, level soffit Up to ~1.2m Concealed lintel in modern practice
Segmental arch Shallow curve, single radius less than semi-circle 1-3m True arch (period) or concealed lintel (modern)
Semi-circular arch Full half-circle, rise = half the span 0.9-2.4m True arch, common in Georgian/Victorian work
Camber arch Very slight upward curve, near-flat appearance Up to ~1.5m Usually concealed lintel
Gothic/pointed arch Two intersecting arcs meeting at a point Variable True arch, specialist/heritage work
Relieving arch Rough arch built above a straight lintel to divert load around it Variable True arch, structural (not decorative)

Detailed Guidance

Setting Out a True Arch

Step 1 — Establish span and rise. Measure the clear opening (span) and decide the rise based on the arch type: a semi-circular arch has rise = span ÷ 2; a segmental arch typically has a rise between 1/6 and 1/4 of the span.

Step 2 — Draw the arch geometry full-size. For a semi-circular arch, strike the curve from a centre point on the springing line using a trammel or string and pencil. For a segmental arch, the centre point is below the springing line — calculate or draw geometrically to find it.

Step 3 — Divide the arch into voussoirs. Divide the curve into an odd number of equal segments (so there is a single central keystone, not a joint at the crown) — typically 7, 9, or 11 voussoirs for a standard domestic-scale arch depending on span and brick size. Each joint must be a true radial line back to the arch centre.

Step 4 — Determine brick cutting or ordering. For gauged brickwork (traditional rubbed arches), bricks are cut and rubbed to the precise taper on site — a specialist skill. For most modern work, purpose-made radial voussoir bricks are ordered to BS 4729 dimensions from a brick manufacturer, or standard bricks are used with a wider mortar joint at the extrados and narrower at the intrados (acceptable for segmental arches with a gentle curve, less acceptable for tight semi-circular arches where the joint variation becomes visually obvious).

Building the Centring

The centring (or "turning piece") is a temporary timber former, cut to the exact profile of the arch soffit, supported on props or the reveal of the opening. It must:

Lay the voussoirs from both skewbacks simultaneously, working up toward the crown, so the centring is loaded symmetrically. The final voussoir (keystone) locks the arch. Do not strike the centring until the mortar has developed adequate strength — minimum 48 hours in good conditions for a cement-lime mortar, longer for pure lime mortar or in cold/damp weather; wider spans and heavier loading above the arch need proportionally longer.

The Concealed-Lintel Approach (Modern Standard Practice)

For the great majority of new-build extensions, garden room openings, and "feature arch" requests over standard-width domestic openings, the concealed steel lintel approach is now standard because it removes the structural risk of the arch entirely:

  1. Install a standard cavity lintel sized exactly as for a straight opening of the same span, following the manufacturer's load tables (see lintel types) — some manufacturers offer a curved/bowed lintel profile to follow the arch shape, otherwise a straight lintel is used and the brickwork below is corbelled/cut to create the arch appearance beneath it
  2. Build the cavity tray DPC above the lintel exactly as for any opening, following the curve with stepped tray sections or a purpose-made curved tray
  3. Build the arch-shaped brickwork as a facing below the lintel soffit line, using purpose-made curved voussoir bricks or standard bricks with tapered joints — this brickwork carries no structural load and does not need centring support for structural reasons (though a temporary support to hold the geometry accurate while the mortar sets is still good practice)
  4. Ensure weep holes are provided at the base of the cavity above the tray exactly as for any lintel

Always tell the client which approach you are using. A "decorative arch on a hidden lintel" is not a lesser job — it is the technically superior, lower-risk approach for most domestic-scale work — but it is a different product from a genuine load-bearing masonry arch, and a client paying for period authenticity (particularly on a listed building repair) may specifically require the true arch method.

Relieving Arches

A relieving arch (sometimes called a discharging arch) is a rough, often semi-circular arch of brickwork built above a straight lintel — historically above timber lintels, which have limited load capacity — to divert the weight of the wall above around the lintel and down into the jambs either side, rather than the lintel carrying the full masonry load. This is a genuinely structural element, commonly found in Georgian and Victorian construction above timber or shallow stone lintels. When repairing or extending a period property, always check for a relieving arch above an existing lintel before assuming the lintel alone is carrying the load — removing or disturbing a relieving arch without understanding its role can lead to lintel overload and cracking.

Frequently Asked Questions

Does a brick arch need building control approval?

Structural masonry work forming part of a load-bearing wall — including a true structural arch — falls under Building Regulations Approved Document A (structure) and should be assessed against the same criteria as any other structural opening. For most domestic scale arches (up to around 1.8-2.4m span), standard masonry design guidance covers the work without needing bespoke structural engineer calculations, but for wider spans, heavy loading above, or any doubt about the abutment strength, a structural engineer should check the design, and building control notification will typically be required as part of the wider project (extension, structural alteration).

Can I build a true arch using standard bricks without ordering special radial voussoirs?

For gently curved segmental arches with a wide radius, standard bricks with a varying mortar joint width (wider at the extrados, narrower at the intrados) can produce an acceptable result, particularly if the joints are kept within a reasonable range (roughly 6-15mm). For tighter curves — semi-circular arches especially — the joint variation becomes visually unacceptable and structurally less reliable, and purpose-made radial voussoir bricks (BS 4729 dimensions) or hand-cut gauged brickwork should be used instead.

How long does the centring need to stay in place?

Minimum 48 hours for a cement-lime mortar in good drying conditions is a reasonable starting point, but this varies significantly with mortar type, weather, span, and load above the arch. Pure lime mortar cures much more slowly and may need the centring left for a week or more. In cold or wet weather, extend the period significantly. When in doubt, leave it longer — striking too early is one of the few genuinely dangerous mistakes possible in arch construction, as a partially-cured arch can collapse without warning when the support is removed.

What's the difference between a keystone and any other voussoir structurally?

In a true semi-circular or segmental arch, the keystone at the crown carries no more load than any other voussoir — every voussoir in a properly built arch is in compression against its neighbours, and the arch only becomes self-supporting once the final (keystone) voussoir is placed, locking the ring. The keystone's structural role is really about completing the ring, not carrying disproportionate load. Decoratively, keystones are often made wider, projecting, or in a contrasting material — this is an aesthetic choice with no bearing on the actual load path.

Regulations & Standards