What Are the Rules for Corbelling Brickwork? Projection Limits and Structural Considerations

Summary

Corbelling is the technique of building out brickwork progressively — each course projects further than the one below — to support a weight above (such as a chimney breast projection, a bay window, an arch, or a decorative feature) or to create architectural detail. It is one of the oldest structural masonry techniques and remains common in both traditional and contemporary brickwork.

The rules governing corbelling derive partly from traditional rules of thumb and partly from structural masonry design principles in Eurocode 6. For small, lightly loaded corbels (such as a chimney flue offset or a decorative string course), the traditional rules are generally sufficient. For more significant structural corbels — supporting a floor, a heavy load, or forming part of a structural frame — an engineer's design is required.

Understanding the distinction between structural and non-structural corbelling, and knowing when to involve a structural engineer, is an important competency for bricklayers, building contractors, and those inspecting existing masonry structures.

Key Facts

• Maximum projection per course — traditionally one-third of the brick length per course; for a 215mm standard UK brick, this is approximately 72mm per course
• Maximum total projection — the total overhang should not exceed the thickness of the wall or pier from which the corbel springs; for a 102.5mm half-brick wall, total projection maximum is approximately 100mm
• Stability rule — the line of action of the load must fall within the middle third of the bearing area; this is the fundamental stability criterion for all corbels
• Bond continuity — corbelled brickwork must be properly bonded; each projecting course must be tied back into the main wall by at least one full brick length (215mm) to prevent overturning
• Minimum bearing — the weight being supported should bear over a zone at least as deep as it is wide
• Structural engineer requirement — corbels supporting load-bearing walls, floors, or large overhangs require a structural design; BS EN 1996-1-1 governs masonry structural design
• Chimney corbelling — standard chimney stacks can be corbelled out at 1:6 maximum slope (one brick every six courses) without engineering; steeper corbelling requires engineer input
• Garden wall corbelling — decorative corbelled coping or feature courses in garden walls can follow the 1/3 rule without engineering provided the wall is non-structural
• Cut corbels — individual cut bricks used as projecting supports (consoles); must be carefully detailed to maintain bond
• Toothed corbels — where alternate courses project and tie back, creating a more complex interlocking pattern; stronger than simple stacked corbels
• Eurocode 6 — BS EN 1996-1-1:2005+A1:2012 — Design of masonry structures; the structural standard for engineered masonry corbels
• Mortar specification — use designation (iii) mortar (1:1:6 or 1:6 masonry cement) minimum for corbels; stronger mortar (designation ii or i) for heavily loaded corbels

Quick Reference Table

Detailed Guidance

The One-Third Rule Explained

The one-third projection rule is based on the geometry of overturning stability. If a brick courses out more than one-third of its length, the centre of gravity of the projecting mass falls beyond the inner edge of the course below, creating an overturning moment that cannot be resisted by the bond alone.

For a standard UK metric brick (215mm × 102.5mm × 65mm), one-third of the length is 71.7mm — rounded to 72mm in practice. Each successive corbelled course projects a maximum of 72mm further than the one below.

The number of courses over which the corbel can be built is therefore limited by the total projection required. If you need to project 300mm, you need at least five courses (5 × 72mm = 360mm — enough) but this assumes the wall is thick enough to tie back each course adequately.

The critical check is that the bond extends at least as far back into the wall as the projection extends outward. If a course projects 200mm, the bricks in that course must bond back 200mm into the wall behind. For a standard brick, this means the projecting brick must overlap with the wall by at least 215mm minus its projection.

Structural vs Non-Structural Corbelling

Non-structural corbelling includes:

• Chimney breast offsets (the chimney stack projecting outwards on an external wall)
• Decorative string courses and cornices
• Garden wall coping stones supported on corbelled brickwork
• Brick quoins and architectural details

These applications carry minimal load (typically only self-weight of the corbelled masonry) and follow the traditional 1/3 rule without requiring engineered design.

Structural corbelling includes:

• Corbels supporting floor beams or joists
• Corbels forming the bearing for lintels or arches
• Corbelling to carry a wall above on a reduced-width supporting wall below
• Any corbel where the load is significant relative to the masonry cross-section

For structural corbels, Eurocode 6 (BS EN 1996-1-1) requires a formal structural design. The engineer will calculate the compression stress in the masonry, the overturning moment, and the shear across the corbel. They will specify mortar strength, brick type and compressive strength, and any reinforcement if required (reinforced masonry corbels are permitted under Eurocode 6).

Chimney Corbelling Standards

Chimney corbelling is one of the most common applications and is covered in detail in HETAS technical guidance and the NHBC Standards. For a chimney stack that projects beyond an external wall (to accommodate a flue offset or to position the stack above the roof ridge):

The maximum slope for chimney corbelling without engineering input is typically quoted as 1:6 (one brick width outward for every six courses in height, approximately one course per 450mm height). This is approximately equivalent to the 1/3 rule per course but expressed as an overall slope.

For chimney cappings and the transition from the stack to the chimney breast, corbelling of two to three courses over a 150–225mm total projection is standard and acceptable without specific engineering input.

Garden Wall Corbelling

Decorative corbelling in garden walls is common — a projecting brick course at the top of the wall provides a coping detail and breaks the water run-off pattern. These projections are typically one course of half-brick on edge (65mm projection) or two courses with 72mm each (144mm total).

For garden walls, the only constraint is the one-third rule per course and ensuring the coping overhangs the wall face sufficiently for weather protection (typically 30–50mm overhang is considered good practice). No engineering assessment is required for standard decorative coping on a non-structural garden wall.

If the garden wall is a boundary wall that needs to resist lateral loading (from fill behind, for example), the entire wall design requires consideration and the corbelling must not compromise the wall's stability.

Cut Corbels and Special Profiles

Cut corbels are individual bricks cut to a shaped profile — the classic Victorian console corbel with a curved lower face. These are used for decorative supports for bay windows, eaves, and cornices. The cutting reduces the effective bearing area and the bond back into the wall.

For cut corbels carrying load, each corbel should be engineered. The critical failure mode is shear across the cut face, and this must be checked using BS EN 1996-1-1 procedures.

Toothed corbels — where alternate bricks in each course project and tie back in an interlocking pattern — are structurally more efficient than simple stacked corbels because the interlock resists overturning. They are commonly used where larger projections are needed, or where the corbel is carrying moderate loads and engineering input is not practical.

Frequently Asked Questions

How do I calculate how many corbelled courses I need for a specific projection?

Divide the required total projection (mm) by 72mm (the maximum per-course projection for standard UK bricks). Round up to the nearest whole number. For example, a 200mm total projection requires at least 3 courses (3 × 72mm = 216mm — just over 200mm, so achievable in 3 courses).

Can I use engineering bricks for a corbel to allow larger projections?

No. The maximum projection per course is governed by the geometry of the bond and overturning stability, not by the brick strength. Engineering bricks have higher compressive strength and lower water absorption than facing bricks, but the 1/3 rule still applies. Engineering bricks may be specified for corbels in aggressive environments (below DPC, in contact with moisture) but do not change the projection limits.

I'm building a Victorian-style bay window with brick corbels — do I need a structural engineer?

Yes. A bay window corbel carries the self-weight of the bay structure, any floor load above, and potentially wall loads. This is structural work. Engage a structural engineer to design the corbel geometry, specify the mortar and brick, and provide calculations. Building Control will require engineer's details for a structural corbel supporting a floor or wall.

What mortar should I use for corbels?

As a minimum, use designation (iii) mortar (approximately 1:1:6 cement:lime:sand or masonry cement equivalent). For structural corbels or corbels in exposed positions, use designation (ii) mortar (1:0.5:4.5). Pre-mixed mortars should be rated to at least M4 (4 N/mm²) and preferably M6 for structural applications. Avoid weak mortars (designation iv or v) in any corbelling application.

Do corbels in a listed building need Listed Building Consent?

If the corbelling forms part of or alters the external appearance or structural character of a listed building, Listed Building Consent is required in addition to any Building Regulations notification. For repairs to existing corbelling using the same mortar and brick specification, consent may not be required — but always check with the Conservation Officer first.

Regulations & Standards

• BS EN 1996-1-1:2005+A1:2012 (Eurocode 6) — Design of masonry structures; general rules for reinforced and unreinforced masonry

• Building Regulations Part A — structure; corbels forming part of a structural frame must be designed under Approved Document A

• BS EN 1996-3:2006 — Simplified calculation methods for masonry structures (useful for simple corbel checks)

• PD 6697:2019 — Recommendations for the design of masonry structures to BS EN 1996; national annex guidance

• The Brick Development Association — technical notes on masonry design and corbelling

• NHBC Technical Standards Chapter 6.2 — masonry wall requirements including corbelling

• ICE — Structural Engineers Guidance on Masonry — Eurocode 6 design guidance

• Historic England — Repair of Traditional Masonry — guidance for corbel repair in historic buildings

• brick bonds — understanding brick bonds and their relationship to corbel stability

• mortar mixes — mortar specification for different applications

• lintel types — structural elements above openings in masonry walls

• part a structure — Building Regulations structural requirements