Lintel Failure Diagnosis: Arch Action vs Lintel Deflection, Spalling Above Openings, Rusted Steel, Propping and Replacement

Q: The cracks above my client's window have been there for 20 years. Are they a problem?

Historical crack stability is an important indicator but not a guarantee. Cracks that have been stable (confirmed by measurements over several months, or by old photos showing no change) and follow the arch action pattern are generally benign. However, before dismissing any crack, check: is there any rust staining (could indicate recent corrosion onset); is the crack width >2 mm (borderline structural); is the window or door now sticking (active movement)? If any of these apply, recommend a str

Quick Answer: Lintel failure presents as cracking above windows or door openings, stepping cracks along mortar joints, spalling brickwork above the opening, or visible deflection of the lintel itself. The critical diagnostic distinction is between true lintel deflection/failure and natural arch action cracking — arch action cracking follows a triangular crack pattern and is generally benign; true lintel failure shows continuous horizontal or diagonal cracks and progressive settlement. A structural engineer should assess before any propping or replacement commences.

Summary

Lintels carry the load of the wall above an opening — windows, doors, and other voids — transferring it to the masonry on either side. In most domestic masonry construction, this is either a precast concrete lintel, a steel lintel (rolled steel joist or proprietary pressed steel), a reinforced brick or stone arch, or in older buildings, a timber lintel. Each fails in different ways and requires different diagnostic approaches.

The most common misdiagnosis is treating arch action cracking as lintel failure. All masonry openings, even those with structural lintels, tend to develop a triangular crack pattern radiating from the upper corners of the opening as the masonry naturally arches over the void. This is a self-limiting process in most cases and does not indicate structural failure. Treating it as lintel failure leads to unnecessary and expensive intervention.

True lintel failure — particularly of corroded steel lintels — is a more serious matter. Rust forms within the lintel, expanding as it oxidises and spalling off the brick or concrete covering it. This type of failure is progressive and self-accelerating — the spalling exposes more steel to moisture, accelerating corrosion further. It requires prompt professional assessment and usually means replacement of the lintel before the cracking extends into the structural walls above.

Key Facts

Typical lintel spans — domestic window openings 900–2400 mm; door openings 900–1200 mm; garage openings 2100–4800 mm
Steel lintel rust expansion — oxidised iron expands to approximately 3× its original volume; this is sufficient to crack and spall 50–75 mm of concrete or brick cover
Corrosion cover requirements (BS 8110) — minimum 25 mm concrete cover to steel in mild exposure; 40 mm in moderate exposure; lintels exposed at the external leaf typically require 40–50 mm
Arch action zone — triangular zone 45° from opening corners; cracking within this zone is typically non-structural
Deflection limit for lintels — BS EN 845-2 specifies maximum deflection of span/500 for prefabricated lintels; visible bow in a lintel is a failure indicator
Precast concrete lintel — typically 100–215 mm depth; cracking along the bottom tension face indicates reinforcement failure or corrosion
Pressed steel lintel (Catnic, IG, Birtley types) — used since 1970s in cavity wall construction; failure by corrosion at the mortar bed or by inadequate bearing (minimum 150 mm each end)
RSJ (Rolled Steel Joist) — older steel lintels; common in pre-1960s construction; very susceptible to corrosion; often protected by concrete encasement which can hide internal corrosion
Timber lintel failure — dry rot or wet rot destroys structural capacity; often found in pre-1920 construction; can fail with no external cracking until significant movement has occurred
Minimum bearing length — for domestic openings: minimum 150 mm bearing each side under BS EN 845-2 and typical lintel manufacturer guidance
Helifix/helical repair — structural stainless steel fixings can stabilise cracking without full lintel replacement in some cases
Propping requirement — always required before removing a failed lintel; typically 2 × acrow props with needles through the wall
Structural engineer — required before any propping, removal, or replacement of a lintel that shows signs of structural failure; a Building Notice is required for structural replacement in England under Building Regulations

Quick Reference Table

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Symptom	Likely Cause	Urgency	Action
Triangular crack at corners of opening	Arch action — normal settlement	Low	Monitor; inject crack if cosmetic concern
Horizontal crack above opening with visible bow in lintel	Lintel deflection failure	High	Structural engineer urgently
Orange/brown staining or flaking from above opening	Steel corrosion / rust jacking	High	Structural engineer; probe brickwork
Vertical crack through brick courses above opening	Differential settlement in masonry above	Medium-High	Structural engineer
Stepping cracks in mortar above opening	Arch action or minor settlement	Low-Medium	Monitor; check lintel bearing
Crumbling concrete below lintel soffit	Concrete carbonation + reinforcement corrosion	High	Probe and assess reinforcement; replace if necessary
Visible bow or sag in lintel	Deflection exceeding span/500	High	Structural engineer; emergency prop if movement continuing
Crack + door/window sticking	Settlement/movement continuing	Medium-High	Structural engineer

Detailed Guidance

Diagnostic Decision Tree

Cracking or distress above opening?
          |
          +-------> Is cracking confined to triangular
                    zone (45° from opening corners)?
                              |
                    YES ------+-----> Monitor; arch action cracks
                                      typically non-structural
                              |
                    NO        v
                    Is lintel visibly deflecting
                    (bow, sag, visible curve)?
                              |
                    YES ------+-----> STOP — structural engineer
                                      Prop immediately if movement
                                      is active or cracking wide
                              |
                    NO        v
                    Is there rust staining,
                    spalling concrete, or
                    orange mortar staining?
                              |
                    YES ------+-----> Probe cover to expose lintel
                                      Assess steel corrosion
                                      Structural engineer
                              |
                    NO        v
                    Are cracks >2 mm wide or
                    widening over weeks/months?
                              |
                    YES ------+-----> Structural engineer
                                      Monitor with crack gauges
                              |
                    NO        v
                    Stabilised crack, no active
                    movement: cosmetic repair
                    acceptable after confirmation
                    of no structural movement

Arch Action vs True Lintel Failure

Arch action cracking: All masonry above an opening tends toward arch behaviour. As the wall settles and compresses, the masonry redistributes loads and cracks propagate diagonally from the upper corners of the opening, typically at 45°–60° to the horizontal, following mortar joints in a stepped pattern.

Characteristics:

Triangular crack pattern radiating from corners
Cracks follow mortar joints (stepped), not through bricks
Cracking is typically stable once the initial settlement is complete (usually within 2–3 years of construction)
Width typically <1 mm; stable over time
Not associated with rust staining or concrete spalling
Window and door operation usually unaffected

True lintel deflection failure: Characteristics:

Visible bow or sag in the lintel soffit or the masonry above
Horizontal crack along the line of the lintel, particularly at the bottom face of a concrete lintel
Cracking is progressive — widens over time
May be associated with rust staining on concrete or brick face
Window or door frame distortion; difficulty opening or closing
Cracks wider than 2 mm or active (measurable change over 1–2 weeks)

Steel Corrosion — Rust Jacking

Rust jacking is the process by which corroding steel expands, fracturing the concrete or masonry encasing it. It is the principal failure mechanism for older rolled steel section lintels and for precast concrete lintels with inadequate cover to reinforcement.

Identifying rust jacking:

Orange/brown staining on brick, mortar, or concrete soffit
Horizontal cracking parallel to the lintel at the mortar bed level
Flaking or spalling of brickwork directly above the lintel
In some cases, rust staining appears on the inner leaf only, indicating the inner lintel component is corroding while the outer leaf appears intact

Assessing severity:

Use a masonry probe (SDS drill, small hammer) to tap the brick/concrete above the lintel — a hollow sound indicates delamination of cover
Carefully break out a small area (100×100 mm) directly above the suspected corrosion to expose the lintel; this requires care not to remove more masonry support than is safe
Visual assessment: surface rust (reddish-brown superficial coating) vs section loss (pitting, delamination of the steel, visible reduction in section thickness)
Use a hammer to test the steel section — a dull thud indicates significant section loss

Decision:

Surface rust only, section intact, cover ≥25 mm: wire brush, apply epoxy-based rust converter and anti-corrosion primer; restore cover with repair mortar — monitor for 12 months
Significant section loss, cracking cover, or spalling: lintel replacement required; structural engineer to confirm

Propping Before Lintel Replacement

Any replacement of a structural lintel requires temporary propping of the structure above. This is not optional — the lintel is carrying load from the wall and any floor or roof structure above the opening.

Standard propping for domestic openings:

A typical domestic window replacement uses:

Two adjustable steel props (acrow props) with base plates
100×100 mm or larger timber "needles" threaded through the wall above the opening — typically 3 courses above the lintel
Props positioned on solid ground or spreader boards on each side of the wall

Process:

Mark needle positions — minimum 450 mm above the lintel and minimum 600 mm from the opening on each side
Drill pilot holes and cut needle slots (if load-bearing wall) — for cavity walls, needle through the inner leaf only may be sufficient for lightweight window lintels; structural engineer to confirm
Insert needles; position props; adjust to snug contact — do not over-jack or you risk lifting the structure
Check all props are plumb and stable before proceeding
Remove old lintel — work carefully; do not remove mortar bedding until new lintel is ready to receive
Insert new lintel; re-bed in mortar; allow 24 hours minimum before removing props
Remove props progressively — loosen both simultaneously in small increments

For openings wider than 2.4 m, beam-and-prop systems or temporary frames are required. Consult a structural engineer.

Lintel Replacement — Product Selection

When replacing a failed lintel, the correct product must be selected based on wall type, opening span, and load conditions.

Cavity wall steel lintels:

Standard pressed steel cavity lintels (Catnic, IG, Birtley, Teplo) accommodate inner and outer leaves
Select by span and load condition — manufacturers publish load tables for their products
Minimum bearing: 150 mm each side for most domestic spans
Galvanised or stainless steel finish depending on exposure category (coastal areas: stainless steel minimum)

Solid wall / single-leaf:

RSJ or precast concrete lintel
RSJ size: for domestic spans, typically 127×76×13 UB (Universal Beam) for openings up to 1.8 m; increase section for longer spans
All steel in exposed positions must be protected with appropriate coating

Precast concrete:

Confirm beam size from manufacturer's span table
Ensure minimum 150 mm bearing
Replace like-for-like as a minimum; upgrade if original was undersized

Building Regulations: Replacement of a structural lintel is a structural alteration requiring Building Regulations approval under Part A (Structure). Submit a Building Notice or Full Plans application before commencing work. The building inspector will want to see the structural engineer's specification or lintel manufacturer's span table confirming the new lintel is adequate.

Frequently Asked Questions

The cracks above my client's window have been there for 20 years. Are they a problem?

Historical crack stability is an important indicator but not a guarantee. Cracks that have been stable (confirmed by measurements over several months, or by old photos showing no change) and follow the arch action pattern are generally benign. However, before dismissing any crack, check: is there any rust staining (could indicate recent corrosion onset); is the crack width >2 mm (borderline structural); is the window or door now sticking (active movement)? If any of these apply, recommend a structural engineer review regardless of history.

Can I replace a lintel without a structural engineer?

For simple lintel-for-lintel replacement of the same product to the same span, in a standard domestic property, many contractors proceed by reference to the lintel manufacturer's load tables and Building Regulations Part A guidance. However, if the opening is being widened, if there is evidence of inadequate bearing on the original, or if the property is unusual (older, non-standard construction, adjacent to previous extensions), a structural engineer's specification provides legal protection and ensures Building Control approval.

What is the difference between a "padstone" and a "bearing block"?

Both terms describe the load-spreading element placed beneath a lintel at its bearing positions. A padstone is typically a precast concrete or natural stone unit designed to distribute the concentrated point load from a lintel bearing into the masonry below. They are required where masonry is low-strength (e.g., aerated concrete block inner leaves) or where the point load is high (e.g., long spans). BS 5628-1 (now BS EN 1996) governs the design of masonry to carry such loads.

I can see rusting on the steel lintel but no cracking. Is it urgent?

Visible rust on a steel lintel without visible cracking suggests early-stage corrosion. Assess the severity: if it is surface rust only with no delamination, expanding cracks, or staining of the brickwork, it can be treated in place with a rust converter and protective coating and monitored. If the rust is associated with any cracking in the masonry or concrete cover, treat as a developing structural issue and get a structural engineer assessment. Do not leave active corrosion unaddressed — it is progressive.

Regulations & Standards

Building Regulations Approved Document A (Structure) — structural requirements; lintel replacement requires Building Regulations compliance
BS EN 845-2:2013 — prefabricated masonry accessories including lintels; deflection limits and bearing requirements
BS EN 1996 (Eurocode 6) — masonry structure design; wall load capacity and bearing design
BS 8500:2015 — concrete specification; cover to reinforcement for durability
BS 7913:2013 — guide to the conservation of historic buildings; relevant for pre-1919 properties with historic lintels
CDM Regulations 2015 — structural propping and temporary works safety
Building Research Establishment (BRE) — Defects in Buildings — Lintel failure and arch action technical guidance
IG Lintels — Technical Span Tables — Product selection and load data
Catnic — Lintel Technical Guide — Cavity wall lintel selection
Historic England — Repair of Traditional Buildings — Guidance on lintels and arches in older masonry
RICS — Structural Defects in Masonry — Professional guidance for surveyors
mortar cracking guide — Distinguishing settlement cracks from structural failure
damp diagnosis — Rust staining often confused with damp ingress
party wall guide — Party Wall Act process for opening-up works to shared walls
building regulations guide — When Building Regulations approval is required for structural works