Lintel Replacement: Concrete, Steel & Cavity — When & How

Summary

Lintel replacement is a structural intervention that is done surprisingly often in UK domestic properties — both as reactive repair (failed original lintel) and as part of alteration work (enlarging openings for bifolds, new windows, garage conversions). The key risks are inadequate temporary propping before removal (which can cause partial collapse), incorrect lintel sizing (causing cracking or structural movement after installation), and inadequate bearing (concentrated load at the end of the lintel causing masonry failure below the bearing point).

The original lintel type tells you a lot about the building's age and the likely failure mode. Concrete lintels with plain bar reinforcement (pre-1970s) corrode and spall; prestressed concrete lintels (post-1970s) are generally more durable but can still fail. Steel lintels in single-leaf construction (used extensively in the 1960s–1980s) corrode and expand, causing cracking in the masonry above. Cavity lintels (used from the 1980s onwards, also called steel boot lintels or catnic lintels) are the modern standard and are less prone to failure — but even these corrode eventually, particularly in coastal areas.

Modern cavity lintels from manufacturers such as Catnic, Birtley, IG, and Expamet are specified by opening width and load type (light/medium/heavy duty). They come with a prescriptive selection chart that allows a builder to specify without full structural calculations for most standard domestic openings. However, for non-standard situations (openings over 3m, unusual load configurations, or where structural calculations are required by building control), a structural engineer should design the lintel specification.

Key Facts

• Temporary propping — MANDATORY before any existing lintel is removed; props must transfer the load above the opening to sound structure on either side; inadequate propping is the principal cause of collapse during lintel work
• Acrow props with spreader boards — standard temporary propping method; 150x50mm timber spreader boards along the course above the lintel; Acrow props at maximum 600mm centres; props on spreader boards below
• Bearing length — minimum 150mm at each end of the lintel is standard practice; BS EN 845-2 specifies minimum 100mm; building control may require 150mm+; check before ordering
• Cavity lintel (boot lintel) — a pressed steel profile that spans the full cavity width; the bottom flange supports the inner leaf; a sloped outer face directs water outward as a cavity tray; standard for all modern cavity wall openings
• Catnic CN type — classic pressed steel cavity lintel; wide outer flange acts as cavity tray; inner and outer leaves both supported; prescriptive selection by loading table
• Concrete lintel — solid reinforced concrete; used historically in single-leaf masonry and in many non-cavity situations; prone to spalling when reinforcement corrodes
• Steel RSJ as lintel — a universal beam (UB) used as a lintel; provides greater spans than standard lintels; requires engineer's calculations; typically boxed in with masonry or fire-protected
• Light duty vs medium duty vs heavy duty — Catnic and similar suppliers classify cavity lintels by the tributary area of masonry above; selection charts give the correct lintel depth for the opening width and loading condition
• Cavity tray — a DPC membrane installed above the lintel to direct any water that penetrates the outer leaf out through weep holes; mandatory over all cavity lintels in external walls
• Weep holes — open vertical joints (every third perpend joint) in the first course above the lintel to allow water to escape from the cavity tray; frequently omitted by builders and then causes water ingress
• Padstone — a concrete or engineering brick pad under the end of a heavy lintel, RSJ, or beam; distributes the point load into the masonry; size from structural calculations
• Building control — required for all structural opening work; Full Plans or Building Notice; may require structural calculations for openings over 1.2m

Quick Reference Table

Detailed Guidance

Assessing the Existing Lintel

Before specifying a replacement, assess what is there and why it is failing:

1. Concrete lintel — spalling: chip away loose concrete to expose the reinforcement. Rust brown staining and flaking concrete around the bottom face indicates corrosion-induced spalling. Replacement is the correct treatment (not repair with bonding compound, which will fail again). Note: if the concrete appears intact but the masonry above is cracking, the lintel may be too shallow (insufficient structural depth) — this can occur where a non-structural lintel was fitted in a location bearing structural load.

2. Steel lintel — corrosion: rust expansion cracks appear as horizontal cracks running along the bottom of the masonry course immediately above the lintel, tracing the shape of the lintel. Probe for corrosion depth. A surface rust bloom may just need treatment, but significant section loss requires replacement.

3. Missing or failed cavity tray: if water is coming in above a window after rain, the cavity tray may be missing, displaced, or bridged. The lintel may be structurally sound but the waterproofing detail is defective. In this case the lintel need not be replaced — but accessing the cavity tray requires significant masonry work.

Temporary Propping Procedure

Propping is the most safety-critical step. Do not cut or remove masonry above an opening until propping is confirmed adequate.

1. Clear the work area — remove furniture and floor coverings inside; protect floors with spreader boards.
2. Fit props on the inside — position Acrow props with timber spreader heads at the ceiling (or floor above) approximately 600mm back from the face of the wall on both sides of the opening. Props at 600mm centres maximum.
3. Fit spreader boards on the floor above — the props must bear through the floor to the structure below, not just onto the floor boards. Place 150x50mm boards across the joists at 90° to spread the load.
4. Fit a spreader beam below the lintel — a 150x50mm (or 225x50mm for heavier loads) timber beam runs horizontally below the lintel, bearing on props. This spreads the load from the masonry above the opening while the lintel is replaced.
5. Confirm all props are tight — hand-tighten each prop; the spreader beam should be bearing firmly against the masonry.
6. Do not remove propping until the new lintel is installed AND the mortar has cured (typically 48 hours minimum).

Removing the Old Lintel and Installing the New

1. With propping in place, carefully chip away the mortar bedding above the lintel from each end inward. Do not use an angle grinder (vibration can dislodge props).
2. With a steel lintel: use a cold chisel to separate the lintel from the masonry; the rust expansion may mean the lintel needs significant force to remove.
3. Clean up the bearing pads on each side — ensure they are level, clean, and capable of carrying the lintel bearing load.
4. Measure the bearing pad dimensions and confirm the replacement lintel has adequate bearing at each end.
5. Lift the new lintel into position (cavity lintels can be heavy — 50–100kg for longer spans; two people minimum). On cavity lintels, confirm the outer flange faces outward and the integral cavity tray slopes to the outer face.
6. Bed in mortar (1:3 cement/sand) on each bearing pad.
7. Check the lintel is level; use packers if needed.
8. Rebuild any masonry that was disturbed during removal.
9. Install a cavity tray above the lintel if not integral, with weep holes at every third perpend.
10. Allow mortar to cure before removing props (minimum 24 hours; ideally 48).

Cavity Tray and Weep Holes

A cavity tray is a DPC membrane (typically lead, flexible DPC, or a proprietary preformed tray) installed across the cavity above the lintel. Its purpose is to intercept any water that penetrates the outer leaf and direct it outward to weep holes in the outer skin.

Weep holes are open perpend joints (no mortar in the vertical joint between two bricks) at the course immediately above the lintel outer flange. They should be at every third perpend — approximately every 450mm. Without weep holes, water accumulates behind the outer skin and enters the building.

This detail is frequently omitted on older construction and often omitted by less experienced builders on new work. If a replacement lintel is being installed, take the opportunity to install cavity tray and weep holes correctly, even if they were not present before.

Frequently Asked Questions

Do I need structural calculations for a lintel over a standard window?

For standard domestic openings up to approximately 1.2m, the manufacturer's prescriptive selection table (e.g. Catnic, Birtley, IG) is generally accepted by building control without separate calculations. For openings over 1.2m (and certainly over 2.4m), building control will usually require structural calculations from a Chartered Structural Engineer. Always check with your building control officer at the pre-application stage.

Can I use a concrete lintel to replace a failed steel one in a cavity wall?

A concrete lintel can only support one leaf of masonry. In a cavity wall, you need a cavity lintel (dual-purpose, supporting both leaves and incorporating the cavity tray). Do not use a single-leaf concrete lintel in a cavity wall opening — the outer leaf will be unsupported.

The masonry above a lintel has cracked — does the lintel need replacing?

Not necessarily. Cracks above a lintel can be caused by the lintel failing (deflecting or corroding), but they can also be caused by foundation movement that has nothing to do with the lintel. Get a structural engineer's opinion if cracks are significant (>5mm, stepped, or growing). A minor cosmetic crack at the end of a lintel bearing area is often caused by mortar shrinkage, not structural failure.

Regulations & Standards

• Building Regulations Approved Document A — Structure: requirements for structural elements including lintels; span tables for standard domestic situations

• BS EN 845-2 — Specification for ancillary components for masonry: lintels; minimum bearing requirements (100mm minimum)

• BS EN 1996 (Eurocode 6) — Design of masonry structures: used by structural engineers for lintel design in non-standard situations

• Building Act 1984 — building control enforcement; structural opening work is notifiable

• Catnic — Lintel Selection Guide — Prescriptive selection tables for all cavity lintel types

• Birtley Group — Lintel Technical Data — Cavity and single skin lintel range

• Approved Document A — Structural Guidance — Spans and bearing requirements

• Structural Timber Association — Temporary Propping Guidance — Temporary works guidance for structural openings

• structural calculations — When structural engineer calculations are required

• structural steel — RSJ beams for larger openings

• window types — Window types and glazing requirements

• building control process — Building control application for structural work