Domestic extensions involve a long list of structural and design decisions. If you are in a conservation area, the local planning authority may restrict materials. If you want a clean, open link between old and new, choosing the right lintel for an extension can feel complex. Most of the complexity comes from understanding the factors that influence lintel selection rather than from the products themselves.
This guide covers common considerations when looking at lintels for house extensions, including span tables, steel and concrete lintels, padstones and temporary support. A structural engineer should always be the one to specify the correct lintel for your project.
Building Regulations for Extension Lintels
Even when an extension qualifies under permitted development, Building Control will still need to approve the structural elements. Lintels form part of the superstructure and must comply with Part A of the UK Building Regulations. Building Control will also need to review the structural calculations used to determine the appropriate lintel type.
While it is not essential to hire an architect, using a structural engineer is strongly recommended. They will determine the lintel sizing, check loading conditions and provide the documentation required for approval.
Using Lintel Span Tables for Extensions
Lintel span tables can help illustrate the range of lintel types available and the kinds of factors that influence selection, such as span width, load conditions and cavity width. They are a useful point of reference for understanding why engineers specify different products for different situations.
It is important not to assume that two properties will require the same lintel. Roof structure, joist direction, point loads and wall construction can differ considerably between houses. A structural engineer should always verify which lintel is needed.
Steel Lintels for Extensions
Steel lintels and RSJs are widely used in domestic extension projects, particularly where the design includes larger openings or where the engineer deems a steel solution appropriate. Steel sections are often selected for situations involving higher loads or wider spans, but the decision rests entirely on engineering calculations.
Steel typically requires protection to meet fire resistance requirements, usually achieved through appropriate cladding materials specified by the designer.
Common pitfalls include assuming that any steel section is suitable, or estimating end bearings without engineering input. Access and handling should also be considered at the planning stage.
Concrete Lintels for Extensions
Concrete lintels are commonly used in smaller openings and areas where the demands on the lintel are lower. Reinforced concrete lintels provide compressive and tensile strength and are a familiar choice in domestic work.
For longer spans, concrete can become heavy and may not provide sufficient capacity depending on the loading. The suitability of concrete lintels for any opening should always be determined by an engineer.
Common misunderstandings include assuming concrete lintels can be used interchangeably for larger openings or that one lintel is adequate in a cavity wall where two might be required.
Do I Need Padstones?
Padstones are often specified where steel beams or heavier lintels are used. Their purpose is to distribute load safely into the supporting masonry. Whether padstones are required, and their size and type, must be confirmed by the structural engineer who designed the opening.
Using alternative materials or blockwork instead of specified padstones can compromise the way the load is transferred, so it is important to follow the engineer’s design.
Temporary Support During Installation
Temporary support, such as using acrow props, is an essential part of many structural alteration projects. The type and number of props required, together with their spacing and arrangement, form part of a temporary works plan that must be developed by a competent person.
General principles such as supporting both leaves of a cavity wall and keeping the work area accessible are widely understood in the trade, but the exact method varies between projects. Temporary works design should not be improvised, and the engineer or contractor responsible for the works should specify a safe method.
How many props will I need for an extension opening?
It is a common question, but one that cannot be answered accurately in an article. The number and arrangement of props depend on several project-specific factors, including wall construction, loading conditions, the size of the opening and the temporary works method selected by the contractor or engineer.
Temporary support must always be designed by a competent person. The engineer or contractor responsible for the works will determine the correct number of props, their spacing and the appropriate method of support for the opening. Generalised figures should not be relied upon, as every extension project has different requirements.
Steel and Concrete Lintels: Typical Characteristics
The following table summarises common characteristics of steel and concrete lintels. These are general observations only and should not be treated as selection guidance.
| Feature | Steel Lintel or RSJ | Concrete Lintel |
|---|---|---|
| Typical application | Often used where higher loads or wider openings are involved | Often used in shorter spans and standard domestic openings |
| Weight | Comparatively lighter for the capacity | Heavier, especially at longer lengths |
| Fire resistance | Usually requires protection | Typically meets fire resistance requirements but must be verified per product |
| Typical construction use | Used in many open-plan designs | Common in window and door openings |
Key Considerations Before Choosing a Lintel
Only a structural engineer can confirm which lintel is appropriate. Factors they will typically assess include:
- Span and clear opening size
- End bearing requirements
- Cavity width
- Roof and floor loading
- Point loads
- Whether single or dual lintels are required for cavity walls
- Handling and access constraints
- Whether padstones or alternative bearing solutions are needed
Questions to Ask Your Structural Engineer
Before work begins on an extension, it can be useful to discuss the lintel design with your engineer so that everyone understands the requirements of the project. Typical questions homeowners and contractors ask include:
- What loads will the lintel need to support?
- How will the span, wall construction and cavity width influence the lintel design?
- Is a single lintel required, or separate lintels for each leaf of a cavity wall?
- Will padstones or other bearing solutions be specified?
- How much end bearing is required for the chosen lintel or beam?
- Are there any point loads or unusual loading conditions to account for?
- Are there temporary works requirements that need to be planned in advance?
- How will the lintel or beam need to be protected to meet fire resistance requirements?
Questions to Ask Your Lintel Supplier Before Ordering
Once your structural engineer has specified the lintel or beam required for your extension, it can be helpful to confirm a few practical details before placing an order. Typical questions customers ask their lintel supplier include:
- Is the specified lintel size and type available from stock?
- Are equivalent products available that match the engineer’s specification?
- What are the physical dimensions of the product, including depth and width, to help plan handling and access?
- What is the approximate weight of the lintel?
- Are there any delivery considerations, such as access restrictions or preferred drop-off points?
- Is manufacturer documentation or a product data sheet available to share with Building Control?
FAQs About Lintels for Extensions
What lintel is used for a 3m opening in an extension?
The lintel for any opening must be specified by a structural engineer. The engineer will assess loads, span, wall type and other design factors before selecting a suitable lintel or beam.
Do I need an engineer to specify a lintel?
Yes. For structural alterations in an extension, an engineer should provide calculations and confirm the lintel type required.
Can a concrete lintel be used for larger extension openings?
Concrete lintels are commonly used for smaller spans. For wider openings, suitability depends entirely on the loading and the engineer’s design. It should not be assumed that concrete lintels are appropriate for larger spans.
What is the difference between an RSJ and a steel lintel?
An RSJ is a solid steel beam. A steel lintel is usually a formed section designed for specific wall constructions. The engineer will decide which is suitable based on structural calculations.
Will Building Control check the lintel?
Yes. Building Control will expect structural calculations and may inspect the lintel installation before approval.
