With over 30 years in the builders’ merchant trade, Andrew brings deep knowledge of everything from civils to timber. Now part of the Gilmore team, he helps customers make the right choices with advice built on decades of hands-on experience.
For lintels for 900mm openings, bear in mind that the lintel will be longer than 900mm and selected based on the wall type and what the wall is carrying, not just the width of the gap.
In most modern UK cavity walls, that usually means a steel cavity lintel designed to support both leaves of masonry. In solid walls or certain internal loadbearing walls, a concrete lintel may be appropriate. The opening width tells you part of the story, but not the whole decision.
Crucially, lintels must extend past the opening to provide adequate bearing at each end. The minimum bearing requirement is set by the lintel manufacturer. Where structural alterations are involved, a structural engineer may specify greater bearing. It is not something that should be guessed on site.
If you want the broader framework behind this decision, our main guide on Which Lintel Do I Need? covers the overall selection process.
900mm is the opening width, not the lintel length. The lintel must be longer to provide adequate bearing at both ends.
Minimum bearing is defined by the manufacturer. In structural work, a structural engineer may require increased bearing.
A lintel for a 900mm opening must allow sufficient end bearing within the masonry on both sides. That means the overall lintel length will always exceed 900mm.
Do not guess bearing dimensions on site. Always refer to the specific product’s published technical guidance, or the engineer’s specification where applicable.
The correct process is straightforward.
A 900mm opening is common for standard window openings, smaller door openings, internal loadbearing walls and typical rear extensions.
In cavity wall construction, a steel cavity lintel is normally required to support both the inner block leaf and the outer brick leaf together. In solid masonry walls, a single lintel spanning the full wall thickness may be suitable.
The same 900mm opening in two different properties can require different lintels depending on wall build-up, number of storeys above, roof and floor loads, and whether the opening is new or altered. The opening width alone never determines the correct lintel.
This article explains principles and selection logic. It does not replace structural design or regulatory approval.
Forming or altering an opening in a loadbearing wall is structural work. In those situations, the lintel forms part of the building’s structural system rather than simply being a product selection.
In these cases, a structural engineer’s specification determines the lintel type, length and bearing requirements. Manufacturer minimum bearing is the starting point, but engineer design takes precedence where structural assessment is required.
If you are unsure whether a wall is loadbearing, treat it as structural until confirmed otherwise. Building Regulations approval is typically required for structural alterations, and compliance with both manufacturer guidance and any engineer’s design will be expected.
Before purchasing a lintel for a 900mm opening, confirm:
If in doubt, provide your supplier with the opening width, wall build-up, number of storeys above and whether the work forms part of an alteration or new build. Clear information leads to appropriate guidance.
To confirm minimum bearing and correct lintel length for your specific product, consult:
A 900mm opening does not mean a 900mm lintel. The lintel must extend beyond the opening to meet the manufacturer’s stated minimum end bearing requirements.
In cavity walls, steel cavity lintels are typically required. In solid masonry or internal loadbearing walls, concrete or steel options may be appropriate.
The correct lintel length and type are determined by wall construction, structural loading and manufacturer guidance. Where structural alterations are involved, a structural engineer’s specification takes precedence.
Please note: The information provided on this website is for general guidance only and should not be relied upon as professional advice.
Building methods, material specifications, and regulations can vary depending on location, project design, and site conditions.
Always refer to the latest Building Regulations, manufacturer data sheets, and consult with a qualified structural engineer, surveyor, or building control officer before starting any construction work or making design decisions.
Gilmore Building Supplies accepts no responsibility for loss, damage, or injury resulting from reliance on the information provided.
