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.
Regarding lintels for 1200mm Openings, the lintel you need depends on the wall type and what that wall is carrying. In most UK housing, a 1200mm span is commonly found over standard windows and internal doors. The right lintel is usually one of three categories: a steel cavity lintel for external cavity walls, a solid wall lintel for single-leaf masonry, or a concrete lintel for certain internal loadbearing walls.
One key point people often miss is that a 1200mm opening does not mean you buy a 1200mm lintel. Lintels need solid support at each end, called bearing, so the lintel must extend past the opening on both sides. In practice, you choose lintel length based on the opening width plus the required end bearing, following the manufacturer’s guidance (and Building Control expectations where the work is structural).
Think: opening width + end bearing, not “opening width = lintel length”.
If you can’t clearly confirm bearing on site, or the wall is loadbearing, treat it as a design decision and involve Building Control or a structural engineer.
If the wall is a typical modern cavity wall with brick outer leaf and block inner leaf, a steel cavity lintel designed to support both leaves is generally appropriate. If it is a solid internal block wall, a concrete lintel is often used. The key decision is not the width of the opening alone, but whether the wall is loadbearing, what sits above it, and how the wall is constructed.
A 1200mm opening is not especially large in domestic construction, but it is large enough that you should not treat it casually. The wall type and load path must dictate the lintel selection. If in doubt, particularly where floors or roof structures bear onto the wall above, you must involve a structural engineer or confirm with Building Control.
This guidance applies to standard low-rise domestic construction in the UK, typically houses up to three storeys.
You will most often encounter a 1200mm opening in:
In a modern cavity wall, the opening will usually require a steel cavity lintel that supports both the outer brick leaf and the inner block leaf. In older properties with solid masonry walls, the solution will differ, often involving a solid wall lintel or a more bespoke structural arrangement.
If the opening is in a non-loadbearing partition wall, the lintel requirements are significantly lighter. However, assumptions are dangerous. Many walls that appear minor are in fact supporting floor joists or roof members above.
Think in terms of structure, not product names.
Start with three questions:
Does it carry floor joists, roof structure, masonry above, or a gable? If yes, the lintel must be suitable for structural loading. If it is purely a partition, the demands are different.
Modern UK houses are typically cavity construction. That means you need a lintel that supports both leaves correctly and maintains the cavity. A steel cavity lintel is commonly used here because it can bridge both leaves while maintaining separation. You may also need to consider cavity wall insulation performance in the design.
In contrast, a single-leaf block wall internally may use a concrete lintel designed for solid masonry construction.
Is there just brickwork? Or is there a floor? A trussed roof? A steel beam bearing somewhere above? The load path determines the structural requirement.
At 1200mm, many standard domestic openings fall within common lintel product ranges, but that does not remove the need to assess the situation properly.
As a general principle:
For broader guidance on lintel types and wall construction, see our parent guide: Which Lintel Do I Need?
The better question is: what wall is that window in, and what loads are coming down through it?
A 1200mm opening in one wall can require a completely different lintel to a 1200mm opening in another. The width is only one variable. Load and wall type matter more.
Internal walls often support floor joists. Removing or altering them without structural consideration is a common error.
In cavity construction, using two independent lintels without proper design can create movement issues and differential settlement. Cavity lintels are designed to work as a system.
Every lintel requires adequate end bearing onto solid masonry such as blocks. Reducing this to “make it fit” is not acceptable.
Just because you are replacing an existing 1200mm window does not mean the original lintel was correct or adequate.
Sometimes, but don’t assume. If the opening size is changing, the wall build-up is changing, or you suspect the existing lintel is corroded, cracked, or undersized, treat it as a redesign decision and confirm the approach properly.
You should involve a structural engineer or obtain formal approval where:
Building Control approval is typically required where structural alterations are made. Even in smaller domestic works, forming or altering loadbearing openings falls under Building Regulations.
Once you have identified the correct lintel type, there are practical factors to consider.
A 1200mm opening does not mean you buy a 1200mm lintel. The lintel must extend beyond the opening to achieve proper bearing at each end. Always account for this when ordering.
Cavity lintels must match the wall construction. Measure cavity width and masonry thickness accurately before purchase.
Many modern cavity lintels include insulation to reduce thermal bridging. This is particularly relevant in external walls.
For external walls, galvanised or coated steel lintels are standard. In exposed environments, higher durability may be necessary.
Even at 1200mm spans, steel lintels can be awkward to handle safely. Plan access and storage properly.
You need extra length beyond the opening for proper bearing at each end. The exact allowance depends on the wall and lintel type, so measure carefully and follow the manufacturer’s requirements. If you’re replacing an existing lintel, confirm the actual bearing available rather than guessing from the opening width.
A lintel for a 1200mm opening is not defined by the width alone. The correct choice depends on:
In most modern UK homes, a 1200mm external opening will require a steel cavity lintel designed to support both masonry leaves. Internal loadbearing walls may use concrete lintels or other structural solutions, depending on the situation.
If you are forming or altering an opening in a structural wall, involve a structural engineer and confirm compliance with Building Control. Approach the decision logically, assess the wall type and load path, and choose the lintel that matches the construction, not just the measurement.
Need help choosing the right lintel? Browse our full range of lintels or speak to our team for practical advice before you buy.
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.
