Lintels are one of those things that get specified on autopilot, and that’s exactly where problems start. I’ve been on jobs where a builder has grabbed a standard catnic off the shelf, fitted it without checking the load above, and six months later the client is ringing up about cracking over the window head. Lintel specification is not complicated, but it does require you to think for a few minutes before you order.
This guide covers what a lintel actually does, which material suits which situation, what the standard UK sizes cover, and the mistakes that end up causing Building Control grief or structural movement down the line.
What a lintel actually does
A lintel is a structural member that spans an opening, whether that’s a window, a door, or a knocked-through internal wall, and transfers the load from the structure above down to the masonry or frame on either side. Without one, the brickwork or blockwork over the opening has nothing to sit on and will crack, drop, or collapse depending on how much load is above it.
The key word there is load. A lintel over a small window in a single-storey extension carries a very different load to one spanning a wide bi-fold door opening in a two-storey house with a bedroom above. This is where a lot of specifiers go wrong: they treat every lintel as the same problem with the same answer.
Steel lintels: the go-to for most modern masonry work
For cavity wall construction, pressed steel lintels are the standard choice on the vast majority of UK domestic jobs. They come pre-formed to match different cavity widths, usually 50 mm, 75 mm, 90 mm and 100 mm cavities, with a leg that supports the outer leaf and a separate inner support for the inner leaf or a combined profile. Catnic, IG Lintels and Birtley are the main UK manufacturers you’ll see on merchants’ shelves.
Steel lintels are light, easy to handle on site, and come in pre-galvanised form so they cope with the moisture exposure in a cavity. The thing to check is the load table. Every reputable UK manufacturer provides load tables showing the safe working load for each lintel profile at a given span. These are calculated to BS EN 845-2, and if Building Control ask you how you specified the lintel, pointing to a manufacturer’s load table for the actual span and load situation is your first line of defence.
For cavity wall construction, also think about the thermal bridge. A steel lintel that runs across the full cavity width creates a cold bridge at the head of the opening. Most modern proprietary lintels have a thin polyurethane thermal break built into the profile. Specify one with it. If you’re working to Part L of Building Regulations, the inspector will look at thermal bridging at junctions, and a lintel without a break is an easy fail flag.
Concrete lintels: solid block and single-leaf walls
Pre-stressed concrete lintels are the right call for solid masonry work, dense blockwork internal walls, and situations where you need a flat soffit without the channel profile of a steel lintel. They’re heavier, which matters on a scaffold or in a tight space, but they’re robust and there’s no corrosion concern.
You’ll see them specified in internal partition walls where the opening is non-structural, and in garages or outbuildings where the masonry is solid rather than cavity. Standard pre-stressed concrete lintels from the likes of Stressline come in depths of 65 mm up to 215 mm, with the deeper sections carrying higher loads over longer spans.
One mistake I see regularly is people using a 65 mm concrete lintel over a 2.4 m garage door opening. That’s underspecified. At that span you need to be looking at the load tables carefully, and for wider openings in load-bearing positions, an engineer’s input is worth getting in writing. If you’re building anything where the span exceeds 1.8 m and there’s significant load above, I’d say it’s worth calling in a structural engineer to sign off the specification. My article on when Building Regulations approval is needed on domestic sites covers the broader picture of when you need professional sign-off.
Timber lintels: timber frame and specific heritage situations
In timber frame construction, the lintel is usually structural timber, either solid C24 grade timber or an engineered product like an LVL (laminated veneer lumber) or glulam beam. The frame manufacturer will often supply these pre-cut and pre-specified, but if you’re building a timber frame panel system on site, you need to make sure the header beam above each opening is sized correctly for the span and load.
Timber lintels also turn up in older properties where you’re doing like-for-like repair or conservation work. Some local planning authorities, particularly in conservation areas or on listed buildings, will want materials matched to what’s there. If you’re working in that context, get advice from the local planning authority before specifying something different.
Standard UK sizes and what they actually cover
For standard domestic openings, most steel lintel ranges cover spans from 600 mm up to 4800 mm. The span is measured as the clear opening plus the minimum bearing at each end, which is typically 150 mm per side for spans up to 1.2 m and 200 mm for anything wider. That bearing is non-negotiable; it’s where the load transfers into the masonry, and skimping it is how you get settlement cracking at the reveals.
In practice, a 900 mm door opening needs a lintel of at least 1200 mm to 1300 mm to get proper bearing. I’ve seen jobs where someone cut a lintel just a touch over the opening width and it had barely 50 mm of bearing each side. That’s a problem waiting to happen.
For anything involving a wide span, a structural opening through a load-bearing wall, or a situation where you’re removing a chimney breast, get a structural engineer involved. The Institution of Structural Engineers has a directory of chartered engineers if you need to find one for a specific job.
Common lintel specification mistakes
The mistakes that cause cracking or Building Control sign-off problems tend to fall into a small number of categories. Insufficient bearing is the most common. Choosing a lintel for the opening size without accounting for the load above is the second. Using a steel lintel without a thermal break in a Part L-assessed build is the third.
Beyond that, watch out for specifying a lintel that’s the wrong profile for the cavity width. A lintel designed for a 50 mm cavity in a wall with a 100 mm cavity means the outer leaf has no support, or the lintel is sitting in the wrong position. This sounds basic, but it goes wrong more than it should.
If you’re handling the business side of managing multiple jobs and specifications at once, some builders I know have found digital tools helpful for keeping documentation in order. One crew I spoke to mentioned they found their website useful for keeping client-facing project information organised. Staying on top of specification records matters when Building Control or a warranty provider asks questions later.
Getting Building Control sign-off on lintels
Building Control inspectors look at lintels during the frame or superstructure inspection. They want to see that the lintel is correctly positioned, has adequate bearing, is the right product for the application, and that you can demonstrate the load has been considered. A manufacturer’s load table reference for the specific product and span is usually sufficient for standard domestic work. For anything non-standard, a structural engineer’s calculation note is what you need in your site file.
If you’re working on a job where the blockwork specification involves different block strengths in different parts of the build, also cross-check that the masonry either side of the opening has the compressive strength to handle the point load from the lintel bearing. A high load transferred into a low-strength block can cause spalling or crushing at the bearing point.
Lintel specification is one of those areas where getting it right costs almost nothing extra in time or money. Getting it wrong costs considerably more when the cracking starts.