What Size Lintel Do I Need?

What Determines the Size of a Lintel

Four things drive every lintel selection, whether it is done by a structural engineer or by the manufacturer’s technical team working from their load tables:

• Clear span.The clear opening between the masonry supports. The lintel supplied will always be longer than the span itself, because the specifier allows a bearing at each end when selecting its length. For background, the product standard for prefabricated lintels, BS EN 845-2:2013+A1:2016, is scoped to straight lintels over clear openings up to 4.5m — one of the reasons larger or unusual openings move out of proprietary tables and into engineer-designed territory altogether.
• The load above.Everything the lintel will carry: the masonry over the opening, any floor joists bearing on the wall, and roof loads such as trusses or attic rooms. Manufacturers declare capacity as a total uniformly distributed load (UDL) in kilonewtons. Concentrated point loads — for example a beam or trimmer bearing directly over the opening — sit outside the standard table assumptions and need specialist assessment.
• Wall construction.Cavity, solid, single-leaf, timber-frame and internal partition walls each use different lintel profiles with their own load tables. For cavity walls the cavity width, the leaf thicknesses and how the load splits between the inner and outer leaf all change the answer — IG’s tables, for example, are published for load ratios from 3:1 to 19:1.
• Bearing.Every lintel must sit on a minimum end bearing at each end, and that minimum is part of the specification — it comes from the manufacturer’s load tables or the engineer’s design for the exact product, not from a rule of thumb on site. The lintel is fully bedded on mortar, and the exact minimum bearing is confirmed by the specifier for the chosen product rather than assumed on site. The wall under each bearing must also take the load without crushing — Planning Portal guidance notes that padstones may be needed to spread it, another point the engineer checks.

How the Selection Process Actually Works

Here is the process the professional follows — note that every step produces numbers specific to your wall, which is exactly why this page does not print a size:

• 1. Confirm what the wall is doing.A structural engineer or surveyor establishes whether the wall is load-bearing and exactly what bears on it — roof, walls above, floor joists.
• 2. Measure the structural opening.The specifier works from the clear span and then allows for the required bearing at each end when selecting the lintel length — which is why the lintel supplied is always longer than the opening it crosses.
• 3. Assess the loads.Dead loads (masonry, floor and roof self-weight) and imposed loads are calculated in kilonewtons. IG’s own guidance says it plainly: “If you are not skilled in assessing loads please contact IG’s Technical Team.”
• 4. Read the safe-working-load table. The wall build-up is matched to a product family, then the declared capacity at that span, cavity width and load ratio must exceed the applied load. Both Catnic and IG run free specification services that do this against their own tested tables.
• 5. Refer anything non-standard to a structural engineer. Point loads, unusual spans, weak or damaged bearing masonry, or removal of a load-bearing wall all require a bespoke design with calculations submitted to Building Control.

Why You Cannot Just Use a Rule of Thumb

Two openings of identical width can need completely different lintels. A window opening in a gable wall may carry little more than the masonry above it; the same-width opening in the neighbouring wall might carry the ends of every first-floor joist plus part of the roof. Copying next door, reusing “what worked last time” or asking the merchant to guess cannot account for loads that nobody has measured.

The stakes are structural: an undersized lintel deflects, cracks the masonry above and can lead to progressive collapse of the wall over the opening. And the consequences are also legal and financial — work done without Building Control approval has no completion certificate, which routinely surfaces as a problem when you sell or remortgage, and the local authority can require unauthorised structural work to be opened up, redone or removed.

Who Must Do This

• A structural engineerdetermines whether the wall is load-bearing, calculates the loads and either specifies a proprietary lintel or designs a bespoke solution. Where a load-bearing wall is being removed or altered, Planning Portal guidance requires the engineer’s structural calculations to be provided for building control approval.
• The lintel manufacturer’s technical team(Catnic, IG and others) selects standard products from their published safe-working-load tables, which are established by testing to BS EN 845-2. This is the correct route for standard lintels in standard situations — and it still needs Building Control.
• Building Controlchecks the proposal against the Building Regulations — the supporting guidance is Approved Document A: Structure (2004 edition incorporating 2004, 2010 and 2013 amendments) — and inspects the work before it is made permanent.