Lintels Explained: Types, Bearing, Cavity Trays, and the Upside-Down Mistake

A builder fits the lintel over your new window, builds the wall up, plasters it, and moves on. Eighteen months later a crack opens above the frame and damp tracks down inside the wall. The lintel went in upside down, with too little bearing, or with no cavity tray above it. None of that is visible once the wall is closed up, and the fix means taking the wall apart again at a cost that runs into four figures. Knowing what a correct lintel looks like before the bricklayer covers it up turns a five-thousand-pound repair into a five-minute check.

What a lintel is (and what it is not)

A lintel is a horizontal structural member that bridges an opening in a wall, a door or a window, and carries the masonry above it across to the wall on either side. Without one, the bricks and blocks over the opening would have nothing to sit on and would collapse into the gap. The lintel takes that load and transfers it sideways into the solid wall at each end. Every door and window opening in a masonry wall needs one. Building control checks for it at every opening, every time.

A lintel is a standard, off-the-shelf component. You order it by length and load rating from a builders' merchant, it arrives on a pallet, and the bricklayer beds it in. Most domestic openings, a 900mm window, a 2.1m back door, take a catalogue lintel sized from the manufacturer's published load tables. No bespoke fabrication required.

That is where a lintel differs from a steel beam, and the distinction matters because people use the words loosely. A steel beam, an RSJ (rolled steel joist) or universal beam, is an engineered member fabricated to a structural engineer's exact schedule to carry a wall, a floor, or a roof across a wide span. Knocking two rooms into one, or fitting a 3m bifold opening, needs a beam, not a lintel. That work involves bespoke steel, padstones, and detailed calculations, and it is covered separately in the structural steel guide and the steels and lintels task. Roughly, if the opening is a normal door or window in an external wall, you are in lintel territory. If you are removing a chunk of load-bearing wall or going beyond about 1.8m of clear span, you are into beam territory and need an engineer.

The two overlap at the edges. A wide patio-door opening might be carried by a heavy-duty steel lintel rather than a fabricated beam, and your structural engineer decides which. But the principle holds: a lintel is a pre-made part for a standard opening, a beam is a designed member for a structural alteration.

Types of lintel and which one you need

Four lintel types turn up in UK domestic work. Most of what you will see is the steel cavity lintel, but it pays to understand the others so you can read your engineer's drawing and check the right product arrived.

Prestressed concrete lintels are dense reinforced-concrete bars with steel wires cast into the bottom edge. They are cheap, strong in the right orientation, and right for internal partition walls and single-leaf openings. They are not the right choice on their own across a full cavity wall, because a single solid lintel only supports one leaf. Use one to carry the inner blockwork leaf, paired with a separate lintel on the outer brick leaf, or skip it and use a combined steel cavity lintel instead.

Steel cavity lintels are the standard answer for an external opening. These are galvanised pressed-steel sections shaped to sit across both leaves of a cavity wall at once, with a built-in slope that acts as the cavity tray and throws water back out. Catnic, IG, Birtley, and Keystone dominate this market. The product code on your engineer's drawing (for example "IG L1/S 95-100 1800mm") tells you the range, the duty, the cavity width it suits, and the length. Match what arrives on site to that code exactly.

Heavy-duty steel lintels look similar but are made from thicker steel to carry a concentrated load, a floor joist bearing down, a roof, an upper-storey wall, on top of the normal brickwork. Your structural engineer specifies heavy duty when the load above the opening is more than just masonry. Do not let a builder substitute a standard lintel for a heavy-duty one to save a few pounds. The duty rating is on the drawing for a reason.

Thermally broken lintels have an insulating layer built into the steel to stop heat escaping through the metal. A plain steel lintel is a cold bridge: its psi value (the heat-loss figure for that junction) is around 0.25 W/mK, while a thermally broken one such as the IG or Keystone Hi-therm+ gets that down to roughly 0.03 to 0.06 W/mK. Part L 2021 made cold bridging harder to ignore in the energy calculation, so on a new extension your SAP assessor may need the better figure to make the numbers pass. The plain-English version: if your extension is being assessed for Part L (it will be), ask whether the lintel needs to be a thermal one. The answer depends on the rest of the build-up, and the manufacturers provide free psi-value calculations for your specific wall.

Timber and stone lintels exist but are heritage or internal-only. Under BS EN 845, the harmonised product standard for masonry lintels (Part 2 covers lintels specifically), timber is permitted only where it is protected from weather and is not supporting masonry. For a standard extension you will not be using one.

Who specifies it, and duty ratings

For a brand-new opening in an extension, your structural engineer specifies the lintel as part of the structural drawings. They calculate the load above the opening (the weight of brickwork, plus any floor or roof bearing on it) and pick a product and length from the manufacturer's load tables to suit. The drawing gives you a code to order against.

For a like-for-like replacement of a failed lintel in an existing wall, off-the-shelf load tables from Catnic or IG are often enough on their own, because the load is already known and the span is unchanged. But the moment you enlarge an opening or knock through, you need an engineer and you need building control. A builder telling you "there was already an opening there, so we do not need building control" is wrong. Enlarging any structural opening is notifiable work.

Duty rating is the one piece of jargon worth nailing down. Standard duty means the only thing above the opening is normal masonry, the brickwork of the wall itself. Heavy duty means there is an additional concentrated load: a floor, a roof, or the wall of the storey above sitting on the lintel. Your engineer's drawing will say which. The cost difference is real but small against the cost of a sagging lintel, so if the drawing says heavy duty, that is what gets ordered.

Sizing and bearing: the 150mm rule

A lintel has to be long enough to span the opening and still rest properly on solid wall at each end. That overlap onto the wall is the bearing, and it is the single most important number in lintel installation.

The working rule is 150mm of bearing at each end. So a lintel for a 1200mm opening needs to be 1200 plus 150 plus 150, which is 1500mm long. The same minimum applies to steel beams: 150mm at each end onto solid masonry.

The regulations are slightly more lenient than that for the smallest openings. Approved Document A (Section 2C24) sets the statutory minimum at 100mm for openings up to 1200mm wide, rising to 150mm for anything wider. But manufacturers and the NHBC recommend 150mm across the board as the practical standard, because it covers every case and removes the chance of a borderline bearing failing inspection. Use 150mm and you are never wrong.

Two things wreck a bearing even when the length is right. First, the lintel must sit on a full masonry unit, a whole brick or block, not a cut piece. A lintel bearing on a cut block has nothing solid under the load path. Second, the bed must be a full bed of mortar, level, with no soft or non-durable packing under the ends. Bricklayers in a hurry sometimes pack a low end with a scrap of timber or a folded DPC offcut. That is a defect and an inspector will fail it.

Installation: orientation, propping, and the upside-down mistake

The most common lintel mistake on UK sites is not the wrong product. It is the right product fitted upside down.

Both steel and prestressed concrete lintels have a correct way up, and both are usually marked. Steel cavity lintels carry an arrow or a "this way up" label, and many concrete lintels are stamped "TOP" on one face. The marking is not cosmetic. A prestressed concrete lintel has its reinforcement wires concentrated near the bottom edge, where the tension is when the lintel bends under load. Flip it over and the steel is now in the compression zone where it does nothing useful, and the lintel can fail. A steel lintel fitted upside down loses its load capacity and its built-in cavity tray stops working, so water runs the wrong way.

There is one wrinkle that causes genuine confusion. Cheap "economy" or "reversible" concrete lintels have no pre-camber and no TOP mark, and those genuinely can go either way up. But any lintel that is marked TOP, or shows a slight upward hog (pre-camber) along its length, must be fitted that face up. The builder who tells you the TOP stamp "just shows which way the bow goes for getting a finish" is talking nonsense to avoid redoing the job. If you can see TOP on the underside of an installed lintel, it is wrong.

The other half of installation is propping. Before any brickwork is cut out to form or enlarge an opening, the wall above has to be supported on Acrow props and needles (steel bars threaded through the wall) or Strongboys (prop-mounted brackets that take the brickwork). For openings over 1.2m the propping is not optional, and it stays in place until the mortar under the new lintel has cured, at least 24 hours for a small opening and longer for a wide one. Knocking out the propping too early, while the bed is still green, is how lintels end up canted (tilted) with cracks appearing upstairs.

When the bricklayer builds the wall back up over a steel cavity lintel, both leaves have to rise together. The inner and outer leaves must never differ in height by more than 225mm, because a steel lintel loaded unevenly on one flange will twist. Watch for one leaf racing ahead of the other.

Cavity tray, weepholes, and the thermal break

Above an external lintel, water that gets into the cavity has to be stopped from reaching the inner leaf and steered back outside. That is the job of the cavity tray and the weepholes, and missing or botched cavity trays are the second most common lintel failure after upside-down fitting.

A combined steel cavity lintel has the tray built into its sloped top face, so on a standard wall the lintel is the tray. Where a separate tray is needed (over concrete lintels, in severe exposure zones, and always in Scotland and Northern Ireland), a preformed cavity tray is laid over the lintel with its upstand turned into the inner leaf and stop ends at each end to dam the water. The single biggest defect here is mortar dropping onto the tray and bridging it: that lets water track straight across to the inside instead of draining out.

For the tray to work, the water needs an exit. That is what weepholes do, open vertical joints (or plastic weep vents pushed into them) in the outer leaf directly above the tray. Count at least two per opening, spaced no wider than 450mm apart. A 1.2m window needs a minimum of two, one near each end. If you cannot see weepholes above a finished opening, the tray has nowhere to drain and water will pool on it.

The thermal break is the third detail. A bare steel lintel conducts heat straight out of the building, and that cold bridge can account for a meaningful slice of the wall's heat loss. On a new extension assessed under Part L, the fix is either a thermally broken lintel or insulation packed correctly around the lintel and an insulated cavity closer at the reveal, so the wall insulation runs continuous past the opening with no gap. Whether you need the upgraded lintel or the standard one plus careful insulation depends on the SAP figures for your specific build, which is your SAP assessor's call, not your builder's.

Cost and where to buy

Lintels are cheap as materials go. The cost that bites is the labour and making-good around them, not the part itself.

A prestressed concrete lintel runs from £10 – £55 depending on length, with a short 600mm internal lintel at the bottom of that range and a 2.4m one at the top. A standard-duty steel cavity lintel sits higher, at £59 – £115 for common window and door lengths. Heavy-duty and thermally broken versions cost more again. Those are material-only prices and they need verifying against a live quote, because retail lintel prices move.

The bigger number is a full lintel replacement as a job: digging out a failed lintel, propping, fitting the new one, and making good. For a standard window or door opening on a brick-and-block cavity wall, that lands at roughly 650 to 1,000 pounds all in, and an upper-floor opening adds 400 to 1,200 pounds in scaffolding. Period properties, stone walls, or awkward access can push a single opening past 2,500 pounds. Get your structural calculations first, then get three builder quotes against the identical specification, because the scope (does it include propping, padstones, making good, decoration?) is where quotes diverge wildly. One homeowner quoted 3,500 pounds for a beam that the market priced under 1,000 found that out the hard way.

Off-the-shelf lintels come from any builders' merchant: Travis Perkins, Jewson, Buildbase, and the trade counters, plus Wickes and Selco for retail quantities. Catnic and IG are the most widely stocked steel brands; Naylor, Supreme, and Stressline cover concrete. Merchants sell concrete lintels off the shelf in 150mm length increments, and steel lintels are cut to standard lengths too, so order the next length up from your calculated minimum rather than asking for a bespoke cut.

External resource

IG Lintels: Lintel Selector and Psi Calculator

Manufacturer load tables and a free psi-value calculator for your specific wall build-up. Useful for checking your engineer's spec or working out whether a thermal lintel is needed for Part L.

iglintels.com

External resource

Catnic: Lintels and Part L Thermal Bridging Guide

Plain explanation of how the Part L 2021 changes affect lintel choice, with the psi values for standard versus thermally broken steel lintels.

catnic.com

Common mistakes

Fitting the lintel upside down. Covered above, and worth repeating because it is the number-one site error. Check for the TOP mark or orientation arrow before the wall is built up.

Too little bearing, or bearing on a cut block. A lintel needs 150mm of solid bearing each end on a full masonry unit. A short bearing or a bearing on a cut block has no proper load path and will eventually move.

Using a concrete lintel where a cavity lintel is needed. A single prestressed concrete lintel only carries one leaf of a cavity wall. Use a combined steel cavity lintel, or two separate lintels, never one solid-wall concrete lintel as the only support across a full cavity construction. A skinny concrete lintel spanning a cavity wall is a classic forum cautionary tale.

Substituting a standard lintel for a specified heavy-duty one. If your engineer's drawing says heavy duty, the load above the opening is more than masonry. A standard lintel in its place will deflect under that load. Do not let the saving tempt anyone.

Missing or bridged cavity tray. No tray, a tray with no stop ends, or a tray buried under mortar droppings all send water inward. Inspect it before the next course goes on, because it is invisible afterwards.

Too few weepholes, or weepholes below ground level. At least two per opening, no more than 450mm apart, and never below the finished external ground level where they will silt up and block.

Chasing too close to the bearing. Cutting a deep vertical chase for a cable or pipe near the lintel ends weakens exactly the bit of wall taking the load. Keep vertical chases shallow: no deeper than 1/3 of the wall thickness, which is about 33mm on a 100mm leaf.

No lintel at all. It happens, usually on an older property where original window frames were used as temporary support during the build and a later window swap removed the support without adding a lintel. If you find cracks tracking diagonally from the corners of a window or door, that is the symptom. Get a structural engineer to look before it spreads.

Where you'll need this

Lintels appear wherever you form, enlarge, or replace an opening, which is to say across the structure stage of any extension or renovation, not just one project type.

• Steels and lintels - choosing between an off-the-shelf lintel and a fabricated steel beam, and managing the steelwork on site
• Windows and doors - every new window and door opening needs a correctly sized and bedded lintel above it before the frame goes in
• What building control inspects - the inspector checks the lintel, bearing, cavity tray, and weepholes over every opening at the structural inspection