Lintel Sizing Guide for Builders

Choosing the Right Lintel

Every opening in a loadbearing wall — doors, windows, serving hatches — needs a lintel to carry the weight above it. Pick the wrong lintel and you get cracking, sagging, or worse. Pick the right one and it disappears into the wall, doing its job quietly for the life of the building.

This guide covers domestic lintels for standard cavity wall and solid wall construction. For anything unusual — wide spans, heavy point loads, or commercial buildings — you need a structural engineer's calculation.

How Lintels Work

A lintel is a beam that spans the opening and transfers the load from the masonry above to the walls either side. The load includes:

• Masonry weight — the bricks or blocks above the opening
• Roof loads — if the wall supports the roof structure
• Floor loads — if the wall supports floor joists
• Wind loads — lateral pressure on the wall

For most domestic openings up to about 2.4m wide, standard manufacturer lintels (Catnic, Birtley, IG, Keystone) are designed to handle typical loading without individual structural calculation. Above 2.4m, or with unusual loads, get it engineered.

Types of Lintel

Steel Lintels (Most Common)

Galvanised pressed steel lintels are the standard choice for domestic construction. They're lightweight, quick to install, and available off-the-shelf for almost every wall type.

| Lintel Type | Wall Construction | Description | |---|---|---| | Standard cavity wall | Cavity wall (100mm outer leaf + cavity + 100mm inner leaf) | Supports both leaves with a built-in cavity tray | | Wide inner leaf | Cavity wall with 140mm block inner leaf | As above, wider profile | | Timber frame | Timber frame with brick outer leaf | Supports outer leaf only, inner leaf carried by timber frame | | Single leaf | Solid wall or internal wall | Supports one skin of masonry | | Heavy duty | Loadbearing with floor/roof loads | Deeper section for higher loads |

Concrete Lintels

Pre-stressed concrete lintels are heavier but cheaper. They're commonly used for internal openings and where the lintel will be rendered or plastered.

| Use | Typical Size | |---|---| | Internal doorway (100mm block wall) | 100 × 65mm | | Internal doorway (140mm block wall) | 140 × 65mm | | External (if rendered/faced) | 100 × 140mm or 100 × 215mm |

Downsides: Much heavier than steel (a 1.2m concrete lintel weighs 20–30kg vs 5–8kg for steel), and they form a cold bridge if used in external walls without insulation.

Stone Lintels

Used in conservation areas or to match existing stonework. Structural capacity is limited — they're often decorative with a steel lintel behind doing the real work.

Sizing by Opening Width

For standard cavity wall construction with typical domestic loading (masonry above, no unusual point loads), here are the common lintel sizes:

Standard Duty (No Floor or Roof Load Directly Above)

| Opening Width | Lintel Length | Minimum Bearing Each End | |---|---|---| | Up to 600mm | 900mm | 150mm | | 601–900mm | 1,200mm | 150mm | | 901–1,200mm | 1,500mm | 150mm | | 1,201–1,500mm | 1,800mm | 150mm | | 1,501–1,800mm | 2,100mm | 150mm | | 1,801–2,100mm | 2,400mm | 150mm | | 2,101–2,400mm | 2,700mm | 150mm |

Lintel length = Opening width + (2 × Bearing)

The minimum bearing is 150mm each end — this is the length of lintel that sits on the wall either side of the opening. Some manufacturers specify 100mm for smaller openings, but 150mm is the safe default.

Heavy Duty (Floor Joists or Roof Loading Above)

When the lintel carries floor or roof loads, you need a heavier section. Most manufacturers offer their lintels in multiple duty ratings:

| Duty | Typical Use | |---|---| | Light/standard | Internal walls, windows with no load above | | Medium | Windows under eaves, doors with limited masonry above | | Heavy | Openings supporting floor joists, openings under the roof | | Extra heavy | Wide openings with combined floor + roof loads |

The manufacturer's load/span tables tell you which duty you need based on the opening width and the loads above. Most merchants stock standard and heavy duty off the shelf. Extra heavy is usually a special order.

Use the Lintel Sizing Guide to look up the right lintel type and size based on your opening width and wall construction.

Bearing and Support

Minimum Bearing

The lintel must bear on solid masonry (or a padstone) for at least 150mm at each end. This means:

• The blocks/bricks either side of the opening must be in good condition
• The bed joint under the lintel bearing must be a full mortar bed
• If the masonry is weak or damaged, fit a concrete padstone to spread the load

Padstones

A padstone is a concrete block or engineering brick placed under the lintel bearing to spread the point load over a wider area. Use padstones when:

• The wall is built from lightweight blocks (below 7 N/mm² compressive strength)
• The lintel carries heavy loads
• The existing masonry is in poor condition

A standard padstone is 215 × 215 × 100mm in dense concrete or Class A engineering brick.

Installation Tips

Propping

Lintels over 1,200mm span should be temporarily propped at mid-span until the masonry above has cured (at least 14 days in normal conditions). This prevents the lintel deflecting under the wet mortar load.

DPC and Cavity Trays

Steel cavity wall lintels have a built-in cavity tray that directs any moisture in the cavity out through weep holes. Make sure:

• Weep holes are formed at every lintel (at least 2, one at each end). Use proprietary weep vents or leave perpend joints open.
• The DPC below the inner leaf bearing laps correctly to prevent moisture bridging.
• Stop ends are fitted at each end of the cavity tray to prevent water running off the end into the wall.

Cutting Lintels

Never cut a steel lintel to length. It's been designed as a structural member at its full length. Cutting it removes material from the bearing area and invalidates the structural performance. Order the correct length from the manufacturer.

Concrete lintels should also not be cut — the prestressing tendons are positioned to work at the full length.

Building Control Considerations

Building control will want to see:

• The lintel type and manufacturer — have the product data sheet on site
• Correct bearing length — 150mm minimum, measured and visible before plastering
• Propping if required by the manufacturer
• Cavity tray and weep holes in external walls
• Padstones if the manufacturer specifies them for the wall type

For openings over about 2.4m, or where there's an unusual loading condition (point loads from steel beams, close-spaced openings, thin walls), building control will typically ask for a structural engineer's calculation. Don't assume a standard lintel will work for large openings — it might, but you need the engineer to confirm it.

If you're building the masonry around the lintel, the Masonry Calculator can help with brick, block, and mortar quantities for the wall.

Common Manufacturer Ranges

| Manufacturer | Common Range | Notes | |---|---|---| | Catnic | CN, CG, CX series | Widely stocked, colour-coded by duty | | Birtley | CB, SB series | Good range, competitive pricing | | IG | Standard, heavy duty | Common in the Midlands and South | | Keystone | Standard, hi-therm | Hi-therm lintels for Part L compliance |

Hi-Therm / Thermal Lintels

Standard steel lintels create a thermal bridge — they conduct heat through the wall faster than the surrounding insulation. Thermally broken lintels (like Keystone Hi-Therm or Catnic Thermally Broken) have an insulated section that reduces heat loss.

For new builds and extensions, Building Regulations Part L increasingly requires thermally broken lintels to meet U-value targets. The cost premium is modest (£10–£20 more per lintel) and they're worth specifying as standard to avoid Part L issues.

Common Mistakes

• Wrong wall type. A cavity wall lintel in a solid wall (or vice versa) doesn't work. Check the wall construction before ordering.
• Insufficient bearing. 100mm might technically work for small openings, but 150mm is the industry standard and what building control expects.
• Missing weep holes. Without weep holes, moisture builds up in the cavity above the lintel and saturates the inner leaf. This causes damp patches inside.
• Forgetting stop ends. Water running off the end of the cavity tray saturates the wall at the lintel ends — a classic damp spot.
• Stacking openings without engineering. Two windows directly above each other, or a door below a window, concentrates loads. Get these checked by an engineer.
• Using the wrong duty. A standard-duty lintel under a gable wall carrying the full roof load will deflect. Check the manufacturer's tables.

Quick Summary

1. Measure the opening width
2. Add 300mm for bearings (150mm each end)
3. Choose the wall type (cavity, solid, timber frame)
4. Check the duty rating based on loads above
5. Specify thermally broken lintels for external walls
6. Ensure weep holes, cavity tray stop ends, and correct DPC laps

The Lintel Sizing Guide gives you recommended lintel types and sizes based on your opening width and wall construction — a quick reference for quoting and ordering.