Box Lintels

What they are

Box lintels are closed-section steel box lintels engineered to span internal door and window openings—typically in 100 mm single-leaf blockwork or loadbearing partitions with a masonry facing. The hollow, closed profile delivers high stiffness with low deflection in a compact, easy-to-handle unit.

Why they’re used

Because the section is fully enclosed, box lintels resist bending and torsion better than simple angles, helping finishes stay crack-free. They’re intended for non-exposed, internal locations where you don’t need built-in insulation or weathering features. In these settings they provide reliable support for masonry self-weight, finishes and modest nearby loads, while keeping installation quick and weight manageable. In short, steel box lintels give you stiffness, tidy levels and dependable performance for internal openings.

Where they’re used

You’ll find box lintels over internal doors and windows in 100 mm blockwork, along corridors and stair cores, and in residential or commercial partitions where angle lintels might deflect too much. They’re not designed for external cavity walls or wet/exposed environments.

Choosing and installing

Select by confirmed clear span, wall thickness/strength and the actual load case from the manufacturer’s tables; internal work often needs tighter deflection control (around L/325), which the closed box helps achieve. Provide at least 150 mm end bearings on sound masonry, lay on a full, level mortar bed with the correct orientation (compression face up), and prop temporarily if required until the wall cures. Don’t notch, drill or chase the section, and specify a suitable coating if used in humid rooms.

Box lintels vs other types

• Steel box lintels: Closed section for high stiffness/low deflection on internal openings. Not designed for external exposure or thermal performance.
• Cavity wall lintels: For external cavity walls; shaped and insulated to manage drainage, stop water tracking, and reduce thermal bridging—use these outside where weathering and U-values matter.
• Solid wall lintels: For external solid masonry (no cavity). Heavier-duty sections and weathering details handle full-wall loads and exposure.
• Single-leaf lintels: For external single-skin masonry (typically 100 mm). Purpose-shaped with drips/weathering; a better choice than internal box sections where the outer leaf is exposed.
• Concrete lintels: Precast, durable, economical, and inherently fire-resistant. Good for simple spans in solid or cavity constructions, but heavier and with less thermal performance than insulated steel cavity lintels.

Compliance notes

• Select and verify capacities from the manufacturer’s load tables for the exact model, span, wall type and load case.
• Provide minimum recommended end bearings (commonly ≥150 mm) on sound masonry and install on a full mortar bed.
• Follow factory production control and product marking requirements (e.g., EN 1090), and lintel performance standards (e.g., EN 845-2).
• Maintain serviceability limits (deflection) appropriate to internal finishes; add temporary propping where specified.

FAQs

Can steel box lintels go in external walls?

Generally no. They’re uninsulated and not detailed for exposure to moisture. Use a cavity (insulated) lintel, or concrete lintel externally.

What span can a box lintel achieve?

Depends on the model and loads; common internal door/window spans are well covered. Always pick from the load table—not guesswork.

Do I need a DPC over an internal lintel?

Not for weathering, but a slip membrane or tape might be used for finish compatibility—follow the product guidance.