Light Steel Framing (LSF): What It Is, Its Limits, and Where Hot-Rolled Steel Fits

Light steel framing (LSF) builds walls and floors from thin, cold-formed galvanized steel sections. It is used for load-bearing frames in houses and apartments and for infill walls in larger buildings. This guide explains what LSF is made of, how it is built, what it does well, where its limits are, and why an LSF project still needs hot-rolled products such as rebar, beams and hollow sections.

What LSF is made of

According to SCI P402, Light Steel Framing in Residential Construction, published by The Steel Construction Institute, LSF sections are typically rolled from 1.0 to 4.0 mm galvanized strip, most often 1.2 to 2.4 mm, in steel grades S350 to S450 (yield strength 350 to 450 N/mm²), with a zinc coating of 275 g/m² in total over both faces.

C and Z sections are the basis of the system. The same guide gives typical C-section depths of 80 to 150 mm for walls and 150 to 300 mm for floor joists. Members are closely spaced and, according to the same guide, joined with self-drilling screws or bolts; it advises against welding on site unless the zinc coating is first removed locally.

A conventional steel frame, by contrast, uses heavier hot-rolled sections such as IPE beams and hollow-section columns, and a few main members carry the loads. In LSF the walls and floors themselves are load-bearing.

How it is built

• Panelised: wall and floor panels are made in a factory and erected on site. SCI P402 notes that panel construction is faster on site than assembling members one by one ("stick-build").
• Stick-built: cut sections arrive on site and are assembled there.

Either way the building sits on reinforced concrete foundations, and SCI P402 stresses tight foundation tolerances, for example ±5 mm on the level of the wall-frame base over the whole wall line.

Strengths

Drawing on the two SCI sources:

• Speed of construction, which SCI P402 lists among the main benefits.
• Light weight: SCI P402 notes that permanent loads are much lower than for concrete or masonry, and the Light Steel Forum says this reduces foundation requirements and lets residential floors sit on a podium.
• Accuracy and quality: the SCI Light Steel Forum points to a high level of quality control, dimensional accuracy and freedom from shrinkage.
• Cleaner sites: fewer deliveries and less on-site waste.
• Thermal performance: in a "warm frame", most insulation sits outside the studs, keeping the steel inside the envelope and reducing cold bridging.

Limits and points to check

• Fire and sound rely on the lining. Fire resistance comes mainly from plasterboard; SCI P402 gives one 12.5 mm fire-resistant board on each face as achieving 30 minutes. Acoustic performance relies on double-leaf walls, mineral wool and resilient bars.
• Height raises the requirements. According to SCI P402, houses and apartments of two to three storeys need 30 minutes of fire resistance for all structural elements, and taller apartment buildings may need 90 or even 120 minutes. In mixed-use buildings, the light steel frames are supported on a steel or concrete structure for the levels below.
• Coating care. Thin galvanized sections must be protected from damage and moisture in storage and on site.
• Design responsibility. As with any structure, LSF must be designed and approved by a structural engineer under the applicable national building regulations.

Where hot-rolled steel still fits

Ahan Asa does not list cold-formed LSF sections. An LSF project still needs:

1. Rebar for the foundations, such as Aj400 (A3) deformed rebar to INSO 3132.
2. A podium or lower storeys in reinforced concrete or a hot-rolled frame of IPE beams and hollow-section columns. For column choices, see box column or double IPE column.
3. Posts and special members: SCI P402 mentions balconies attached to square hollow section posts supplied with the light steel package. Our square hollow sections (SHS) and rectangular hollow sections (RHS) are made to EN 10219-2 in grade S235JRH; see the hollow section category.

Summary

LSF is a light, fast and accurate way to build houses, apartments and infill walls from thin galvanized cold-formed sections. Fire resistance, acoustics and building height need to be settled with the designer early. Foundations, podiums and some posts still call for rebar, beams and hollow sections; to request a quote for those, contact us.