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Every SFS wall is either infill or loadbearing. That one distinction changes the head detail, the stud selection, the engineering requirements, and the fire performance of the wall. Yet it's often missed in specifications, which leads to costly site problems. This guide explains what infill and loadbearing actually mean, how they differ, and how to specify correctly.

What is infill SFS?

Infill SFS is a non-loadbearing steel frame that fills the gap between the primary structure — usually the floor slab or steel frame of a building — and the external cladding or internal finish. The infill SFS does not carry any load from the floor above or the roof; it merely supports its own weight, the cladding or boards fixed to it, and any wind loads acting on the wall.

Infill SFS is the most common type of SFS construction. It's used in:

• External walls of mid-rise residential and commercial buildings (filling between the floor slabs and steel frame)
• Internal partitions in commercial and industrial buildings
• Shaftwall construction
• Most drylining applications where a stud wall is built between existing structural elements

Key characteristics of infill SFS:

• No structural load from above — the load above is carried by the primary structure, not by the SFS
• Head track is slotted — to allow the primary structure to deflect under load without crushing the SFS wall
• Deflection allowance at head — typically 20–30mm, depending on the anticipated slab deflection
• Base track is fixed — bolted to the floor below, using anchors or shot-fired fixings
• Lateral restraint only — the SFS resists wind loads acting on the wall, and transfers them to the primary structure

What is loadbearing SFS?

Loadbearing SFS is a steel frame that carries structural loads from above — typically from the floor above, or from the roof, or from other walls. The SFS is part of the structural load path, not just an infill between structure.

Loadbearing SFS is used in:

• Multi-storey residential buildings where the walls carry floor loads
• Some industrial and commercial buildings where the SFS is the primary structure
• Roof-supporting walls
• Any wall that has structural loads applied to it from above

Key characteristics of loadbearing SFS:

• Structural load from above — the SFS is designed to carry that load safely
• Head track is plain — no deflection allowance, because the SFS is part of the load path and can't move independently
• Base track is fixed and designed for load transfer — the load from above is transferred through the SFS to the structure below
• Structural engineering required — the wall must be designed by a qualified engineer for the specific loads
• Larger, heavier studs — typically 100mm–300mm, with 1.5mm or 2.0mm gauge
• Fire performance includes loadbearing capacity (R) — tested to BS EN 1365-1 or BS 476 Part 21, not just BS EN 1364-1

The key difference — the head detail

The single most important difference between infill and loadbearing SFS is the head detail:

Infill — slotted head track

The head track in an infill SFS wall is slotted (also called a deflection head or movement head). The slots allow the track to slide vertically relative to the slab above, absorbing the deflection of the primary structure.

Why does this matter? Primary structures deflect under load — a concrete slab might deflect 20mm under its design load. If the SFS wall were rigidly connected to the top and bottom slabs, that deflection would either push the wall down (cracking boards) or pull the top of the wall away (failing the connection). The slotted head track prevents this by allowing movement.

Standard deflection allowance is 20mm, but can be 30mm or more depending on the project. The allowance is specified by the structural engineer.

Loadbearing — plain head track

The head track in a loadbearing SFS wall is plain (not slotted). The wall is part of the load path, so it must be rigidly connected to the structure above. Load is transferred through the wall to the structure below, and no relative movement is allowed.

This means:

• The wall doesn't allow for primary structure deflection — the wall is designed to accommodate the deflection through its own structural behaviour
• Any movement in the primary structure is transferred through the wall, not absorbed by it
• The wall must be designed to withstand both the applied loads and any differential movement

Stud selection for infill vs loadbearing

Stud selection follows a different logic for each type:

Infill stud selection

• Wind loading is the primary load — the SFS must resist wind pressure acting on the external wall
• Height is a key constraint — taller walls need wider studs
• Deflection limit — the wall must not deflect too much under wind load (typically L/360 or L/500)
• Stud width typically 70mm–150mm depending on height and wind exposure
• Gauge typically 1.2mm for most heights, 1.5mm or 2.0mm for taller walls

Loadbearing stud selection

• Axial load from above is the primary load — the wall must carry the floor or roof above safely
• Bending from wind or eccentricity is also considered
• Buckling is a key constraint — tall, thin studs can buckle under axial load
• Stud width typically 100mm–300mm depending on loads
• Gauge typically 1.5mm or 2.0mm, sometimes heavier
• Structural engineering required — the design must be carried out by a qualified engineer

Fire performance

Fire testing is different for infill and loadbearing SFS:

Infill — BS EN 1364-1

Infill SFS walls are tested to BS EN 1364-1 (or BS 476 Part 22 for older systems). These tests measure integrity (E) and insulation (I) — the wall must not allow flames or hot gases through, and must limit the temperature rise on the unexposed side.

Loadbearing — BS EN 1365-1

Loadbearing SFS walls are tested to BS EN 1365-1 (or BS 476 Part 21 for older systems). These tests measure integrity (E), insulation (I), AND loadbearing capacity (R) — the wall must maintain its structural load for the duration of the fire test.

This means a loadbearing wall's fire rating is more demanding than an equivalent infill wall. A wall that achieves 60 minutes as an infill wall might only achieve 30 minutes as a loadbearing wall, because of the additional structural requirements during the fire.

Engineering requirements

Infill — no structural engineering required

Infill SFS walls are typically designed by the SFS contractor using manufacturer span tables. No structural engineering input is required as long as the design is within the tabulated range. This makes infill SFS fast and cost-effective.

Loadbearing — structural engineering required

Loadbearing SFS walls must be designed by a qualified structural engineer. The engineer will:

• Calculate the axial loads on the wall (from floors, roof, and any other loads above)
• Calculate bending moments from wind and eccentric loads
• Check buckling capacity of studs under axial load
• Design the connection to the structure above and below
• Verify fire performance (structural adequacy during fire)
• Provide a design statement and calculations for Building Control

This is a significant additional cost and time requirement, but it's essential for safety. A loadbearing wall that fails under load can bring down the building.

What happens on site if you get it wrong

Using a plain head track on an infill wall

• Boards crack along joints as the primary structure deflects
• Wall bows outward as the deflection pushes the track
• Screws pull out at joints and board edges
• In extreme cases, the head track tears away from the slab
• Fire and acoustic performance compromised
• Costly remedial work

Using a slotted head track on a loadbearing wall

• Wall sinks under load as the slotted track allows movement
• Wall may be crushed or buckle as load is transferred
• Boards crack or fall away
• Floors above may sag
• In extreme cases, structural failure
• Building potentially unsafe

Using the wrong stud size or gauge

• Wall deflects more than intended (serviceability failure)
• Wall fails under load (structural failure — for loadbearing walls)
• Fire test invalidated (if the tested system specified a different stud)
• Acoustic performance reduced
• Costly remedial work

How to specify correctly

1. State the type clearly — "infill" or "loadbearing". Ambiguity is a common cause of problems.
2. Specify the head detail — slotted track with deflection allowance for infill; plain track for loadbearing. State the deflection allowance in mm.
3. Specify the stud and gauge — unless the design is being carried out by the SFS contractor, in which case specify the design loading and deflection limit.
4. State the structural requirements — for loadbearing walls, confirm the wall has been designed by a structural engineer.
5. State the fire rating — EI for infill, REI for loadbearing. Confirm tested build-up.
6. Provide engineer drawings — for loadbearing walls, the structural engineer should provide drawings showing the design.

If you're not sure which you need, err on the side of asking. The cost of an engineering input is small compared to the cost of a loadbearing wall failing.

Summary

Infill SFS is a non-loadbearing steel frame that fills the gap between primary structure and cladding/finish. Loadbearing SFS carries structural loads from above. The distinction is critical — it changes the head detail (slotted for infill, plain for loadbearing), the stud selection, the fire testing requirements, and the engineering input needed.

Specify clearly: "infill" or "loadbearing", with the head detail, stud size, and engineering requirements stated. Never use a plain head track on infill or a slotted head track on loadbearing — the consequences range from cosmetic damage to structural failure.