For architects, structural engineers and main contractors specifying SFS infill walls, no single detail causes more on-site problems than the deflection head. Get it right, and the SFS package accommodates primary frame movement invisibly for the life of the building. Get it wrong, and you see cracked plasterboard, displaced cavity barriers, compromised fire compartmentation and — in the worst cases — buckled studs.
This guide explains what an SFS deflection head is, why it matters, the system options available in 2026, and the detailing decisions that separate a compliant specification from one that fails inspection.
For background on how deflection heads sit within the wider SFS system, our pillar guide on SFS construction and how infill framing connects to primary structures explains the structural relationship between the SFS frame and the building’s primary steel or concrete structure.
What is a deflection head, and why does it matter?
An SFS infill wall fixes between the floor slab below and the soffit (underside of the slab) above. The primary structural frame deflects under load — composite steel beams sag a few millimetres under imposed load, concrete slabs creep over time, and the whole building moves marginally with wind and thermal effects.
The SFS infill wall, by contrast, is a non-loadbearing element. It must not pick up the vertical load of the slab above. If it does, two things happen:
- The studs buckle or the boards crack. SFS sections are designed for lateral wind load, not vertical compression. Picking up slab load from above will overstress them.
- The detail fails its fire test. Most fire-tested SFS deflection details are tested with a specific deflection allowance. If the detail used in practice doesn’t match the test, the fire rating is invalidated.
The deflection head is the engineered detail at the top of the SFS panel that allows the slab above to move vertically — typically ±15mm to ±25mm — without transferring load into the studs.
Typical deflection allowances
Required deflection allowance varies by primary frame type:
- Concrete slab construction: 10–15mm typical, accounting for creep over the building life
- Composite steel-and-concrete slab: 15–25mm typical, depending on span and loading
- Long-span steel frame buildings: up to 30mm or more — bespoke calculation required
Your structural engineer specifies the allowance based on the calculated deflection of the primary structure. The SFS subcontractor’s job is to install a head detail that accommodates that allowance with margin to spare. Our SFS infill walls technical guide covers the wider design parameters that drive deflection head selection.
Three deflection head systems in common UK use
1. Slotted head track (proprietary systems)
The most common solution in 2026 is a manufacturer’s slotted head track — Metsec’s slotted head, Intrastack’s equivalent, or similar from other suppliers. The track has vertical slots, and the studs are screw-fixed through the slots. As the slab above deflects, the slot allows the screw to slide within the track, isolating the stud from vertical load.
- Pros: Fast to install, well-tested, fire-rated detail readily available.
- Cons: Slightly more expensive than fixed track. Specific screw type and torque required for the slot to function correctly.
- Typical fire rating: 60, 90 or 120 minutes when used with the manufacturer’s tested through-wall system.
2. Double-track (two-track) deflection head
A traditional alternative: two head tracks, one fixed to the soffit and one fixed to the studs, with the stud track sliding telescopically inside the soffit track. The studs do not connect directly to the soffit track at all.
- Pros: Larger deflection allowances achievable (up to 30mm+). Robust for heavy commercial applications.
- Cons: More site labour, more risk of incorrect installation, larger head void requiring careful fire stopping.
- Typical fire rating: Achievable to 120 minutes with correct cavity barrier and fire stop detailing.
3. Engineered deflection bracket systems
For high-deflection situations or unusual geometries (cantilevered slabs, large spans, transfer structures), engineered bracket systems with engineered slip joints are used. These are bespoke per project.
- Pros: Handles any deflection allowance the engineer specifies.
- Cons: Lead time. Requires careful coordination with structural design.
Fire performance — where most details fail
Here is the detail most architects and contractors miss: a deflection head is a hole in your fire compartment until it is properly fire-stopped.
A 60-minute compartment wall with a 20mm deflection void at the head is, technically, a 60-minute wall with a 20mm hole at the top — unless the head void is treated as a fire-rated linear gap seal.
Compliant detailing requires:
- A tested fire-rated deflection head detail specific to the manufacturer’s system
- An intumescent or compressible fire stop in the head void, sized for the maximum deflection
- Cavity barriers at the slab edge if the SFS sits proud of the slab
- Documentation showing the as-built detail matches the tested detail
Under the Building Safety Act and the Golden Thread requirements for Higher-Risk Buildings, this documentation is no longer optional. Our fire-rated drylining specification guide covers the same compliance principles for the internal lining elements.
The five most common deflection head mistakes on site
After two decades installing SFS, we see the same errors repeatedly. If you are a main contractor’s project manager, look for these on your inspections:
- Studs hard-fixed to a non-slotted head track. The most common error — usually because a fixed track was easier to find on site than the specified slotted track. Result: vertical load transferred into studs.
- Slot fully populated with the screw at one end. The screw should sit roughly central in the slot to allow movement in both directions. Inspectors check this with a torch.
- Cavity barrier omitted or compressed. If the cavity barrier is squeezed too tight, it cannot accommodate the deflection. If it is missing entirely, the fire compartment fails.
- Substitution of a non-tested fire stop. “Equivalent” intumescent foam is not equivalent unless it has been tested in the specific deflection head detail. Manufacturer-specified fire stops are not substitutable.
- No deflection allowance documented in the handover pack. When the building is occupied and the slab deflects over a year, no one remembers what allowance was designed in. Documentation should record the specified deflection and the as-built detail.
Specifying a deflection head correctly
If you are writing the SFS specification for a project, your deflection head clause should cover:
- Calculated deflection allowance (from the structural engineer)
- Permitted system types (e.g., “slotted head track to manufacturer’s tested detail, or approved equal”)
- Fire rating required at the head detail (must match the wall fire rating)
- Cavity barrier detail at the slab edge
- Acoustic seal requirement at the head void (often missed)
- Tested system reference (the manufacturer’s test report number)
- As-built documentation requirement (sample sign-off, photographic record)
Our SFS supply service sources only manufacturer-tested deflection head systems with full traceability and compliance documentation. For installation, our SMAS and CHAS-accredited installation teams install to the tested detail with sign-off photography at every head.
Why this single detail matters disproportionately
In a typical mid-rise commercial building, the deflection head represents perhaps 2–3% of the total facade area. But it accounts for a disproportionate share of facade defects, fire compartmentation failures, and post-occupancy issues.
A facade contractor who treats the deflection head as a routine detail will deliver problems. One who treats it as the single most important fire-and-movement detail in the SFS package will deliver a facade that performs for 60+ years.
If you are specifying or procuring an SFS package for a project where deflection head performance matters — which is essentially every project — our team can review your structural drawings and recommend a tested detail before tender. Get in touch with your structural drawings and head condition details.
Related reading
- SFS infill walls — technical design guide — the wider context for deflection head decisions.
- Fire-rated drylining specification guide — the same compliance logic applied to the internal wall lining build-up.
- What is included in an SFS package — where deflection head detailing sits in the full SFS scope.
