Key takeaways
- SFS through-wall systems achieve 30, 60, 90 or 120-minute fire resistance only as complete, manufacturer-tested build-ups — the rating belongs to the tested system, not to the steel.
- Know the notation: REI adds load-bearing resistance (R) to integrity (E) and insulation (I); SFS infill is normally EI-rated, as it is not load-bearing.
- Five site errors invalidate a rating: board substitution, mixing systems, omitting cavity barriers at slab edges, wrong deflection-head detailing, and untreated service penetrations.
- Over 18 m (Higher-Risk Buildings), the whole external wall must be evidenced to Approved Document B / BS 8414, with tested-system documentation kept in the golden thread.
For architects, fire engineers and main contractors specifying external walls in 2026, fire performance is no longer just one consideration among many — it is the consideration that overrides all others. The Building Safety Act, the post-Grenfell regulatory framework, and the September 2026 amendments to Approved Document B have made fire-rated SFS specification a precision engineering exercise.
This guide explains what 60, 90 and 120-minute SFS fire resistance ratings actually mean, how they are achieved through tested through-wall systems, and the specification decisions that decide whether your building passes Gateway 2 — or sits in the regulator’s pile.
For the foundational context on how SFS systems work in the broader building envelope, our pillar guide on SFS construction and infill framing sets out the structural principles. This article focuses specifically on fire performance.
What does “60-minute fire resistance” actually mean?
Fire resistance for an SFS wall is measured against three criteria, tested in accordance with BS EN 1364 (for non-loadbearing walls):
- R — loadbearing capacity (only applicable to loadbearing walls — most SFS infill is non-loadbearing, so this doesn’t apply)
- E — integrity (the wall must not allow flames or hot gases to pass through)
- I — insulation (the unexposed face must not heat up beyond a defined limit)
A “60-minute fire-rated SFS wall” is shorthand for EI 60 — the wall must maintain integrity and insulation for 60 minutes when tested to the standard fire curve. The same logic applies for EI 90 and EI 120 ratings.
Critically, the rating applies to a specific tested build-up, not to “SFS in general.” Change one element — the board layer count, the manufacturer of the plasterboard, the stud spacing, the deflection head detail — and the rating no longer applies. Substitutions kill ratings.
Typical 60, 90 and 120-minute SFS through-wall build-ups
The exact build-up varies by manufacturer’s tested system. The descriptions below are representative of typical specifications under British Gypsum’s GypLyner Xternal range or equivalent systems from Siniat, Knauf and others.
60-minute fire-rated SFS through-wall (EI 60)
- 100mm Intrastack (or equivalent) SFS framework at 600mm centres
- 5mm Glasroc X Sheathing Board to external face
- 50mm Isover Acoustic Partition Roll (or equivalent) in cavity
- One layer 15mm Gyproc FireLine to internal face
- 20mm deflection head with tested fire stop
90-minute fire-rated SFS through-wall (EI 90)
- Same SFS framework
- 5mm Glasroc X Sheathing Board external
- 50mm mineral wool insulation in cavity
- Two layers 15mm Gyproc FireLine to internal face
- Tested deflection head detail with appropriate cavity barrier
120-minute fire-rated SFS through-wall (EI 120)
- Same SFS framework
- 5mm Glasroc X Sheathing Board external
- 50mm mineral wool insulation in cavity
- Three layers 15mm Gyproc FireLine (or two FireLine + one FireLine MR) internal
- Enhanced cavity barrier and fire stop detail
The pattern is clear: higher fire ratings are achieved primarily by adding plasterboard layers internally, rather than by changing the steel framing itself. The SFS frame is essentially the same across ratings — what changes is the board build-up.
When you need each rating — UK regulatory framework
Under Approved Document B (Volume 2, Buildings Other Than Dwellinghouses) and Volume 1 (Dwellinghouses), the required fire resistance is set by:
- Building height (over 18m and over 30m thresholds)
- Building use (residential, commercial, healthcare, etc.)
- Compartmentation strategy (compartment floor and wall locations)
- Higher-Risk Building (HRB) classification under the Building Safety Act 2022
Typical applications for each rating in 2026:
- EI 60: Commercial offices and lower-rise residential below 11m
- EI 90: Mid-rise residential (11–18m), most schools, healthcare facilities
- EI 120: High-rise residential and commercial above 18m, all HRBs, compartment walls between residential dwellings in stacked configurations
Our Fire-Rated Drylining 2026 article covers the wider regulatory context post-September 2026 amendments. The same Building Safety Act framework that drives drylining decisions drives SFS fire specification.
The five things that invalidate an SFS fire rating
Under regulatory scrutiny, here are the most common reasons SFS systems fail to deliver their specified fire rating:
1. Board substitution
Specified Gyproc FireLine, installed standard Wallboard. The visual difference is minimal; the fire test difference is total. Specifications must call out the manufacturer and product reference, not just “fire-rated plasterboard.”
2. Mixed-system installation
Specified Intrastack framework with British Gypsum boards (a matched, tested system). Installed Metsec framework with British Gypsum boards (not tested as a system). Each manufacturer’s test data applies only to their tested combination.
3. Cavity barriers omitted at slab edge
When the SFS sits proud of the slab edge, the cavity between the slab and the external sheathing is a route for fire spread. Cavity barriers are mandatory and frequently missed.
4. Deflection head detailing
Covered in detail in our SFS deflection heads guide — but worth repeating. A non-fire-rated deflection head invalidates the wall’s fire rating regardless of how good the rest of the wall is.
5. Service penetrations untreated
Every cable, pipe and duct that passes through the SFS wall must be fire-stopped to the wall’s rating. A 120-minute wall with a 30-minute service penetration is a 30-minute wall.
Specification language that protects fire performance
When writing the SFS fire specification for a project, the clause should include:
- The required fire rating (EI 60, EI 90 or EI 120)
- Reference to a specific tested system (manufacturer + test report number)
- Substitution clause prohibiting product substitution without re-test evidence
- Penetration sealing requirement to the same rating as the wall
- Cavity barrier requirement at all slab edges and around openings
- As-built compliance evidence required at handover
Loose specifications (“60-minute fire-rated SFS system”) give the supply chain too much latitude. Precise specifications close that gap.
Sourcing fire-rated SFS components
A competent SFS subcontractor sources components from a single tested system family — Intrastack with British Gypsum, Metsec with Siniat or Knauf, or another tested combination. Mixing brands in pursuit of cheaper components is a false economy that can invalidate the entire fire rating.
Our SFS supply service sources fully-traceable, BS-certified components from tested system families with documentation pack handover. For complex projects with bespoke fire performance requirements, our structural engineering team coordinates the through-wall specification with the fire engineer.
The Higher-Risk Building (HRB) overlay
For buildings over 18m or seven storeys, additional Building Safety Act requirements apply:
- Gateway 2 approval before construction begins, including fire strategy
- Golden Thread of digital information maintained throughout the build
- Competence evidence for designers, contractors and subcontractors
- As-built compliance documentation signed off by the principal designer and contractor
- Sign-off at Gateway 3 before occupation
For SFS specifically, this means every fire-rated detail must be documented as installed, with photographic evidence, material delivery notes, and operative competence certificates. This is the documentation our SMAS and CHAS-accredited install teams produce as standard on HRB projects.
Practical advice for specifiers
If you are an architect or fire engineer writing an SFS fire specification in 2026:
- Specify the tested system, not the rating alone. “EI 90 to British Gypsum tested specification GLEX-XYZ” is enforceable. “EI 90 SFS system” is not.
- Coordinate the deflection head with the fire rating. They are inseparable.
- Specify cavity barriers explicitly at every slab edge and around every opening.
- Require as-built documentation as a contractual obligation.
- Walk the site at first-fix. The first 100m² of SFS will reveal whether the gang is delivering to specification. Catch the substitution problems early.
For project-specific advice on SFS fire specification, get in touch with your fire strategy, structural drawings and external wall sections, and our team will recommend a tested specification before tender.
Related reading
- SFS deflection heads explained — the single detail that most often invalidates a fire rating.
- Fire-Rated Drylining in 2026 — the wider regulatory context for fire-rated wall systems.
- SFS infill walls — technical design guide — the structural and detailing context that frames fire decisions.
Further reading:Steel Construction Institute — Fire Protection | Approved Document B
Related:Cost | Infill | What Is | Thermal
Frequently asked questions
What fire resistance ratings are available for SFS external walls?
SFS external wall systems are available in 30, 60, 90 and 120-minute fire resistance ratings when specified in accordance with tested systems. The rating depends on the stud gauge and depth, sheathing board type, plasterboard specification (number of layers and board type), and cavity barrier provision. All fire-rated SFS systems must be specified using tested system references from approved manufacturers.
What is the difference between REI-60 and EI-60 in SFS construction?
REI-60 means the wall provides 60 minutes of load-bearing resistance (R), integrity (E) and insulation (I). EI-60 means the wall provides 60 minutes of integrity and insulation only — it is not a load-bearing element. SFS infill walling is typically classified as EI rather than REI because it is not a structural element; the primary frame carries the loads.
What cavity barriers are required in SFS external walls?
Approved Document B requires cavity barriers at every floor level, at the tops and sides of openings, and at maximum 20-metre horizontal intervals in SFS external walls. Cavity barriers must be of a material that provides 30 minutes of fire resistance and must be installed to close the cavity in the event of a fire. Intumescent cavity barriers are commonly used in SFS construction.
Can SFS achieve the fire performance required for buildings over 18 metres?
Yes — SFS external wall systems with appropriate fire-rated sheathing, cavity barriers and cladding systems can achieve the fire performance required for buildings over 18 metres under Approved Document B and BS 8414. The entire external wall system — not just the SFS frame — must be assessed and documented. BAS Frames works with fire engineers and cladding specialists to ensure compliant specifications on higher-risk buildings.
