SFS Fire Resistance Ratings: 60, 90 & 120 Minute Systems Explained

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.