The Specifier’s Checklist: What Architects Need from Their SFS Subcontractor at RIBA Stage 3
When a building goes wrong on site, the post-mortem almost always traces back to a decision made — or not made — at RIBA Stage 3. Concept design has been signed off, the structural strategy is firming up, and the specialist subcontractors are about to be appointed. For Steel Frame System (SFS) packages on mid-rise residential, student, and commercial schemes, Stage 3 is the moment when the architect’s specification either gets ahead of the build or falls behind it.
The problem is that most published guidance on SFS is written for end-clients or contractors. There is very little that speaks directly to the architect about what to actually request from the SFS specialist at this stage. This article fills that gap. It is a practical checklist of the deliverables, evidence, and competence indicators an architect should expect from a Steel Frame System subcontractor before they’re appointed — and the warning signs that mean the firm in front of you isn’t ready for what’s coming under the Building Safety Act regime.
Why Stage 3 is the SFS pinch point
At Stage 3 the architect is firming up the building envelope, finalising the cladding strategy, and resolving how the secondary structure interfaces with the primary frame. Decisions made now lock in the load paths, the deflection criteria, the fire compartmentation strategy, and the cavity barrier zones. Get those wrong and Stage 4 becomes a long, expensive sequence of value-engineering meetings.
For Steel Frame Systems specifically, three things have changed in the last eighteen months that make Stage 3 rigour more important than ever. The Building Safety Regulator’s Gateway 2 process now demands that safety-critical detailing is fully resolved before construction begins. The September 2026 amendments to Approved Document B have raised the bar on fire performance evidence. And Part L 2025 has tightened the thermal performance demands on the wall build-up that sits within the SFS frame.
An SFS subcontractor who is ready for that environment has a defined set of capabilities. One who is not will reveal themselves in the gaps.
The seven things to ask for before you appoint
1. Engineering calculations methodology
Ask the subcontractor to walk you through how they calculate wind loads, deflection limits, and stud spacing for your specific building. They should be referencing British Standards and Eurocodes (BS EN 1993-1-3 for cold-formed steel, the relevant Eurocode 1 parts for wind loading), and they should be able to explain how site-specific factors — exposure category, building height, terrain — feed into the design. Generic calculation sheets that don’t reference your scheme are a red flag.
This is exactly the level of detail covered in the design service at BAS Frames, where engineering calculations are typically issued within one to two weeks of brief.
2. A clear fabrication and drawing schedule
A competent SFS specialist should commit to a drawing programme that aligns with your Stage 4 deliverables. Expect an initial set of drawings two to four weeks after calculations sign-off, followed by structured revision rounds against architect’s comments. Ask for sample drawing packages from previous comparable projects so you can see the level of detail and how connection details are documented.
3. Deflection head detailing for every condition
Deflection heads are where SFS infill projects go wrong on site more than any other detail. Ask the subcontractor to walk through their standard detail for slotted heads, double-track systems, and bracket systems, and explain when each is appropriate. They should be able to discuss fire-rated detailing at the head condition specifically, because this is one of the most common Gateway 2 evidence gaps.
If you want a deeper technical reference for the conversation, the BAS Frames piece on SFS deflection heads and compliance detailing covers the three system types and the five mistakes that destroy compliance.
4. Fire test evidence — building-specific, not generic
The Building Safety Regulator has been explicit that generic test data and product brochures are no longer acceptable for Gateway 2 submissions on higher-risk buildings. Ask the SFS subcontractor to demonstrate that they have access to through-wall build-up test evidence that matches your specified configuration — not a similar one. EI 60, EI 90, and EI 120 ratings each require different evidence packages. The BAS Frames guide to SFS fire resistance ratings sets out what genuine compliance looks like.
5. Thermal performance modelling against Part L 2025
The SFS frame itself doesn’t determine the wall’s U-value, but the stud spacing, web depth, and thermal bridging at floor slab interfaces all influence the achievable performance. Ask the subcontractor to show their typical wall build-ups against Part L 2025 targets and how they handle the thermal break detailing at slab edges and head conditions. The SFS U-values and thermal performance overview is a useful conversation reference.
6. BIM and digital deliverables
For most schemes above £5m, expect to receive an IFC model from the SFS subcontractor that integrates with your central coordination model. Ask what level of detail (LOD) they typically deliver, how they handle clash detection with M&E packages, and whether they can supply their components as parametric families. A subcontractor who can’t deliver to your BIM execution plan is going to create coordination work for you that should have been resolved in the model.
7. Accreditation and competence evidence
SMAS, CHAS, and ConstructionLine accreditations are the baseline. For Higher Risk Buildings under the Building Safety Act, you should also be looking at evidence of personnel competence — CSCS at the appropriate level, supervisory training, and ideally engagement with the Construction Leadership Council’s competence framework. Ask for the actual certificates rather than logos on a brochure.
The four red flags
There are four patterns that almost always indicate an SFS subcontractor isn’t ready for a specifier-led project.
The first is brochure-level technical answers. If you ask about a specific deflection condition and the response is a generic statement about flexibility and quality, the firm doesn’t have engineering depth in-house. The second is a refusal to share previous calculation packages or drawing samples on the grounds of confidentiality. Reasonable redaction is fine; complete opacity is not. The third is no clear answer on Gateway 2 evidence packaging — the firm should have processes that produce evidence in the format BSR expects, not produce it reactively when asked. The fourth is sales-led communication structure. If your first three conversations are with a business development manager and no engineer is present, the technical interrogation hasn’t really happened.
What this looks like in practice
A well-run SFS appointment process at Stage 3 should produce a clear technical proposal, a sample of relevant previous deliverables, a programme commitment, and a competence dossier within about three weeks of initial enquiry. The architect should come out of those three weeks with enough confidence in the subcontractor’s capability to write them into the specification by name, or with enough doubt to look elsewhere.
The Steel Frame Systems market in the UK has expanded fast over the last five years, and not every firm that calls itself an SFS specialist has the engineering and compliance depth to operate at HRB level. The checklist above is the filter. Use it before Stage 4, when changing your mind costs nothing, rather than after, when it costs the programme.
To discuss a specific project at Stage 3 or to request a sample drawing package, get in touch with the BAS Frames team.
