With increasing focus on façade performance and boundary separation, assessing the risk of fire spreading externally across a building is now a crucial component of fire safety design—especially in high-rise and high-risk environments.
This article explores how CFD (Computational Fluid Dynamics) is used to assess external fire spread, including its role in regulatory compliance, façade material justification, and planning approvals.
What Is External Fire Spread?
External fire spread refers to the movement of flame, smoke, and thermal radiation across the outer surfaces of a building. It may occur:
- Vertically (e.g., up the façade)
- Horizontally (e.g., to adjacent structures)
- Through window openings and balconies
Why Is It Important?
- Life Safety: Prevents re-entry of smoke or flame to upper levels
- Compartmentation Integrity: Protects escape routes and adjacent dwellings
- Planning & Approval: Local authorities often request evidence that external spread is mitigated, especially near site boundaries
- Insurance & Liability: A key concern in post-Grenfell scrutiny and façade safety reform
How CFD Supports External Fire Spread Assessment
CFD simulation tools such as FDS (Fire Dynamics Simulator) model:
- Flame height and heat flux
- Thermal radiation impact on nearby surfaces or openings
- Re-entry of hot gases through windows
- Impact of wind and ventilation
Inputs typically include:
- Fire source size and duration
- Façade geometry and materials
- Window location and opening factor
- Proximity to adjacent buildings or property boundaries
Example Scenario
A residential development proposes timber-effect cladding below balconies on all floors.
CFD is used to:
- Simulate a bin fire starting on a ground floor balcony
- Assess how heat and flame impinge on the façade
- Measure re-entry temperatures at upper-level windows
Outcome: Revised materials and installation strategy are recommended to prevent vertical fire spread.
Key Metrics in External Spread CFD Analysis
- Heat Flux (kW/m²): Determines whether the surrounding surfaces could ignite
- Flame Extension (m): Assesses vertical or horizontal reach
- Temperature at Openings: Verifies tenability for re-entry
- Radiation to Boundaries: Evaluates impact on adjacent properties.
Regulatory Context
- Approved Document B (ADB): Requires control of external fire spread, especially on buildings over 18m
- BS 8414 / BR 135: Fire performance testing of cladding systems
- BS 9991 / BS 9999: Allow for engineered approaches where appropriate
- Building Safety Act: Increased scrutiny of cladding and façades
CFD allows justification of façade decisions when BS 8414 testing is impractical or when performance-based validation is preferred.
How FDS Consult UK Supports You
- CFD Models built in Pyrosim
- External fire spread simulations using FDS and Smokeview
- Analysis for both new builds and remediation projects
- Collaboration with façade engineers and fire strategy teams
- Regulator-ready reporting and visualisation for planning submission
Visual Outputs We Provide
- Heat flux maps on façade elevations
- 3D flame envelope visualisation
- Smoke re-entry and spread overlays
- Plan views with thermal impact zones
CFD from FDS Consult UK
CFD for external fire spread analysis is a powerful tool for assessing façade safety, justifying design decisions, and preventing costly delays at the approval stage.
FDS Consult UK combines CFD expertise with deep fire engineering experience to help you deliver safer, compliant buildings.
Request a CFD-based façade analysis or speak to our team about your project’s external fire spread strategy.
