Underground car parks present unique fire safety challenges including ventilation, structural fire resistance, and the growing risk from electric vehicle fires. This comprehensive guide covers all aspects.. Underground Car Parks: A Growing Challenge Underground car parks are found beneath virtually every major new development in the UK. They present unique fire safety challenges: no natural ventilation, limited fire service access, structural fire resistance requirements, and increasingly, the risk of electric vehicle battery fires that behave very differently from conventional vehicle fires. Fire Behaviour in Underground Car Parks Heat release rate : single car fire 5 MW, multi car 15 20 MW Peak temperature : 800 1,100°C in immediate vicinity Smoke production : massive — single car fire produces enough smoke to fill 2,000m³ Spread : car to car fire spread typically occurs at 12 15 minutes Duration : vehicle fires can burn for 60 90 minutes EV fires : higher peak temperatures (up to 1,200°C), longer duration (hours), with re ignition risk Ventilation Design BS 7346 7 Smoke Ventilation Parameter Naturally Ventilated Mechanically Ventilated Ventilation openings ≥5% of floor area each side N/A Extract rate Natural (buoyancy) 10 air changes/hour minimum Inlet air ≥50% of openable area Dedicated supply at low level Fan rating N/A F300 (300°C for 60 minutes) Duct fire rating N/A EI 120 Impulse fans Optional for air distribution Critical for smoke management Impulse (Jet) Fan Systems Replace traditional ductwork — ceiling mounted jet fans creating air movement CFD design — computational modelling to verify smoke management effectiveness Flexibility — fans can be reversed to direct smoke toward extraction points Cost savings — 40 60% reduction compared to ducted systems Height savings — no duct bulkheads, reducing floor to ceiling excavation EV Charging Areas — Enhanced Protection Additional Fire Risks Thermal runaway — lithium ion battery failure during charging Toxic gases — hydrogen fluoride, hydrogen cyanide from battery fires Extended duration — EV fires can burn for 3 4 hours with re ignition Water consumption — firefighting requires 10,000 40,000 litres per EV fire Environmental — contaminated firewater containing battery electrolyte Enhanced Protection Measures Linear heat detection — along EV charging bays for rapid response Enhanced sprinkler density — 15mm/min over charging area (vs standard 5mm/min) Gas detection — hydrogen fluoride and hydrogen monitoring Enhanced ventilation — 1.5× standard extraction rate for EV zones Drainage — contained and bunded charging areas for contaminated water Structural protection — enhanced fire resistance beneath EV zones Structural Fire Resistance Requirements Approved Document B — REI 120 for elements supporting other building parts above BS EN 1992 1 2 — concrete design for fire resistance Spalling risk — high strength concrete susceptible to explosive spalling Cover — minimum 40mm to reinforcement for 120 minute fire resistance Fibre reinforced concrete — polypropylene fibres reducing spalling risk Post Tensioned Structures Higher sensitivity to fire exposure Tendon temperatures — critical at 300°C (vs 500°C for mild steel) Increased cover — minimum 50mm to prestressing tendons Board protection — may be required for critical transfer elements Fire Detection Point heat detectors — BS 5839 1 Category P2 minimum Beam detectors — for large open span car parks VESDA — aspirating detection for earliest warning Video smoke detection — AI powered CCTV analytics Carbon monoxide — combined CO/fire detection reducing false alarms Sprinkler flow switch — secondary detection from sprinkler activation Fire Service Access Dry rising main — minimum 100mm bore, outlet at every level Fire fighting lift — BS EN 81 72 compliant, serving all basement levels Fire fighting lobby — protected lobby at every level Access routes — minimum 2.1m headroom for firefighters with BA Hydrant — within 18m of dry rising main inlet Premises information box — at entrance with car park plans Ventilation controls — fire service override at ground level Magnus Opifex SEVEN LTD — UK's Leading Fire Safety & Fire Engineering Consultancy 🌐 magnus opifex.co.uk 📞 +44 7486 691724 ✉️ office@magnus opifex.co.uk Founders: Nicoleta Vasile, Baroness of Brattleby — CEO, Lawyer and Barrister, Legal & Administrative Director Alina — Technical Director & Expert Fire Engineer (BEng) Head Office: Ealing Cross, 85 Uxbridge Road, London W5 5BW Magnus Opifex SEVEN LTD delivers engineering led fire engineering, fire risk assessments, CFD modelling, and building safety consultancy across the United Kingdom and internationally. With over 20 years of combined experience and a UK portfolio spanning healthcare, residential and infrastructure, we bring truly engineered solutions with a personal touch. © 2026 Magnus Opifex SEVEN LTD. All rights reserved.