Flat Roofing

Why cold deck roofs fail: interstitial condensation, 1:150 ventilation requirement rarely achievable, timber rot progression, blistering diagnosis, conversion to warm deck options

EPDM bonded vs mechanically fixed: seam tape (75mm min overlap, EPDM primer then butyl tape), termination bars at ≤200mm centres, 150mm upstands, 1:40 recommended fall

Part C weather resistance: 150mm minimum upstands, lead/aluminium flashings with 75mm overlap and 25mm chase, drainage falls, penetration detailing, NFRC CoP compliance satisfies Part C

Part L 2021 U-value target 0.18 W/m²K: 130–150mm PIR or 175–210mm EPS, tapered insulation average U-value calculation, thermal bridging at parapets (ψ values)

BS EN 12056-3 design rainfall (75mm/hr primary, 150mm/hr overflow), minimum falls 1:80, mandatory overflow provision, outlet sizing by roof area, green roof drainage considerations

Systematic survey methodology, felt/EPDM/GRP failure modes, IR thermography and ELD techniques, RICS three-grade condition rating, condition report format for customer-facing use

Parapet upstand minimum 150mm above finished roof level (BS 6229:2018), coping stone DPC and drip detail, lead/GRP/EPDM flashing at parapet base, thermal bridging Ψ-values, five most common failure modes

Patch repair limitations on aged felt, 20% blistering threshold, overlay vs strip (Part L 25% renovation trigger, 0.18 W/m²K), Party Wall Act notice, cost-benefit framework

150mm upstand at rooflight kerb (BS 6229:2018), timber vs prefabricated kerb, membrane termination, U-value ≤1.6 W/m²K for replacement (Part L 2021), permitted development 150mm projection limit, condensation diagnosis

Layer-by-layer green roof build-up (VCL → insulation → FLL-tested root barrier → drainage cell → filter fleece → substrate), extensive 60–150 kg/m² vs intensive 200+ kg/m², BS 8579:2019, BNG/Environment Act 2021

GRP installation: 450g/m² CSM, 1–2% MEKP catalyst, key coat timing (hardened but not fully cured), topcoat application, common defects (delamination, crazing, dry patches)

XPS insulation above waterproofing membrane, 100kg/m² minimum ballast, ISO 13165 moisture correction factor (fX = 0.04 W/m²K), 200–250mm XPS for 0.18 W/m²K target, no VCL needed

NFRC CoP 1 (built-up felt), CoP 2 (single-ply EPDM/PVC), CoP 3 (mastic asphalt), CoP 4 (cold-applied liquid/GRP): falls 1:80 minimum, 150mm upstands, warranty provider alignment

SBS vs APP modified bitumen: 2-layer minimum, 75mm side laps, hot works permit, dedicated fire watcher, 1-hour post-completion fire watch, NFRC CoP 1 compliance

Warm deck build-up: deck → VCL → PIR insulation → waterproofing membrane; VCL position (below insulation, not above); tapered insulation for falls; 0.18 W/m²K Part L target