Flat Roof Conversion and Replacement in London Victorian Terraces: Sarnafil, EPDM, and Warm Roof Build-Up

THE FIRST DESIGN DECISION FOR A NEW FLAT ROOF ON A LONDON VICTORIAN TERRACE IS THE BUILD-UP TYPE — WARM ROOF, COLD ROOF, OR INVERTED ROOF — EACH WITH DIFFERENT INSULATION POSITIONS RELATIVE TO THE STRUCTURAL DECK AND WATERPROOFING MEMBRANE.

WARM ROOF (THE RECOMMENDED BUILD-UP FOR ALL NEW FLAT ROOFS IN LONDON VICTORIAN TERRACES — SATISFIES PART L 2021 THERMAL REQUIREMENTS AND ELIMINATES THE RISK OF INTERSTITIAL CONDENSATION): IN A WARM ROOF BUILD-UP, THE INSULATION IS PLACED ABOVE THE STRUCTURAL DECK (TYPICALLY 18MM ORIENTATED STRAND BOARD — OSB 3 — OR 18MM WBP PLYWOOD) AND BELOW THE WATERPROOFING MEMBRANE. THE DECK, INSULATION, AND WATERPROOFING MEMBRANE ARE ALL ON THE WARM SIDE OF THE THERMAL ENVELOPE — HENCE THE NAME WARM ROOF. TYPICAL WARM ROOF BUILD-UP FROM STRUCTURAL DECK UPWARD: (1) STRUCTURAL DECK: 18MM OSB 3 (STERLING OSB3 — NORBORD — THE STANDARD STRUCTURAL DECKING FOR UK FLAT ROOFS — TONGUE AND GROOVE — STAGGERED JOINTS — FIXED WITH 3.5MM X 65MM RING SHANK NAILS AT 150MM CENTRES ALONG JOISTS AND AT EDGES); (2) AIR AND VAPOUR CONTROL LAYER (AVCL — A VAPOUR BARRIER MEMBRANE APPLIED DIRECTLY TO THE TOP OF THE STRUCTURAL DECK — PREVENTS MOISTURE-LADEN INTERNAL AIR RISING INTO THE INSULATION AND CONDENSING — TYPICALLY SELF-ADHESIVE SBS MODIFIED BITUMEN AVCL SUCH AS FIRESTONE VAPOUR BARRIER SA OR IKO TORCHMASTER VAPOUR BARRIER); (3) PIR BOARD INSULATION (POLYISOCYANURATE RIGID INSULATION BOARD — THE MOST WIDELY USED FLAT ROOF INSULATION IN UK RESIDENTIAL FLAT ROOF WARM BUILDS — CELOTEX GA4000 OR IKO ENERTHERM ALU): CELOTEX GA4000 (CELOTEX LTD — HADLEIGH, SUFFOLK — THE UK'S LARGEST PIR RIGID INSULATION MANUFACTURER): GA4000 IS THE PIR BOARD SPECIFICALLY DESIGNED FOR FLAT ROOF WARM ROOF APPLICATION — TAPERED EDGE PROFILE FOR DRAINAGE FALLBACK — FOIL FACINGS — RECOMMENDED THICKNESS FOR PART L 2021 COMPLIANCE (U-VALUE 0.18 W/M²K FOR NEW FLAT ROOFS — PART L 2021 VOLUME 2 TARGET): APPROXIMATELY 100MM CELOTEX GA4000 (LAMBDA 0.022 W/MK). IKO ENERTHERM ALU (IKO PLC — THE SECOND MOST WIDELY SPECIFIED UK PIR FLAT ROOF BOARD): SIMILAR PERFORMANCE AND THICKNESS TO CELOTEX GA4000; (4) WATERPROOFING MEMBRANE (SEE BELOW — SARNAFIL, EPDM, OR TORCH-ON BITUMEN).

COLD ROOF (THE TRADITIONAL BUILD-UP — INSULATION BELOW THE STRUCTURAL DECK — IN THE JOIST VOID — VENTILATED VOID BETWEEN THE INSULATION AND THE DECK ABOVE — NOT RECOMMENDED FOR NEW FLAT ROOFS IN LONDON VICTORIAN TERRACES — NOT EASILY ACHIEVABLE TO PART L 2021 U-VALUE REQUIREMENTS WITHOUT VERY DEEP JOISTS — AND THE VENTILATED COLD ROOF IS MORE PRONE TO CONDENSATION RISK IF THE VENTILATION GAP IS BLOCKED): RETAINED IN SOME RENOVATION PROJECTS WHERE THE EXISTING COLD ROOF STRUCTURE IS ADEQUATE AND THE WATERPROOFING ONLY IS BEING REPLACED — BUT FOR ALL NEW FLAT ROOF STRUCTURES IN LONDON VICTORIAN TERRACE EXTENSIONS: SPECIFY WARM ROOF.

INVERTED ROOF (THE INSULATION IS PLACED ON TOP OF THE WATERPROOFING MEMBRANE — PROTECTING THE MEMBRANE FROM UV AND THERMAL SHOCK): USED PRIMARILY ON COMMERCIAL FLAT ROOFS AND ON ACCESSIBLE ROOFTOP TERRACES (WHERE THE INSULATION BOARDS ARE BALLASTED BY PAVING SLABS OR GRAVEL) — NOT COMMONLY SPECIFIED FOR DOMESTIC FLAT ROOF RENOVATION IN LONDON VICTORIAN TERRACES.

SARNAFIL G410 (SIKA-SARNAFIL — ZURICH, SWITZERLAND — THE WORLD'S LEADING MANUFACTURER OF PVC (POLYVINYL CHLORIDE) SINGLE-PLY WATERPROOFING MEMBRANES — SIKA-SARNAFIL IS A DIVISION OF SIKA AG) IS THE MOST WIDELY SPECIFIED PREMIUM SINGLE-PLY WATERPROOFING MEMBRANE FOR FLAT ROOFS IN UK RESIDENTIAL RENOVATIONS AND EXTENSIONS — INCLUDING LONDON VICTORIAN TERRACE REAR EXTENSIONS.

SARNAFIL G410 TECHNICAL SPECIFICATION: MATERIAL: THERMOPLASTIC PVC (POLYVINYL CHLORIDE) REINFORCED WITH A POLYESTER FABRIC (THE WOVEN POLYESTER SCRIM EMBEDDED IN THE PVC PROVIDES DIMENSIONAL STABILITY AND TENSILE STRENGTH — PREVENTING THE MEMBRANE FROM STRETCHING OR CREEPING UNDER MECHANICAL LOAD AND THERMAL MOVEMENT); THICKNESS: 1.5MM (STANDARD FOR DOMESTIC FLAT ROOF APPLICATION — THE BBA CERTIFIED MINIMUM FOR SARNAFIL G410 IN RESIDENTIAL USE) OR 1.8MM (HEAVY-DUTY — USED FOR TRAFFICKED ROOFS OR WHERE ADDITIONAL PUNCTURE RESISTANCE IS REQUIRED — NOT TYPICALLY NEEDED FOR A DOMESTIC REAR EXTENSION OR OUTRIGGER ROOF IN LONDON). COLOUR: STANDARD COLOURS ARE GREY (THE MOST COMMON — ARCHITECTURALLY NEUTRAL) AND WHITE (THE REFLECTIVE OPTION — REDUCES SOLAR HEAT GAIN INTO THE ROOF — RELEVANT FOR GREEN ROOF OR SOLAR PV SUBSTRATE APPLICATIONS). LIFE EXPECTANCY: SARNAFIL G410 IS BBA (BRITISH BOARD OF AGRÉMENT) CERTIFIED WITH A MINIMUM LIFE EXPECTANCY OF 30 YEARS (WHEN CORRECTLY INSTALLED BY A SIKA SARNAFIL APPROVED INSTALLER AND MAINTAINED APPROPRIATELY). IN PRACTICE: SARNAFIL G410 INSTALLATIONS FROM THE 1980S AND 1990S ARE STILL PERFORMING SATISFACTORILY — LIFE EXPECTANCY OF 30-50 YEARS IS REALISTIC FOR A CORRECTLY INSTALLED SARNAFIL G410 ROOF. INSTALLATION METHOD: HOT-AIR WELDING (THE SARNAFIL G410 MEMBRANE IS INSTALLED BY LAP-WELDING THE OVERLAPPING EDGES OF ADJACENT MEMBRANE PANELS USING A HOT-AIR WELDER — THE HEAT FUSES THE TWO PVC SURFACES TOGETHER — CREATING A SEAM AS STRONG AS THE PARENT MEMBRANE — THERE IS NO SEPARATE ADHESIVE USED FOR THE LAPS — THE HOT-AIR WELD IS THE JOIN). FIXINGS: MECHANICALLY FIXED TO THE DECK WITH SARNAFIL FASTENERS AND SEAM PLATES (THROUGH THE MEMBRANE FLANGE — THE FASTENER IS COVERED BY THE OVERLAPPING PANEL — SO NO EXPOSED FASTENERS IN THE FINISHED ROOF). ALTERNATIVELY, FULLY ADHERED TO THE INSULATION WITH SARNAFIL BONDING ADHESIVE (THE FULLY ADHERED SYSTEM — THE MEMBRANE IS GLUED TO THE INSULATION — PROVIDES BETTER WIND-UPLIFT RESISTANCE THAN MECHANICAL FIX IN HIGH-WIND ENVIRONMENTS OR ON LOW-PITCH FLAT ROOFS).

FIRESTONE RUBBERCOVER EPDM (FIRESTONE BUILDING PRODUCTS — THE WORLD'S LARGEST MANUFACTURER OF EPDM (ETHYLENE PROPYLENE DIENE MONOMER) RUBBER ROOFING MEMBRANES — FIRESTONE BUILDING PRODUCTS LLC IS A SUBSIDIARY OF BRIDGESTONE CORPORATION) IS THE MOST WIDELY SPECIFIED EPDM FLAT ROOF MEMBRANE IN UK DOMESTIC RENOVATION AND EXTENSION PROJECTS — INCLUDING LONDON VICTORIAN TERRACE REAR EXTENSIONS AND OUTRIGGER ROOFS.

FIRESTONE RUBBERCOVER EPDM TECHNICAL SPECIFICATION: MATERIAL: EPDM (ETHYLENE PROPYLENE DIENE MONOMER) — THE SYNTHETIC RUBBER — INHERENTLY FLEXIBLE (ELONGATION AT BREAK >300%) — THE FLEXIBILITY MAKES EPDM VERY TOLERANT OF SUBSTRATE MOVEMENT AND THERMAL CYCLING (EPDM CAN ACCOMMODATE MOVEMENT WITHOUT CRACKING THAT WOULD SPLIT A MORE RIGID MEMBRANE). THICKNESS: 1.14MM (THE STANDARD FIRESTONE RUBBERCOVER THICKNESS FOR DOMESTIC APPLICATION — THINNER THAN SARNAFIL G410 AT 1.5MM BUT EPDM HAS DIFFERENT PROPERTIES — EPDM IS NOT REINFORCED BUT IS INHERENTLY FLEXIBLE AND PUNCTURE-RESISTANT). COLOUR: BLACK (THE NATURAL COLOUR OF EPDM — ALL FIRESTONE RUBBERCOVER MEMBRANES ARE BLACK — HIGHER SOLAR HEAT ABSORPTION THAN GREY OR WHITE PVC SARNAFIL — CAN INCREASE THE SURFACE TEMPERATURE OF THE ROOF ON A HOT SUMMER DAY). WIDTH: FIRESTONE RUBBERCOVER IS AVAILABLE IN WIDTHS UP TO 15.25M — ALLOWING MANY DOMESTIC FLAT ROOF AREAS TO BE COVERED IN A SINGLE PIECE WITH NO SEAMS — THIS IS A SIGNIFICANT ADVANTAGE OVER SARNAFIL G410 (WHICH IS SUPPLIED IN 2.0M WIDE ROLLS AND ALWAYS HAS MULTIPLE LAPPED SEAMS — EACH SEAM IS A POTENTIAL FAILURE POINT). LIFE EXPECTANCY: FIRESTONE RUBBERCOVER IS BBA CERTIFIED WITH A MINIMUM LIFE EXPECTANCY OF 30 YEARS. INSTALLATION METHOD: FULLY BONDED WITH FIRESTONE BONDING ADHESIVE (THE EPDM MEMBRANE IS GLUED TO THE INSULATION BOARDS — IN CONTRAST TO SARNAFIL WHERE EITHER MECHANICAL FIX OR FULL ADHESIVE IS USED — EPDM IS ALWAYS BONDED). SEAMS (WHERE PRESENT): FIRESTONE QUICKSEAM TAPE (A SELF-ADHESIVE SEAM TAPE — APPLIED OVER THE LAPPED MEMBRANE EDGES — NO HOT-AIR WELDING REQUIRED — THIS IS AN ADVANTAGE FOR SMALLER CONTRACTORS WHO DO NOT OWN OR OPERATE HOT-AIR WELDING EQUIPMENT). TYPICAL APPLICATION FOR LONDON VICTORIAN TERRACE: THE MOST COMMON APPLICATION FOR FIRESTONE RUBBERCOVER EPDM IN LONDON VICTORIAN TERRACE FLAT ROOF RENOVATION IS THE REAR OUTRIGGER OR SINGLE-STOREY KITCHEN EXTENSION ROOF — WHERE THE SIMPLE RECTANGULAR PLAN ALLOWS A SINGLE PIECE OF EPDM TO BE LAID WITH NO SEAMS — PROVIDING THE MOST RELIABLE WATERPROOFING.

TORCH-ON BITUMEN FELT (MODIFIED BITUMEN — SBS OR APP — APPLIED TO THE DECK AND INSULATION BY PROPANE TORCH — THE HEAT MELTS THE BITUMEN ON THE UNDERSIDE OF THE FELT ROLL WHICH BONDS TO THE SUBSTRATE OR UNDERLYING FELT LAYER — MULTI-LAYER BUILD-UP WITH TYPICALLY 2 OR 3 LAYERS OF MODIFIED BITUMEN FELT) REMAINS THE MOST WIDELY USED FLAT ROOF WATERPROOFING SYSTEM IN UK RESIDENTIAL RENOVATION OVERALL — DESPITE THE PREMIUM ALTERNATIVES OF SARNAFIL G410 AND FIRESTONE RUBBERCOVER EPDM — BECAUSE IT IS LOWER COST, WELL UNDERSTOOD BY UK ROOFING CONTRACTORS, AND REPAIRABLE WITH COMMONLY AVAILABLE MATERIALS.

BAUDER TORCHON (BAUDER LTD — IPSWICH, SUFFOLK — THE UK'S MARKET-LEADING MANUFACTURER OF MODIFIED BITUMEN WATERPROOFING FELT FOR THE UK ROOFING MARKET): THE BAUDER TORCHON (SBS MODIFIED BITUMEN — FIRE-RESISTANT TOPSHEET — AVAILABLE IN 3MM AND 4MM MINERAL-FINISH UNDERLAY AND OVERLAY VERSIONS) IS THE PREMIUM TORCH-ON SPECIFICATION FOR A DOMESTIC FLAT ROOF IN LONDON — AVAILABLE IN THE BAUDER TWO-LAYER (UNDERLAY + OVERLAY) SYSTEM FOR RESIDENTIAL APPLICATIONS. SBS MODIFIED BITUMEN (THE SBS — STYRENE BUTADIENE STYRENE — MODIFICATION OF BITUMEN): SIGNIFICANTLY MORE FLEXIBLE AND COLD-TEMPERATURE-RESISTANT THAN UNMODIFIED BITUMEN — SBS BITUMEN CAN WITHSTAND LOWER TEMPERATURES WITHOUT BECOMING BRITTLE AND CRACKING (IMPORTANT FOR UK WINTER CONDITIONS). APP MODIFIED BITUMEN (ATACTIC POLYPROPYLENE): ALTERNATIVE TO SBS — STIFFER AT LOW TEMPERATURES THAN SBS BUT MORE RESISTANT TO SOLAR HEAT DISTORTION — LESS COMMONLY SPECIFIED FOR RESIDENTIAL FLAT ROOFS IN THE UK THAN SBS. MINERAL SURFACED (THE EXPOSED TOPSHEET OF THE TORCH-ON SYSTEM IS MINERAL-SURFACED — FINE SLATE AGGREGATE FUSED INTO THE BITUMEN SURFACE — PROVIDES UV RESISTANCE FOR THE BITUMEN — THE STANDARD DOMESTIC FLAT ROOF TORCH-ON FINISH). LIFE EXPECTANCY OF TORCH-ON BITUMEN FELT: TYPICALLY 15-25 YEARS FOR A WELL-INSTALLED TWO OR THREE-LAYER SBS TORCH-ON SYSTEM — SIGNIFICANTLY SHORTER THAN SARNAFIL G410 OR FIRESTONE RUBBERCOVER EPDM AT 30+ YEARS. THE SHORTER LIFE EXPECTANCY AND THE FIRE RISK OF TORCH APPLICATION (PROXIMITY TO TIMBER DECK AND COMBUSTIBLE ROOF COMPONENTS) ARE THE MAIN REASONS WHY PREMIUM SINGLE-PLY SYSTEMS (SARNAFIL, EPDM) ARE PREFERRED FOR NEW FLAT ROOF WORKS IN LONDON VICTORIAN TERRACE EXTENSIONS.

A FLAT ROOF ON A LONDON VICTORIAN TERRACE REAR EXTENSION OR OUTRIGGER PROVIDES AN OPPORTUNITY FOR THE INSTALLATION OF ROOFLIGHTS (FLAT ROOF FIXED ROOFLIGHTS — SUCH AS THE BRETT MARTIN SKY-FRAME, FAKRO FTT U8 TRIPLE GLAZED FLAT ROOF WINDOW, OR THE KORNICHE ALUMINIUM LANTERN ROOF) TO BRING NATURAL LIGHT INTO THE KITCHEN OR LIVING SPACE BELOW.

FLAT ROOF DRAINAGE (CRITICAL DETAIL — A FLAT ROOF THAT DOES NOT DRAIN FREELY AND QUICKLY WILL POND WATER — PONDING ACCELERATES THE DETERIORATION OF ALL FLAT ROOF MEMBRANES — PARTICULARLY TORCH-ON BITUMEN FELT): A FLAT ROOF MUST HAVE A MINIMUM FALL OF 1:40 (25MM PER METRE) IN THE STRUCTURAL DECK DIRECTION — ACHIEVED EITHER BY: (1) TAPERED INSULATION (THE INSULATION BOARDS ARE SUPPLIED IN TAPERED THICKNESSES — CREATING THE FALL FROM THICKER TO THINNER BOARD ACROSS THE ROOF AREA — ALLOWING A COMPLETELY HORIZONTAL STRUCTURAL DECK — THE PREFERRED SPECIFICATION FOR NEW WARM ROOF FLAT ROOFS IN LONDON VICTORIAN TERRACE EXTENSIONS); OR (2) FIRRINGS (TAPERED TIMBER PIECES FIXED TO THE TOP OF THE FLAT ROOF JOISTS TO CREATE THE FALL BEFORE THE OSB DECK IS FIXED — THE TRADITIONAL METHOD IN TIMBER-JOIST FLAT ROOFS). DRAINAGE OUTLETS: SARNAFIL AND EPDM SYSTEMS INCORPORATE PROPRIETARY DOME OUTLETS (TYPICALLY 75MM OR 110MM DRAINPIPE DIAMETER — INSULATED DOME OUTLET WITH GRAVEL GUARD — CONNECTED TO A 70MM OR 110MM PVC RAINWATER DOWNPIPE — THE OUTLET IS FACTORY-BONDED INTO THE MEMBRANE TO MAINTAIN WATERPROOFNESS AT THE PENETRATION).

FLAT ROOF REPLACEMENT COSTS IN LONDON (2025 — SPECIALIST ROOFING CONTRACTOR OR EXPERIENCED BUILDING CONTRACTOR — SUPPLY AND FIT — INCLUDING REMOVAL OF EXISTING MEMBRANE, NEW OSB DECK IF NEEDED, INSULATION, MEMBRANE, AND DRAINAGE OUTLETS).

SARNAFIL G410 PVC SINGLE PLY (WARM ROOF — 100MM PIR INSULATION + SARNAFIL G410 1.5MM — PER M²): INSULATION (CELOTEX GA4000 100MM): APPROXIMATELY £20-30/M². SARNAFIL G410 1.5MM (SUPPLY): APPROXIMATELY £8-12/M². INSTALLATION LABOUR (SARNAFIL APPROVED INSTALLER): £25-45/M². TOTAL WARM ROOF SARNAFIL G410 (SUPPLY AND FIT — COMPLETE WARM ROOF BUILD-UP): APPROXIMATELY £55-90/M².

FIRESTONE RUBBERCOVER EPDM (WARM ROOF — 100MM PIR INSULATION + EPDM 1.14MM — PER M²): EPDM MEMBRANE (SUPPLY): APPROXIMATELY £5-8/M². INSTALLATION LABOUR: £20-35/M². TOTAL WARM ROOF EPDM: APPROXIMATELY £45-75/M².

BAUDER TORCH-ON FELT (WARM ROOF — 100MM PIR INSULATION + TWO-LAYER BAUDER TORCHON — PER M²): BAUDER TORCHON (SUPPLY): APPROXIMATELY £6-9/M². INSTALLATION LABOUR: £18-30/M². TOTAL WARM ROOF TORCH-ON: APPROXIMATELY £40-70/M².

FOR A TYPICAL LONDON VICTORIAN TERRACE REAR OUTRIGGER FLAT ROOF (25M² — INCLUDING REPLACEMENT OSB DECK, AVCL, PIR INSULATION, AND SARNAFIL G410 MEMBRANE): TOTAL SUPPLY AND FIT: APPROXIMATELY £1,375-2,250 (AT £55-90/M²).