Flat Roof Types and Membranes for London Victorian Terrace Extensions: Sarnafil, EPDM, GRP, and Felt

BEFORE SELECTING A FLAT ROOF MEMBRANE SYSTEM, THE FLAT ROOF CONSTRUCTION TYPE MUST BE UNDERSTOOD — THE LOCATION OF THE INSULATION LAYER RELATIVE TO THE STRUCTURAL DECK AND THE WATERPROOFING LAYER DETERMINES WHETHER THE ROOF IS A WARM ROOF, COLD ROOF, OR INVERTED ROOF — EACH HAS DIFFERENT CONDENSATION RISK PROFILES AND DIFFERENT SUITABILITY FOR THE MAIN MEMBRANE SYSTEMS.

WARM ROOF (THE CURRENT STANDARD AND RECOMMENDED BUILD-UP FOR NEW-BUILD FLAT ROOFS ON LONDON VICTORIAN TERRACE EXTENSIONS — BUILDING CONTROL REQUIRES THIS APPROACH FOR NEW FLAT ROOF CONSTRUCTION): THE INSULATION IS PLACED ABOVE THE STRUCTURAL DECK AND BELOW THE WATERPROOFING MEMBRANE — SO THE STRUCTURAL DECK AND JOISTS ARE ON THE WARM SIDE OF THE INSULATION — THEY ARE KEPT AT OR ABOVE DEW POINT TEMPERATURE AT ALL TIMES — DRAMATICALLY REDUCING INTERSTITIAL CONDENSATION RISK COMPARED TO A COLD ROOF BUILD-UP; TYPICAL WARM ROOF BUILD-UP (FROM STRUCTURAL DECK UPWARD — MODERN LONDON EXTENSION FLAT ROOF): STRUCTURAL JOISTS (C16 SAWN TIMBER JOISTS — TYPICALLY 195MM OR 220MM DEEP C16 STRUCTURAL TIMBER AT 400MM CENTRES — OR POSI-JOIST OPEN WEB TIMBER FLOOR TRUSSES FOR LONGER SPANS) + STRUCTURAL DECK (18MM OSB3 TONGUE AND GROOVE — GLUED AND NAILED — OR 18MM STRUCTURAL PLYWOOD — TO BS EN 636 CLASS 3) + VAPOUR CONTROL LAYER (VCL — TYPICALLY FAKRO VCL + (OR SIMILAR SELF-ADHESIVE VCL) — APPLIED TO TOP FACE OF DECK, FULLY TAPED AT ALL JOINTS AND LAPS — PREVENTS MOISTURE-LADEN WARM AIR FROM THE INTERIOR RISING INTO THE INSULATION AND CONDENSING) + CONTINUOUS PIR INSULATION BOARD (KINGSPAN THERMAROOF TR27 LPC/FM — 120MM TO 200MM DEPTH DEPENDING ON TARGET U-VALUE AND ROOF JOIST DEPTH — SEE BELOW) + SEPARATING LAYER (GEOTEXTILE FLEECE) + WATERPROOFING MEMBRANE (SARNAFIL G410, FIRESTONE EPDM, GRP, OR SBS FELT — CHOICE DEPENDS ON SPECIFICATION AND BUDGET — SEE BELOW); COLD ROOF (OLDER AND NOW LARGELY SUPERSEDED BUILD-UP — INSULATION BETWEEN THE JOISTS BELOW THE STRUCTURAL DECK — THE DECK IS ON THE COLD SIDE OF THE INSULATION — HIGH INTERSTITIAL CONDENSATION RISK — REQUIRES CROSS-VENTILATION OF THE ROOF VOID AT EAVES — BUILDING CONTROL REQUIRES PROOF THAT ADEQUATE VENTILATION IS MAINTAINED): NOT RECOMMENDED FOR NEW-BUILD FLAT ROOFS — ONLY RELEVANT FOR REPAIR OF EXISTING COLD ROOF CONSTRUCTION; INVERTED ROOF (ALSO CALLED UPSIDE-DOWN ROOF — THE WATERPROOFING MEMBRANE IS BELOW THE INSULATION — THE INSULATION PROTECTS THE MEMBRANE FROM THERMAL SHOCK, UV, AND FOOT TRAFFIC — REQUIRES EXTRUDED POLYSTYRENE INSULATION (ROOFMATE SL-A OR ROOFMATE LG-A — XPS — WHICH IS WATER-RESISTANT UNLIKE PIR) — THE TOP SURFACE IS BALLASTED WITH WASHED RIVER GRAVEL (50MM DEPTH MINIMUM) OR PAVING SLABS (ON PEDESTAL SUPPORTS) — USEFUL FOR GREEN ROOFS OR ROOF TERRACES): TECHNICALLY AN INVERTED ROOF — ONLY APPROPRIATE WHERE THE ROOF TERRACE OR GREEN ROOF USE MAKES IT THE APPROPRIATE BUILD-UP.

SARNAFIL (SIKA SARNAFIL — MANUFACTURED BY SIKA AG, BAAR, SWITZERLAND — PRODUCED IN THE UK AT SIKA'S UK OPERATIONS — THE MARKET-LEADING SINGLE-PLY ROOFING MEMBRANE SYSTEM FOR COMMERCIAL AND HIGH-SPECIFICATION RESIDENTIAL FLAT ROOFING IN THE UK) IS THE PREMIUM SPECIFICATION FOR A SINGLE-PLY PVC (POLYVINYL CHLORIDE) FLAT ROOF MEMBRANE ON A LONDON VICTORIAN TERRACE EXTENSION — WIDELY REGARDED AS THE BEST SINGLE-PLY MEMBRANE SYSTEM AVAILABLE FOR COMMERCIAL AND HIGHER-SPECIFICATION RESIDENTIAL FLAT ROOF APPLICATIONS IN THE UK MARKET.

SARNAFIL G410 SYSTEM (THE CURRENT STANDARD SARNAFIL PRODUCT FOR MECHANICALLY ATTACHED OR FULLY ADHERED FLAT ROOFING — G410 DESIGNATION: G = GLASS FIBRE REINFORCED — 410 = APPROXIMATE PRODUCT MASS g/m²): THICKNESS: G410-12EL (1.2MM — LIGHT DUTY — NOT RECOMMENDED FOR AREAS SUBJECT TO FOOT TRAFFIC); G410-15EL (1.5MM — THE STANDARD SPECIFICATION FOR LONDON RESIDENTIAL EXTENSION FLAT ROOFS — GOOD PUNCTURE RESISTANCE — SUITABLE FOR OCCASIONAL FOOT TRAFFIC DURING MAINTENANCE); G410-20EL (2.0MM — HEAVY DUTY — FOR EXPOSED ROOF TERRACES WITH REGULAR FOOT TRAFFIC OR IN HIGHLY EXPOSED LOCATIONS); COLOUR: STANDARD SUPPLY IN LIGHT GREY — ALSO AVAILABLE IN ANTHRACITE (DARK GREY) — SARNAFIL COLOUR RANGE (BESPOKE COLOURS AVAILABLE TO ORDER — MINIMUM QUANTITIES APPLY); SARNAFIL SYSTEM APPLICATION: THE MEMBRANE IS SUPPLIED IN ROLLS (TYPICALLY 1.8M OR 2.0M WIDE) — LAID OVER THE PIR INSULATION + GEOTEXTILE FLEECE ON THE WARM ROOF DECK — MECHANICALLY FASTENED (METAL FASTENING BARS THROUGH THE INSULATION TO THE STRUCTURAL DECK AT LAPS — THE SARNAFIL MEMBRANE IS THEN HOT-AIR WELDED OVER THE FASTENING BARS AT ALL LAPS) OR FULLY ADHERED (BONDED DIRECTLY TO THE INSULATION WITH SIKA SARNACOL ADHESIVE — USED WHERE MECHANICAL FASTENING IS NOT POSSIBLE OR WHERE A MORE SECURE BOND IS REQUIRED); ALL LAPS AND JOINTS ARE HOT-AIR WELDED USING A LEISTER TRIAC AUTOMATIC HOT-AIR WELDER OR LEISTER TRIAC HAND WELDER AT A MINIMUM LAP OF 40MM — PRODUCING A FULLY WATERTIGHT, HOMOGENEOUS THERMOPLASTIC WELD — WELD STRENGTH APPROACHES THE TENSILE STRENGTH OF THE MEMBRANE; SARNAFIL UPSTANDS AND DETAILS: AT ALL ABUTMENTS (EXISTING REAR WALL OF PROPERTY, PARAPET WALLS, CHIMNEY STACKS): THE SARNAFIL MEMBRANE IS TURNED UP A MINIMUM 150MM ABOVE THE FINISHED ROOF LEVEL — THE UPSTAND IS HOT-AIR WELDED TO A SARNAFIL REINFORCING STRIP (SIKA SARNAFIL S73 ELF OR SIMILAR DETAIL MEMBRANE) — CAPPED WITH A SARNAFIL OVER-FLASHING OR SIKA SIKAFLEX SEALANT AT THE TOP EDGE — MEETING THE REQUIREMENTS OF BS 8747 AND NHBC CHAPTER 7.1; GUARANTEE: SARNAFIL G410 INSTALLED BY AN APPROVED SARNAFIL CONTRACTOR (SIKA SARNAFIL APPROVED ROOFING CONTRACTOR — LISTED ON SIKA'S UK WEBSITE) CARRIES A SIKA 20-YEAR SYSTEM GUARANTEE (COVERING MEMBRANE, ACCESSORIES, AND WORKMANSHIP — A MAJOR ADVANTAGE OVER GENERIC SINGLE-PLY OR FELT ROOFING PRODUCTS WHERE SYSTEM GUARANTEES ARE SHORTER OR NOT AVAILABLE).

EPDM (ETHYLENE PROPYLENE DIENE MONOMER — A SYNTHETIC RUBBER MEMBRANE — THE MAIN MARKET COMPETITORS TO SARNAFIL PVC IN THE UK RESIDENTIAL FLAT ROOFING MARKET) AND GRP (GLASS REINFORCED POLYESTER — FIBREGLASS FLAT ROOFING — THE MOST COMMON FLAT ROOF SYSTEM FOR LOWER-BUDGET RESIDENTIAL EXTENSIONS IN THE UK) ARE TWO FURTHER COMMON FLAT ROOF MEMBRANE OPTIONS FOR LONDON VICTORIAN TERRACE EXTENSIONS.

EPDM RUBBER MEMBRANE (MAIN UK PRODUCTS: FIRESTONE RUBBERCOVER — FIRESTONE BUILDING PRODUCTS LLC, INDIANAPOLIS USA — THE MARKET LEADER IN EPDM ROOFING FOR UK RESIDENTIAL; CARLISLE SYNTEC EPDM; CLASSICBOND EPDM — ICOPAL UK; RUBBER4ROOFS — UK EPDM DISTRIBUTOR): THICKNESS: TYPICALLY 1.14MM (FIRESTONE RUBBERCOVER 1.14MM — THE STANDARD UK RESIDENTIAL SPECIFICATION) — ALSO AVAILABLE 1.52MM (GREATER THICKNESS — BETTER PUNCTURE RESISTANCE); SYSTEM: EPDM MEMBRANE IS SUPPLIED IN LARGE SHEETS (UP TO 15.25M X 61M IN FIRESTONE RUBBERCOVER — ENABLING WHOLE-ROOF COVERAGE WITHOUT SEAMS ON ALL BUT THE LARGEST ROOFS — ELIMINATING THE MAIN WEAK POINT IN TRADITIONAL FELT ROOFING) — FULLY ADHERED TO THE INSULATION BOARD USING BONDING ADHESIVE (FIRESTONE WATER-BASED BONDING ADHESIVE — APPLIED TO BOTH MEMBRANE AND SUBSTRATE); WHERE SEAMS ARE REQUIRED: EPDM SEAM TAPE (SELF-ADHESIVE BUTYL TAPE — TYPICALLY 75MM WIDE — APPLIED WITH A ROLLER) — NOTE: EPDM SEAM ADHESION IS TYPICALLY NOT AS STRONG AS A FULLY WELDED PVC SARNAFIL LAP — THE QUALITY OF EPDM SEAM TAPING IS MORE DEPENDENT ON WORKMANSHIP QUALITY AND SURFACE PREPARATION THAN SARNAFIL HOT-AIR WELDING; EPDM ADVANTAGES: EXCELLENT UV RESISTANCE (EPDM IS INHERENTLY UV RESISTANT — DOES NOT REQUIRE UV-PROTECTIVE COATING — LONG PROVEN TRACK RECORD FOR UV DURABILITY); EXCELLENT FLEXIBILITY AT LOW TEMPERATURES (EPDM REMAINS FLEXIBLE DOWN TO -45°C — CRITICAL FOR THERMAL MOVEMENT IN COLD WEATHER); COST (LOWER INSTALLED COST THAN SARNAFIL FOR SMALLER ROOFS); EPDM DISADVANTAGES: TYPICALLY SUPPLIED IN BLACK ONLY (GREY ALTERNATIVES ARE AVAILABLE BUT LESS COMMON THAN SARNAFIL COLOUR OPTIONS); SOLVENTED ADHESIVE VERSIONS (OLDER EPDM SYSTEMS USED SOLVENT-BASED BONDING ADHESIVE — MODERN WATER-BASED ALTERNATIVES HAVE ADDRESSED THIS); GRP (GLASS REINFORCED POLYESTER — FIBREGLASS FLAT ROOFING — A COMPOSITE SYSTEM WHERE WOVEN GLASS FIBRE MATTING IS SATURATED IN CATALYSED POLYESTER RESIN ON SITE AND CURED IN PLACE TO FORM A RIGID MONOLITHIC WATERPROOFING LAYER): SYSTEM: GRP TRIM BOARDS (FIBREGLASS ANGLE TRIM AT ALL ABUTMENTS AND PERIMETERS) FIXED TO SUBSTRATE — WET-ON-WET APPLICATION OF RESIN-SATURATED GLASS FIBRE MATTING TO SUBSTRATE — TOPCOAT APPLIED — PRODUCES A RIGID, SEAMLESS, JOINTLESS LAYER; GRP ADVANTAGES: SEAMLESS (NO LAPS OR SEAMS — THE MAIN FAILURE POINT IN OTHER FLAT ROOF SYSTEMS — STRONG RESISTANCE TO WATER INGRESS AT DETAILS); SUITABLE FOR COMPLEX SHAPES AND CURVED DETAILS; AESTHETICALLY CLEAN AND SMOOTH; GRP DISADVANTAGES: BRITTLE (MORE PRONE TO CRACKING UNDER IMPACT OR MOVEMENT AT JUNCTIONS WITH DISSIMILAR MATERIALS THAN FLEXIBLE MEMBRANE SYSTEMS — REQUIRES A VERY STABLE, RIGID SUBSTRATE — MOVEMENT IN THE SUBSTRATE WILL CAUSE GRP CRACKING); TEMPERATURE SENSITIVITY (POLYESTER RESIN CURE IS SENSITIVE TO TEMPERATURE — SHOULD NOT BE APPLIED BELOW 5°C OR ABOVE 25°C — RESTRICTS INSTALLATION IN COLD LONDON WINTERS); ODOUR (STYRENE MONOMER IN POLYESTER RESIN PRODUCES STRONG ODOUR DURING APPLICATION — REQUIRES ADEQUATE VENTILATION AND NOTIFYING NEIGHBOURS IN A TERRACE SETTING).

TORCH-ON SBS (STYRENE-BUTADIENE-STYRENE) MODIFIED BITUMEN FELT (ALSO CALLED TORCH-ON FELT OR SBS MODIFIED BITUMEN MEMBRANE — A TWO-LAYER OR THREE-LAYER SYSTEM WHERE LAYERS OF SBS-MODIFIED BITUMEN FELT ARE TORCH-FUSED TO EACH OTHER AND TO THE SUBSTRATE USING A PROPANE TORCH — PRODUCING A HOT-FUSED CONTINUOUS WATERPROOFING LAYER) IS THE TRADITIONAL FLAT ROOF MEMBRANE SYSTEM AND CONTINUES TO BE USED IN LONDON, PARTICULARLY FOR BUDGET-SENSITIVE RESIDENTIAL EXTENSION PROJECTS.

SBS TORCH-ON FELT SYSTEM (TYPICAL TWO-LAYER SPECIFICATION FOR LONDON RESIDENTIAL EXTENSION): BONDING LAYER (BASE LAYER — FIRST PLY): SOPREMA ELASTOPHENE 25 (SOPREMA GROUP — STRASBOURG FRANCE — THE MAIN EUROPEAN SBS FELT MANUFACTURER) OR BAUDER TORCHON 4KG (BAUDER LTD UK) OR IKO PERMATORCH (IKO GROUP — MANUFACTURER UK AND CANADA) — 3MM-4MM THICKNESS — TORCH-APPLIED DIRECTLY TO THE PRIMED PIR INSULATION BOARD SURFACE (USING SOPREMA ALSAN FLASHING PRIMER OR BAUDER BITUMEN PRIMER); CAP SHEET (TOP LAYER — SECOND PLY): SOPREMA ELASTOPHENE FLAM PLUS (4MM — SLATE/MINERAL GRANULE FINISH — THE STANDARD SPECIFICATION FOR AN EXPOSED SBS CAP SHEET — THE MINERAL GRANULE SURFACE PROVIDES UV PROTECTION AND REDUCES THERMAL MASS OF THE MEMBRANE RELATIVE TO AN EXPOSED BLACK BITUMEN SURFACE) OR BAUDER FLEXIFLEX PLUS; SBS FELT ADVANTAGES: LOWER INSTALLED COST THAN PVC SARNAFIL OR EPDM ON SMALLER ROOFS; WELL-UNDERSTOOD BY GENERAL ROOFING CONTRACTORS (WIDER INSTALLER BASE THAN SPECIALIST SINGLE-PLY PVC); TORCH-FUSED LAPS ARE MORE RELIABLE THAN COLD-BOND ADHESIVE LAPS WHEN PROPERLY EXECUTED; SBS FELT DISADVANTAGES: SHORTER SERVICE LIFE THAN HIGH-QUALITY PVC SARNAFIL (TYPICALLY 10-20 YEARS FOR A GOOD TWO-LAYER SBS SYSTEM VS 25-30+ YEARS CLAIMED FOR SARNAFIL G410 WITH 20-YEAR GUARANTEE); FIRE RISK DURING INSTALLATION (PROPANE TORCH ON A COMBUSTIBLE SUBSTRATE — REQUIRES FIRE WATCH AND FIRE BLANKET DURING APPLICATION — MORE RISK IN AN OCCUPIED TERRACE SETTING); PERFORMANCE DEPENDENT ON INSTALLER QUALITY; WARM ROOF THERMAL SPECIFICATION — PART L 2021 TARGETS FOR A NEW FLAT ROOF: THE BUILDING REGULATIONS PART L 2021 NOTIONAL U-VALUE BACKSTOP FOR A NEW FLAT ROOF IS 0.15 W/M²K MAXIMUM (U-VALUE = THE RATE OF HEAT TRANSFER THROUGH A BUILDING ELEMENT — LOWER IS BETTER) — THIS REQUIRES APPROXIMATELY 120-150MM DEPTH OF PIR INSULATION (KINGSPAN THERMAROOF TR27 LPC/FM AT 120MM = U-VALUE APPROXIMATELY 0.16 W/M²K WITH TYPICAL TIMBER JOIST CORRECTION — 140MM THERMAROOF TR27 = APPROXIMATELY 0.14 W/M²K); INSULATION CORRECTION FACTOR FOR JOIST BRIDGING (TIMBER THERMAL BRIDGING): WHEN INSULATION IS ONLY BETWEEN JOISTS (COLD ROOF) THE EFFECTIVE U-VALUE IS SIGNIFICANTLY WORSE THAN THE INSULATION MATERIAL U-VALUE ALONE — IN A WARM ROOF WHERE THE INSULATION IS CONTINUOUS ABOVE THE DECK, THE ONLY THERMAL BRIDGING IS AT THE FASTENING POINTS OF THE MECHANICAL FASTENERS (RELATIVELY MINOR) — WARM ROOF SIGNIFICANTLY OUTPERFORMS COLD ROOF FOR SAME INSULATION DEPTH.

FLAT ROOF SUPPLY AND FIT COSTS FOR LONDON VICTORIAN TERRACE SINGLE-STOREY REAR EXTENSION (LONDON 2025 — APPROXIMATE RANGE — SUPPLY AND FIT — SPECIALIST FLAT ROOFING CONTRACTOR — MEMBRANE AND INSULATION ONLY — EXCLUDES STRUCTURAL DECK, JOISTS, PARAPET WALLS, ROOFLIGHTS).

SARNAFIL G410 SINGLE-PLY PVC WARM ROOF (APPROVED SIKA SARNAFIL CONTRACTOR — 1.5MM G410-15EL GREY — 120MM KINGSPAN THERMAROOF TR27 PIR INSULATION — GEOTEXTILE FLEECE — VCL — MECHANICAL FASTENING OR FULL BOND — WELDED LAPS AND UPSTANDS — 20-YEAR SIKA GUARANTEE): APPROXIMATELY £80-130 PER M² (SUPPLY AND FIT — MEMBRANE, INSULATION, ACCESSORIES, UPSTANDS, TRIMS — MINIMUM CHARGE APPLIES — SMALLER ROOFS HIGHER PER-M² RATE DUE TO FIXED MOBILISATION AND SETUP COSTS). EPDM RUBBER (FIRESTONE RUBBERCOVER 1.14MM — WARM ROOF — 120MM PIR INSULATION — WATER-BASED BONDING ADHESIVE FULLY ADHERED): APPROXIMATELY £55-90 PER M² (SUPPLY AND FIT — MEMBRANE, INSULATION, ACCESSORIES — SMALLER ROOFS HIGHER PER-M² RATE). GRP FIBREGLASS (TWO-COAT GRP SYSTEM — WARM ROOF SUBSTRATE — GRP ANGLE TRIM — RESIN AND GLASS FIBRE MATTING — TOPCOAT — MINERAL AGGREGATE OR SMOOTH FINISH): APPROXIMATELY £50-85 PER M² (SUPPLY AND FIT — INCLUDING SUBSTRATE PRIMER, RESIN, MAT, TOPCOAT — COMPLEX SHAPES AND MANY DETAILS INCREASE COST). TORCH-ON SBS TWO-LAYER FELT (TWO-PLY SBS SYSTEM — WARM ROOF — PRIMER + ELASTOPHENE 25 BASE LAYER + ELASTOPHENE FLAM PLUS MINERAL CAP SHEET): APPROXIMATELY £40-70 PER M² (SUPPLY AND FIT — MORE CONTRACTORS AVAILABLE FOR SBS FELT — LOWER BASELINE COST BUT SHORTER EXPECTED SERVICE LIFE THAN SARNAFIL). TOTAL FLAT ROOF COST FOR A TYPICAL 20M² SINGLE-STOREY LONDON REAR EXTENSION (MEMBRANE + INSULATION ONLY): SARNAFIL: APPROXIMATELY £1,600-2,600 (20M²); EPDM: APPROXIMATELY £1,100-1,800 (20M²); GRP: APPROXIMATELY £1,000-1,700 (20M²); SBS FELT: APPROXIMATELY £800-1,400 (20M²). ROOFLIGHTS (VELUX OR FAKRO FLAT ROOF WINDOWS — ADDITIONAL COST): VELUX CFU FIXED FLAT ROOF WINDOW 60X60CM TRIPLE GLAZED: APPROXIMATELY £350-550 SUPPLY — APPROXIMATELY £200-350 FIT; VELUX CVP OPAQUE FLAT ROOF WINDOW (WALK-ON) 60X60CM: APPROXIMATELY £1,200-2,000 SUPPLY + FIT. GREEN ROOF (SEDUM BLANKET ON WARM ROOF DECK — ADDITIONAL TO MEMBRANE — SUB-STRATE AND SEDUM BLANKET): APPROXIMATELY £50-90 PER M² ADDITIONAL (FOR SEDUM BLANKET SYSTEM) — STRUCTURAL ASSESSMENT OF ROOF DECK FOR ADDITIONAL LOAD REQUIRED.