Flat Roof Insulation for London Extensions and Homes: Warm Deck, Cold Deck, Inverted Roof

THE CHOICE OF FLAT ROOF CONSTRUCTION TYPE IS THE MOST FUNDAMENTAL DECISION IN FLAT ROOF SPECIFICATION — AND THE WRONG CHOICE, OR CORRECT CHOICE POORLY EXECUTED, IS THE MOST COMMON CAUSE OF FLAT ROOF FAILURE IN LONDON RESIDENTIAL EXTENSIONS.

WARM DECK FLAT ROOF (THE RECOMMENDED APPROACH FOR LONDON EXTENSIONS): IN A WARM DECK FLAT ROOF, THE THERMAL INSULATION IS PLACED ABOVE THE STRUCTURAL DECK (THE PLYWOOD OR TIMBER DECK ON TOP OF THE ROOF JOISTS) AND BELOW THE WATERPROOFING MEMBRANE. BECAUSE THE INSULATION IS ON THE WARM SIDE OF THE DECK, THE DECK AND ROOF STRUCTURE ARE KEPT WARM — ABOVE THE DEW POINT TEMPERATURE. THIS PREVENTS CONDENSATION FORMING WITHIN THE ROOF BUILDUP. WARM DECK FLAT ROOF IS THE RECOMMENDED AND MOST WIDELY SPECIFIED APPROACH FOR LONDON RESIDENTIAL EXTENSION FLAT ROOFS — IT IS THERMALLY EFFICIENT, ELIMINATES CONDENSATION RISK IN THE STRUCTURAL DECK, AND COMPLIES WITH APPROVED DOCUMENT L.

COLD DECK FLAT ROOF (NOT RECOMMENDED FOR NEW CONSTRUCTION): IN A COLD DECK FLAT ROOF, THE INSULATION IS PLACED BELOW THE STRUCTURAL DECK (INSIDE THE CEILING BUILDUP, BETWEEN AND BELOW THE ROOF JOISTS). THE DECK AND ROOF STRUCTURE ARE ON THE COLD SIDE OF THE INSULATION. THIS MEANS THE DECK IS COLD — AND UNLESS VERY EXTENSIVE CROSS-VENTILATION OF THE VOID ABOVE THE INSULATION IS PROVIDED (50MM MINIMUM VENTILATED AIR GAP TO CROSS-VENTILATED EAVES — DIFFICULT TO ACHIEVE IN LONDON TERRACE EXTENSION LAYOUTS), CONDENSATION WILL FORM ON THE UNDERSIDE OF THE COLD DECK AND EVENTUALLY ROT THE PLYWOOD. COLD DECK FLAT ROOFS ARE NOT RECOMMENDED FOR NEW CONSTRUCTION — APPROVED DOCUMENT C NOTES THAT COLD DECK ROOFS SHOULD NOT BE USED WHERE ADEQUATE VENTILATION CANNOT BE PROVIDED.

INVERTED ROOF (ALSO CALLED UPSIDE-DOWN ROOF): IN AN INVERTED ROOF, THE WATERPROOFING MEMBRANE IS LAID DIRECTLY ON THE STRUCTURAL DECK (AS WITH A WARM DECK), BUT THE INSULATION IS PLACED ABOVE THE MEMBRANE RATHER THAN BELOW IT. THE MEMBRANE IS THEREFORE PROTECTED FROM THERMAL SHOCK (NO LARGE TEMPERATURE SWINGS AS THE INSULATION IS ABOVE IT) — SIGNIFICANTLY EXTENDING MEMBRANE LIFE. THE INSULATION MUST BE MOISTURE-RESISTANT (EXTRUDED POLYSTYRENE XPS — ROOFMATE SL-A, STYRODUR C, OR FLOORMATE 300) AS IT IS EXPOSED ABOVE THE MEMBRANE. INVERTED ROOFS ARE USED ON COMMERCIAL AND AMENITY ROOF TERRACES IN LONDON — LESS COMMON IN DOMESTIC SINGLE-STOREY EXTENSIONS.

FLAT ROOFS IN LONDON RESIDENTIAL BUILDINGS MUST MEET THE THERMAL PERFORMANCE REQUIREMENTS OF APPROVED DOCUMENT L (CONSERVATION OF FUEL AND POWER). THE RELEVANT U-VALUE TARGETS CHANGED WITH THE 2021 EDITION OF APPROVED DOCUMENT L.

PART L 2021 U-VALUE TARGETS FOR FLAT ROOFS (ENGLAND — APPLICABLE TO ALL LONDON EXTENSIONS AND NEW DWELLINGS RECEIVING BUILDING REGULATIONS APPROVAL FROM JUNE 2022): NEW FLAT ROOF (EXTENSION OR NEW DWELLING): U = 0.15 W/M²K (LIMITING FABRIC U-VALUE). RENOVATION OF EXISTING FLAT ROOF (ROOF OF AN EXISTING DWELLING BEING REPAIRED OR REROOFED): U = 0.18 W/M²K.

PIR INSULATION THICKNESS REQUIRED FOR A WARM DECK FLAT ROOF TO ACHIEVE U = 0.15 W/M²K: KINGSPAN THERMAROOF TR27 (LAMBDA 0.027 W/MK): REQUIRED THICKNESS = 140MM (SINGLE LAYER — TECHNICALLY). IN PRACTICE, TWO LAYERS ARE SPECIFIED FOR IMPROVED THERMAL BRIDGING THROUGH JOINTS: 80MM BOTTOM LAYER + 60MM TOP LAYER (JOINTS STAGGERED). CELOTEX XR4000 (LAMBDA 0.022 W/MK): REQUIRED THICKNESS = 110MM SINGLE LAYER (OR 60MM + 50MM IN TWO STAGGERED LAYERS). XTRATHERM THERMACLASS CAVITY 32 (LAMBDA 0.032 W/MK): REQUIRES APPROXIMATELY 170MM FOR U = 0.15 W/M²K — THICKER BUT LOWER COST PER MM THAN KINGSPAN OR CELOTEX.

PRACTICAL IMPLICATION FOR LONDON EXTENSION FLAT ROOFS: FOR A LONDON SINGLE-STOREY REAR EXTENSION WITH FLAT ROOF (COMMON EXTENSION TYPE), THE WARM DECK FLAT ROOF BUILDUP ABOVE THE STRUCTURAL DECK IS: VAPOUR CONTROL LAYER (E.G. FAKRO PROFI FOIL OR SIGA MAJREX VCL) — APPLIED TO TOP OF DECK. 140MM PIR INSULATION (KINGSPAN THERMAROOF TR27 IN TWO STAGGERED LAYERS). BALLASTED LOOSE STONE FINISH (20MM RIVER GRAVEL — PROTECTS MEMBRANE FROM UV), OR FIRESTONE EPDM/SARNAFIL G410 DIRECTLY ON INSULATION (MECHANICALLY FIXED OR FULLY ADHERED).

THE VAPOUR CONTROL LAYER (VCL) IS THE MOST COMMONLY OMITTED OR INCORRECTLY INSTALLED ELEMENT IN WARM DECK FLAT ROOF CONSTRUCTION IN LONDON — AND ITS ABSENCE OR POOR INSTALLATION IS A FREQUENT CAUSE OF PREMATURE ROOF FAILURE.

WHY A VCL IS REQUIRED IN A WARM DECK FLAT ROOF: IN A WARM DECK FLAT ROOF, WARM MOIST AIR FROM THE INTERIOR (TYPICALLY 18 TO 22°C, RELATIVE HUMIDITY 50 TO 70% IN LONDON RESIDENTIAL KITCHENS AND BATHROOMS) RISES TOWARDS THE ROOF STRUCTURE. IF THIS MOIST AIR IS ALLOWED TO REACH THE COLD UNDERSIDE OF THE INSULATION LAYER (OR PENETRATE THROUGH GAPS IN THE INSULATION), IT WILL COOL BELOW ITS DEW POINT AND CONDENSATE. A VCL BELOW THE INSULATION (ABOVE THE STRUCTURAL DECK) PREVENTS MOISTURE-LADEN AIR FROM REACHING THE INSULATION AND CONDENSATING.

VCL SPECIFICATION: MINIMUM VCL PERFORMANCE FOR WARM DECK FLAT ROOF: SD VALUE (EQUIVALENT AIR LAYER THICKNESS) OF AT LEAST 100 METRES. RECOMMENDED PRODUCTS: PROOFATHERM (ROOFING TRADE VCL MEMBRANE — WIDELY USED IN LONDON EXTENSION FLAT ROOFS). FAKRO PROFI FOIL (SD >100M). SIGA MAJREX (SD 19M — INTELLIGENT VAPOUR CONTROL MEMBRANE — CLOSES IN COLD WEATHER, OPENS IN WARM WEATHER TO ALLOW SEASONAL DRYING). CRITICAL VCL INSTALLATION DETAILS: ALL JOINTS MUST BE LAPPED AND TAPED WITH COMPATIBLE TAPE (SIGA SICRALL OR PROOFATHERM-COMPATIBLE TAPE). VCL MUST BE TURNED UP AT WALLS AND SEALED TO THE AIR BARRIER AT JUNCTIONS. ALL SERVICE PENETRATIONS (DOWNLIGHTS, EXTRACT DUCTS) THROUGH THE FLAT ROOF CEILING MUST BE INDIVIDUALLY SEALED.

THE WATERPROOFING MEMBRANE OVER THE FLAT ROOF INSULATION IS THE FINAL ELEMENT THAT COMPLETES THE WARM DECK BUILDUP — AND ITS SPECIFICATION AND INSTALLATION QUALITY IS CRITICAL TO FLAT ROOF LONGEVITY.

EPDM RUBBER MEMBRANE (FIRESTONE RUBBERCOVER, IKO RUBERSHIELD, PERMAROOF — THE MOST WIDELY USED FLAT ROOF MEMBRANE FOR LONDON EXTENSION ROOFS): EPDM (ETHYLENE PROPYLENE DIENE MONOMER) IS A SYNTHETIC RUBBER SINGLE-PLY MEMBRANE AVAILABLE IN WIDTHS UP TO 15.25M — ALLOWING A TYPICAL LONDON EXTENSION FLAT ROOF (5M X 4M) TO BE COVERED IN A SINGLE PIECE WITHOUT SEAMS. SEAM-FREE COVERAGE DRAMATICALLY REDUCES THE RISK OF WATER INGRESS. FIRESTONE RUBBERCOVER EPDM 1.14MM THICKNESS: THE STANDARD SPECIFICATION FOR LONDON RESIDENTIAL EXTENSION FLAT ROOFS — 25-YEAR PRODUCT WARRANTY. EPDM IS UV-STABLE (NO UV DEGRADATION OVER ITS LIFE), FLEXIBLE IN LOW TEMPERATURES (IMPORTANT IN LONDON WINTER CONDITIONS), AND COMPATIBLE WITH BALLASTED AND UNBALLASTED APPLICATIONS. EPDM ADHESIVE APPLICATION: FIRESTONE BONDING ADHESIVE (WATER-BASED CONTACT ADHESIVE) APPLIED TO BOTH THE INSULATION AND THE MEMBRANE BACK; CONTACT BOND — PRESS AND ROLL FIRMLY.

SARNAFIL G410 PVC SINGLE-PLY MEMBRANE (THE ALTERNATIVE PREMIUM ROOF MEMBRANE FOR LONDON EXTENSION PROJECTS): SARNAFIL G410 1.5MM PVC MEMBRANE IS A HOT-AIR WELDED SINGLE-PLY MEMBRANE — JOINTS BETWEEN PANELS ARE HEAT-WELDED (NOT SELF-ADHESIVE), PROVIDING A WELD STRONGER THAN THE PARENT MATERIAL. SARNAFIL G410 IS PARTICULARLY APPROPRIATE WHERE THE FLAT ROOF HAS COMPLEX GEOMETRY (MULTIPLE OUTLETS, PENETRATIONS) OR WHERE A MECHANICALLY FIXED (RATHER THAN FULLY ADHERED) SYSTEM IS PREFERRED.

FLAT ROOFS MUST BE DESIGNED WITH ADEQUATE FALLS TO DRAIN RAINWATER TO OUTLETS OR GUTTERS — STANDING WATER ON A FLAT ROOF ACCELERATES MEMBRANE DEGRADATION AND INCREASES PONDING LOADS.

MINIMUM DESIGN FALLS: BS EN 16237 (DESIGN AND CONSTRUCTION OF FLAT ROOFS WITH FLEXIBLE WATERPROOFING) REQUIRES A MINIMUM DESIGN FALL OF 1:80 (12.5MM PER METRE) FROM THE HIGH POINT TO THE OUTLET. IN PRACTICE, A STRUCTURAL FALL OF 1:40 IS OFTEN SPECIFIED TO ALLOW FOR TOLERANCES IN CONSTRUCTION — AFTER DEFLECTION AND TOLERANCE, A MINIMUM RESIDUAL FALL OF 1:80 IS ACHIEVED. FALLS IN LONDON EXTENSION FLAT ROOFS ARE CREATED BY: TAPERED INSULATION (KINGSPAN THERMAROOF TR PIR IN TAPERED BOARD FORM — TAPERED FROM 25MM MINIMUM AT GUTTER TO FULL THICKNESS AT HIGH POINT, PRE-CUT TO THE REQUIRED GRADIENT). FIRRING PIECES (TIMBER FIRRING PIECES FIXED TO FLAT JOISTS — TAPERED SECTION CREATES THE FALL; THE INSULATION IS THEN LAID FLAT ON THE SLOPED DECK).

ROOF OUTLETS AND GUTTERS: FOR A LONDON SINGLE-STOREY REAR EXTENSION FLAT ROOF DISCHARGING TO A REAR GUTTER: THE GUTTER SHOULD BE SIZED FOR THE ROOF CATCHMENT AREA. MINIMUM 100MM HALF-ROUND FASCIA GUTTER (ALUMINIUM OR UPVC) FOR ROOF WIDTHS UP TO 6M — DOWNPIPE 63MM OR 68MM TO BELOW-GROUND DRAIN.

FLAT ROOF INSULATION COSTS FOR LONDON SINGLE-STOREY REAR EXTENSIONS VARY BASED ON INSULATION TYPE, THICKNESS, AND WATERPROOFING SYSTEM.

KINGSPAN THERMAROOF TR27 140MM PIR INSULATION (SUPPLY, DELIVERED TO LONDON EXTENSION SITE): £35 TO £50 PER M² SUPPLY ONLY. FOR A 20 M² FLAT ROOF: £700 TO £1,000 INSULATION SUPPLY.

CELOTEX XR4000 110MM (SUPPLY): £25 TO £40 PER M².

FULL WARM DECK FLAT ROOF SYSTEM (VCL + 140MM PIR KINGSPAN THERMAROOF + FIRESTONE EPDM 1.14MM — SUPPLY AND INSTALL INCLUDING GUTTERS AND OUTLETS): £75 TO £130 PER M². FOR A 20 M² LONDON EXTENSION FLAT ROOF: £1,500 TO £2,600.

SARNAFIL G410 1.5MM SYSTEM (SUPPLY AND INSTALL): £90 TO £150 PER M² — £1,800 TO £3,000 FOR 20 M².

TAPERED INSULATION SYSTEM (WHERE FALLS ARE CREATED IN INSULATION RATHER THAN FIRRING — KINGSPAN TAPERFIT): ADDITIONAL COST OF £5 TO £15 PER M² OVER FLAT BOARD INSULATION, OFFSET BY SAVING ON FIRRING PIECES AND ASSOCIATED LABOUR.