Composite Decking for London Victorian Terrace Gardens: Specification and Installation

THE UK COMPOSITE DECKING MARKET IS DOMINATED BY SEVERAL WELL-ESTABLISHED BRANDS — EACH WITH DIFFERENT BOARD PROFILES, SURFACE TEXTURES, COLOUR RANGES, AND WARRANTY TERMS.

TREX TRANSCEND (THE MARKET-LEADING COMPOSITE DECKING BRAND IN THE UK AND USA): TREX TRANSCEND BOARDS ARE MANUFACTURED FROM 95% RECYCLED CONTENT (WOOD FLOUR AND POLYETHYLENE FILM — TYPICALLY RECLAIMED CARRIER BAGS AND INDUSTRIAL PLASTIC FILM). BOARD PROFILE: GROOVED EDGE (FOR HIDDEN FIXING WITH TREX HIDEAWAY FASTENER SYSTEM) OR SOLID EDGE (FOR FACE-SCREW FIXING). BOARD DIMENSIONS: TREX TRANSCEND GROOVED: 139MM WIDE X 25MM DEPTH. SPAN RATING: MAXIMUM 406MM (16 INCH) JOIST SPACING FOR 90° BOARD INSTALLATION OR 305MM FOR DIAGONAL INSTALLATION. COLOUR RANGE: TIKI TORCH (WARM CEDAR), ISLAND MIST (GREY-GREEN), SPICED RUM (WARM BROWN), LAVA ROCK (DARK CHARCOAL). WARRANTY: 25-YEAR FADE AND STAIN WARRANTY (TREX RESIDENTIAL WARRANTY). TREX TRANSCEND COMPOSITE PRICE: £40 TO £55 PER M² BOARD SUPPLY ONLY (EX-DISTRIBUTOR).

MILLBOARD (PREMIUM UK COMPOSITE DECKING — MOST REALISTIC TIMBER APPEARANCE): MILLBOARD IS A PREMIUM COMPOSITE DECKING BRAND MANUFACTURED IN THE UK BY OLDBURY BOARDS. MILLBOARD ENHANCED GRAIN (POLYMER-COATED MINERAL COMPOSITE — DURABLE, SOFTER TEXTURE THAN STANDARD COMPOSITE). BOARD DIMENSIONS: 176MM WIDE X 32MM DEEP. JOIST SPACING: MAXIMUM 400MM CENTRES. REALISTIC WOOD GRAIN TEXTURE AND COLOUR VARIATION — AVAILABLE IN COPPERED OAK, LIMED OAK, DRIFTWOOD GREY, ANTIQUE OAK. WARRANTY: 25-YEAR PRODUCT WARRANTY. MILLBOARD PRICE: £80 TO £110 PER M² BOARD SUPPLY ONLY — THE PREMIUM UK COMPOSITE BOARD.

FIBERON SYMMETRY (MID-RANGE COMPOSITE — POPULAR WITH LONDON LANDSCAPING CONTRACTORS): FIBERON SYMMETRY BOARDS: 140MM WIDE X 25MM DEEP, GROOVED EDGE, HIDDEN FASTENER COMPATIBLE. LIFETIME WARRANTY AGAINST FADING, STAINING, AND MOULD. COLOUR RANGE: ARCTIC WHITEFISH (VERY LIGHT GREY), HAZEL, IPE (DARK BROWN). FIBERON PRICE: £35 TO £50 PER M² BOARD SUPPLY ONLY.

TIMBERTECH AZEK (PREMIUM PVC-CAPPED COMPOSITE — UV RESISTANT): TIMBERTECH AZEK IS A PVC-CAPPED COMPOSITE BOARD — THE POLYMER OUTER LAYER IS A SOLID PVC CAP (RATHER THAN POLYMER BLENDED INTO THE WOOD FLOUR) — PROVIDING SUPERIOR STAIN AND MOISTURE RESISTANCE. BOARD: 140MM WIDE X 25MM DEEP. PRICE: £55 TO £75 PER M² SUPPLY ONLY.

THE PERFORMANCE AND LONGEVITY OF A COMPOSITE DECK IN A LONDON GARDEN DEPENDS SIGNIFICANTLY ON THE QUALITY OF THE SUBFRAME — THE STRUCTURAL JOIST SYSTEM THAT SUPPORTS THE COMPOSITE BOARDS.

JOIST SPACING: COMPOSITE DECKING BOARDS TYPICALLY HAVE A MAXIMUM SPANNING CAPABILITY OF 300MM TO 406MM (DEPENDING ON BOARD PROFILE AND INSTALLATION ANGLE). THE STANDARD UK COMPOSITE DECK JOIST SPACING IS 400MM CENTRES (FOR STRAIGHT INSTALLATION) AND 300MM CENTRES (FOR DIAGONAL INSTALLATION — DECKING BOARDS LAID AT 45° TO THE JOIST DIRECTION, WHICH IS POPULAR IN LONDON GARDEN DESIGN FOR VISUAL INTEREST AND SPACE PERCEPTION).

JOIST MATERIAL OPTIONS: TIMBER JOISTS (TREATED SOFTWOOD — C16 KILN-DRIED REGULARISED — THE MOST COMMON CHOICE FOR LONDON GARDEN DECKING SUBFRAME): PRESSURE-TREATED SOFTWOOD JOISTS (TANALISED OR OSMOSE-TREATED TO UC4A HAZARD CLASS) AT 47MM X 100MM (2"|X4") OR 47MM X 150MM (2"X6") DEPENDING ON SPAN. ADVANTAGES: LOW COST, EASY TO CUT AND FIX, WIDELY AVAILABLE. DISADVANTAGE: STILL SUBJECT TO SOME TIMBER MOVEMENT AND LONG-TERM DECAY RISK (EVEN TREATED — INSPECTION EVERY 3 TO 5 YEARS ADVISABLE). COMPOSITE OR ALUMINIUM JOISTS: TREX ELEVATIONS STEEL SUBFRAME (HOT-DIP GALVANISED STEEL JOIST SYSTEM DESIGNED FOR USE WITH TREX COMPOSITE BOARDS — LONG-TERM MAINTENANCE-FREE SUBFRAME) OR EASYDECK COMPOSITE JOIST (HOLLOW COMPOSITE JOIST — MOISTURE-RESISTANT, WILL NOT ROT): MORE EXPENSIVE BUT ELIMINATES THE SUBFRAME MAINTENANCE CONCERN.

GROUND CLEARANCE: A MINIMUM OF 50MM CLEAR AIRWAY BETWEEN THE UNDERSIDE OF THE COMPOSITE BOARDS AND THE GROUND SURFACE IS REQUIRED — THIS ALLOWS DRAINAGE AND AIR CIRCULATION, PREVENTING MOISTURE ACCUMULATION UNDER THE BOARDS (WHICH WOULD ENCOURAGE ALGAE ON THE UNDERSIDE AND ACCELERATE JOIST TIMBER DECAY).

DECK FEET AND PEDESTALS: WHERE THE GROUND LEVEL IS UNEVEN (COMMON IN LONDON REAR GARDENS WITH SLOPING PROFILES OR RAISED AREAS), ADJUSTABLE DECK FEET (BUZON DPH ADJUSTABLE PEDESTAL 25MM TO 700MM HEIGHT) OR COMPOSITE BLOCK BEARERS ALLOW THE SUBFRAME TO BE LEVELLED WITHOUT EXCAVATION OR CONCRETE WORK.

COMPOSITE DECKING BOARDS IN A LONDON REAR GARDEN SHOULD BE INSTALLED WITH A HIDDEN FIXING SYSTEM — FACE-SCREWING COMPOSITE BOARDS IS A COMMON AMATEUR INSTALLATION ERROR THAT LEAVES SCREW HEADS VISIBLE ON THE BOARD SURFACE, CREATES WATER INGRESS POINTS AT THE SCREW HOLES, AND DOES NOT ALLOW FOR THERMAL EXPANSION OF THE COMPOSITE BOARD.

HIDDEN FIXING SYSTEMS: TREX HIDEAWAY FASTENER (STAINLESS STEEL OR CARBON STEEL CLIP — DESIGNED FOR TREX GROOVED-EDGE BOARDS): THE HIDEAWAY CLIP CLIPS INTO THE GROOVE ON THE BOARD EDGE AND IS SCREWED TO THE TOP OF THE JOIST BETWEEN EACH BOARD — HOLDING THE BOARD DOWN WHILE LEAVING THE SURFACE COMPLETELY SCREW-FREE. FIBERON PHANTOM HIDDEN FASTENER (SIMILAR CLIP SYSTEM FOR FIBERON GROOVED BOARDS). MILLBOARD FIXED FASTENER (FACE-FIX COLOUR-MATCHED SCREW THROUGH THE BOARD SURFACE — MILLBOARD BOARDS DO NOT HAVE GROOVED EDGES): FOR MILLBOARD, THE INSTALLATION USES A STAINLESS STEEL COLOUR-MATCHED SCREW (MILLBOARD COLOUR MATCHED FIXING SCREW) THROUGH THE BOARD SURFACE AT AN ANGLE OF 45° INTO THE JOIST — MINIMISING VISIBILITY.

BOARD GAP (EXPANSION JOINT): A 6MM TO 8MM GAP BETWEEN ADJACENT BOARDS IS MAINTAINED BY THE HIDDEN FIXING CLIP SPACING — THIS GAP ALLOWS: THERMAL EXPANSION OF THE COMPOSITE BOARD (COMPOSITE BOARDS EXPAND APPROXIMATELY 1.5MM PER LINEAR METRE FOR A 25°C TEMPERATURE RISE). DRAINAGE OF RAINFALL THROUGH THE DECK SURFACE TO THE GROUND BELOW (IMPORTANT IN LONDON'S WET CLIMATE — WATER MUST NOT POND ON THE DECK SURFACE). END GAPS: A 6MM GAP MUST ALSO BE MAINTAINED AT THE BOARD ENDS (AGAINST WALLS, FASCIA BOARDS, OR STRUCTURES) TO ALLOW END EXPANSION.

GARDEN DECKING IN A LONDON VICTORIAN TERRACE REAR GARDEN IS GENERALLY WITHIN PERMITTED DEVELOPMENT (CLASS E GPDO 2015) — BUT THERE ARE HEIGHT AND AREA LIMITATIONS THAT MUST BE OBSERVED.

PERMITTED DEVELOPMENT RULES FOR GARDEN DECKING (CLASS E GPDO 2015): DECKING IS PERMITTED DEVELOPMENT (NO PLANNING APPLICATION REQUIRED) PROVIDED: THE DECK IS WITHIN THE CURTILAGE (GARDEN) OF A SINGLE DWELLINGHOUSE. THE DECK IS NOT MORE THAN 300MM ABOVE NATURAL GROUND LEVEL (MEASURED FROM THE LOWEST ADJACENT GROUND LEVEL TO THE TOP OF THE DECK SURFACE). THE DECK DOES NOT OCCUPY MORE THAN 50% OF THE TOTAL AREA OF THE GARDEN (MEASURED FROM THE REAR WALL OF THE HOUSE EXCLUDING ANY SIDE EXTENSION).

WHEN PLANNING PERMISSION IS REQUIRED FOR LONDON DECKING: DECK HEIGHT EXCEEDS 300MM ABOVE GROUND LEVEL (COMMON ON SLOPING NORTH OR SOUTH LONDON GARDENS WHERE THE GARDEN DROPS AWAY FROM THE HOUSE). IN LONDON CONSERVATION AREAS — DECKING IN FRONT GARDENS (VISIBLE FROM HIGHWAY) REQUIRES CONSENT. DECKING THAT COVERS MORE THAN 50% OF THE REAR GARDEN AREA REQUIRES PLANNING APPLICATION.

REAR GARDEN DECKING IN LONDON CONSERVATION AREAS: PERMITTED DEVELOPMENT RIGHTS FOR GARDEN DECKING ARE NOT REMOVED BY CONSERVATION AREA STATUS (CONSERVATION AREAS AFFECT DEMOLITION AND CLADDING OF PRINCIPAL ELEVATIONS — NOT TYPICALLY REAR GARDEN DECKING). HOWEVER, IF THE DECK IS ELEVATED (ABOVE 300MM) AND VISIBLE FROM A PUBLIC AREA OR OVERLOOKING NEIGHBOUR, LOCAL PLANNING AUTHORITIES MAY REQUEST A PERMITTED DEVELOPMENT CERTIFICATE BEFORE WORK BEGINS.

COMPOSITE DECKING IN A LONDON REAR GARDEN REQUIRES A DRAINAGE STRATEGY — BOTH FOR THE DECK SURFACE AND FOR THE WATER THAT DRAINS THROUGH THE BOARD GAPS TO THE GROUND BELOW.

SURFACE DRAINAGE: THE 6-8MM BOARD GAPS IN A COMPOSITE DECK ALLOW RAINFALL TO DRAIN THROUGH THE DECK SURFACE TO THE GROUND BELOW — THE DECK SURFACE SHOULD ALSO BE INSTALLED WITH A MINIMUM 1:100 FALL (1CM PER METRE) TO PREVENT PONDING ON THE BOARD SURFACE. WATER PONDING ON A COMPOSITE DECK SURFACE ENCOURAGES ALGAE AND BIOFILM GROWTH — THIS IS THE MAIN MAINTENANCE REQUIREMENT FOR COMPOSITE DECKING IN LONDON'S HUMID, SHADED GARDEN ENVIRONMENTS.

GROUND DRAINAGE UNDER THE DECK: THE SOIL OR PAVING UNDER A COMPOSITE DECK MUST HAVE ADEQUATE DRAINAGE. IF THE GROUND UNDER THE DECK IS COMPACTED SOIL OR LONDON CLAY (WHICH DRAINS POORLY), CONSIDER: HARDCORE BASE (100MM COMPACTED TYPE 1 AGGREGATE) UNDER THE JOIST SUBFRAME — PROVIDES FREE-DRAINING SURFACE. GRAVEL FILL (20MM CLEAN SHINGLE) SPREAD OVER THE GROUND UNDER THE DECK — PREVENTS WEED GROWTH AND IMPROVES DRAINAGE.

MAINTENANCE REQUIREMENTS: COMPOSITE DECKING (TREX TRANSCEND, FIBERON SYMMETRY, MILLBOARD) REQUIRES ANNUAL CLEANING WITH A COMPOSITE DECK CLEANER (TREX DECK CLEANER OR SIMPLE GREEN COMPOSITE DECK CLEANER — APPLIED BY BRUSH OR LOW-PRESSURE HOSE) TO REMOVE ALGAE, BIOFILM, AND ORGANIC DEBRIS FROM THE BOARD SURFACE. THIS TYPICALLY TAKES 1 TO 2 HOURS PER YEAR FOR A 20 M² DECK — A SIGNIFICANTLY LOWER MAINTENANCE COMMITMENT THAN NATURAL HARDWOOD DECKING (WHICH REQUIRES ANNUAL SANDING, OILING, AND PERIODIC TREATMENT).

COMPOSITE DECKING COSTS IN LONDON INCLUDE MATERIAL (BOARDS, SUBFRAME JOISTS, HIDDEN FIXINGS, FASCIA BOARDS) AND INSTALLATION LABOUR.

BOARD COSTS (SUPPLY ONLY): FIBERON SYMMETRY OR ENTRY-LEVEL COMPOSITE: £35 TO £50 PER M². TREX TRANSCEND: £40 TO £55 PER M². TIMBERTECH AZEK (PVC-CAPPED): £55 TO £75 PER M². MILLBOARD ENHANCED GRAIN (PREMIUM): £80 TO £110 PER M².

SUBFRAME COST (TREATED SOFTWOOD JOISTS + CONCRETE BEARER BLOCKS OR ADJUSTABLE DECK FEET — SUPPLY AND INSTALL): £15 TO £25 PER M².

HIDDEN FIXINGS (SUPPLY — PER M² OF DECK): £4 TO £8 PER M².

INSTALLATION LABOUR (CUTTING, FITTING, FASCIA BOARDING — DECK INSTALLATION PER M²): £20 TO £35 PER M².

TOTAL INSTALLED COST PER M² (MATERIAL + LABOUR): FIBERON OR ENTRY-LEVEL: £74 TO £118 PER M². TREX TRANSCEND: £79 TO £123 PER M². MILLBOARD PREMIUM: £119 TO £178 PER M².

TYPICAL LONDON REAR GARDEN DECKING PROJECT (20 M² COMPOSITE DECK WITH TREX TRANSCEND — INCLUDING SUBFRAME, HIDDEN FIXINGS, FASCIA BOARDS, AND TWO STEPS FROM THE REAR DOOR): £1,800 TO £2,800 MATERIAL + £800 TO £1,400 LABOUR = £2,600 TO £4,200 FULLY INSTALLED.