External Cladding for London Homes: Fibre Cement, Timber, and Rainscreen Systems Guide

A CORRECTLY DESIGNED CLADDING SYSTEM IS ALWAYS A RAINSCREEN — THE CLADDING PANEL IS NOT SEALED AGAINST THE STRUCTURE BENEATH IT BUT IS SEPARATED BY A VENTILATED CAVITY OF MINIMUM 25MM CLEAR WIDTH. THIS CAVITY SERVES MULTIPLE FUNCTIONS.

FIRST, DRAINAGE: ANY RAINWATER THAT PENETRATES BEHIND THE OUTER CLADDING (THROUGH JOINTS, FIXINGS, OR IMPACT) CAN DRAIN DOWN THE BACK OF THE PANEL THROUGH THE CAVITY AND EXIT AT THE BASE THROUGH A VENTILATED CAVITY TRAY, RATHER THAN BEING TRAPPED AGAINST THE STRUCTURE.

SECOND, VENTILATION: AIR CIRCULATES UPWARD THROUGH THE CAVITY (DRIVEN BY THERMAL BUOYANCY), DRYING ANY RESIDUAL MOISTURE AND PREVENTING CONDENSATION BUILDUP ON THE INNER FACE OF THE CLADDING OR THE OUTER FACE OF THE STRUCTURE.

THIRD, THERMAL BUFFER: THE VENTILATED AIR LAYER REDUCES THE TEMPERATURE DIFFERENTIAL BETWEEN THE OUTER CLADDING SURFACE (WHICH MAY REACH 60'C IN SUMMER SUN) AND THE INSULATED STRUCTURE BEHIND, REDUCING DIFFERENTIAL THERMAL MOVEMENT.

THE TECHNICAL REQUIREMENTS FOR A COMPLIANT RAINSCREEN CAVITY ARE: MINIMUM 25MM CLEAR CAVITY (MEASURED BETWEEN THE BACK OF THE PANEL AND ANY INSULATION OR BREATHER MEMBRANE SURFACE); OPEN AT THE BASE FOR DRAINAGE AND AT THE TOP FOR VENTILATION (NET FREE AREA NOT LESS THAN 500MM' PER METRE WIDTH AT EACH LEVEL); CAVITY FIRE BARRIERS AT EVERY FLOOR LEVEL AND WINDOW PERIMETER (IN BUILDINGS OVER TWO STOREYS).

FIBRE CEMENT (FIBRE REINFORCED CEMENT) CLADDING IS THE DOMINANT LOW-MAINTENANCE CLADDING MATERIAL IN UK RESIDENTIAL CONSTRUCTION. IT IS COMPOSED OF PORTLAND CEMENT, SILICA, AND WOOD FIBRE REINFORCEMENT, AUTOCLAVED TO PRODUCE A DIMENSIONALLY STABLE, DURABLE BOARD THAT IS NOT COMBUSTIBLE (EUROCLASS A2-S1,D0 — IMPORTANT FOR FIRE SAFETY COMPLIANCE UNDER PART B), MOISTURE-RESISTANT, FROST-RESISTANT, AND TERMITE/ROT-RESISTANT.

CEDRAL CLICK (BY ETEX) IS THE UK MARKET LEADER FOR RESIDENTIAL FIBRE CEMENT CLADDING. THE CEDRAL CLICK SYSTEM IS A HORIZONTAL LAPPED PROFILE (WEATHERBOARDING) IN 12MM THICK X 190MM FACE WIDTH BOARDS, WITH A CONCEALED CLICK FIXING THAT ALLOWS BOARDS TO BE CLIPPED TOGETHER WITHOUT EXPOSED SCREW HEADS ON THE FACE. CEDRAL CLICK IS AVAILABLE IN A RANGE OF 20+ FACTORY-APPLIED COLOURS AND A WOOD-EFFECT SMOOTH FINISH OR SAWN TIMBER TEXTURE. THE BOARDS ARE NAILED OR SCREWED INTO VERTICAL TIMBER BATTENS AT 600MM CENTRES FIXED TO THE SUBSTRATE, WITH THE BATTENS CREATING THE MINIMUM 25MM CAVITY.

MARLEY ETERNIT CEDRAL (THE MARLEY BRAND VERSION OF ESSENTIALLY THE SAME PRODUCT) AND JAMES HARDIE HARDIEPLANK ARE DIRECT COMPETITORS WITH SIMILAR PERFORMANCE CHARACTERISTICS. ALL THREE ARE AVAILABLE THROUGH NATIONAL BUILDERS' MERCHANTS AND CARRY BBA CERTIFICATES.

KEY PRACTICAL POINTS FOR CEDRAL CLICK INSTALLATION: ALL CUT EDGES MUST BE SEALED WITH CEDRAL WEATHERBOARD EDGE SEAL (PRIMER) TO PREVENT MOISTURE PENETRATION INTO THE CUT CEMENT FACE. ALL INTERNAL AND EXTERNAL CORNERS USE PROPRIETARY CORNER TRIM PROFILES (PVC OR ALUMINIUM) — NOT TIMBER. THE HORIZONTAL BATTENS MUST BE PRESSURE-TREATED TO USE CLASS 3 (TANALITH E) OR BE CONSTRUCTED FROM NATURALLY DURABLE SPECIES. STARTER TRIM AND DRIP EDGE AT THE BASE OF THE CLADDING MUST DIRECT WATER AWAY FROM THE WALL FACE.

NATURAL AND ENGINEERED TIMBER CLADDING IS POPULAR FOR REAR EXTENSIONS AND GARDEN STRUCTURES IN LONDON, PARTICULARLY WHERE A WARMER AESTHETIC IS DESIRED. TIMBER CLADDING REQUIRES MORE MAINTENANCE THAN FIBRE CEMENT BUT CAN BE SPECIFIED IN HIGH-DURABILITY SPECIES OR MODIFIED TIMBERS TO REDUCE MAINTENANCE REQUIREMENTS.

ACCOYA (ACETYLATED RADIATA PINE) IS THE PREMIUM ENGINEERED TIMBER FOR EXTERNAL CLADDING. THE ACETYLATION PROCESS MODIFIES THE WOOD AT A MOLECULAR LEVEL, REDUCING ITS MOISTURE ABSORPTION TO LESS THAN 5% (COMPARED TO 30% FOR UNTREATED PINE) AND MAKING IT DIMENSIONALLY STABLE, ROT-RESISTANT, AND TERMITE-RESISTANT. ACCOYA IS CERTIFIED USE CLASS 4 (GROUND CONTACT) AND CARRIES A 50-YEAR ABOVE-GROUND GUARANTEE. IT CAN BE FINISHED WITH TRANSPARENT OR OPAQUE MICROPOROUS PAINTS OR OIL SYSTEMS AND WILL WEATHER TO A SILVER-GREY IF LEFT UNCOATED. ACCOYA FEATHER-EDGE OR SHADOWCLAD PROFILES ARE AVAILABLE FROM FINNFOREST/MEDITE SMARTPLY.

WESTERN RED CEDAR (THUJA PLICATA) IS A NATURALLY DURABLE TIMBER (USE CLASS 3 WITHOUT TREATMENT) WITH A WARM REDDISH-BROWN TONE THAT WEATHERS TO SILVER-GREY. IT IS COMMONLY USED IN LONDON FOR REAR EXTENSION AND GARDEN STUDIO CLADDING. CEDAR OIL (WATCO DANISH OIL OR SIKKENS CETOL BL) APPLIED ANNUALLY PRESERVES THE COLOUR AND EXTENDS SERVICE LIFE. CEDAR IS AVAILABLE AS FEATHER-EDGE, TONGUE-AND-GROOVE, OR REBATED PROFILE.

LARCH (LARIX SPP.) — PARTICULARLY SIBERIAN LARCH (LARIX SIBIRICA) — IS ANOTHER NATURALLY DURABLE OPTION (USE CLASS 3 UNTREATED) WITH A GOLDEN-AMBER COLOUR. IT IS DENSER AND HARDER THAN CEDAR AND TAKES CLEAR FINISHES WELL.

KEBONY (THERMALLY MODIFIED RADIATA PINE) IS A SCANDINAVIAN PRODUCT THAT USES HEAT AND STEAM TO MODIFY THE WOOD STRUCTURE, ACHIEVING USE CLASS 3 DURABILITY WITH AN ATTRACTIVE BROWN-GREY TONE. IT IS AN FSC-CERTIFIED, SUSTAINABLE ALTERNATIVE TO TROPICAL HARDWOODS.

FIRE SAFETY IS THE MOST IMPORTANT REGULATORY CONSIDERATION IN EXTERNAL CLADDING SPECIFICATION FOR UK RESIDENTIAL BUILDINGS, PARTICULARLY FOLLOWING THE GRENFELL TOWER TRAGEDY IN 2017 AND SUBSEQUENT CHANGES TO BUILDING REGULATIONS.

FOR HOUSES (TYPICALLY UNDER 11M HEIGHT AND NO MORE THAN 3 STOREYS): ALL EXTERNAL CLADDING ON RESIDENTIAL BUILDINGS BELOW 11M HEIGHT MUST BE OF LIMITED COMBUSTIBILITY — MINIMUM EUROCLASS B-S3,D2 OR BETTER — OR USE A THIRD-PARTY CERTIFIED SYSTEM WHERE THE SYSTEM AS A WHOLE (PANELS + INSULATION + CAVITY BARRIERS + FIXINGS) HAS BEEN ASSESSED FOR FIRE PERFORMANCE. FIBRE CEMENT CLADDING (CEDRAL, HARDIE) IS CLASS A2-S1,D0 (ESSENTIALLY NON-COMBUSTIBLE) AND COMPLIES WITHOUT FURTHER ASSESSMENT. ACCOYA AND MODIFIED TIMBER CLADDING IS CLASS C OR D (COMBUSTIBLE) — ON MOST HOUSES (UNDER 11M) THIS IS PERMISSIBLE PROVIDED CAVITY FIRE BARRIERS ARE INSTALLED AT EACH FLOOR LEVEL.

FOR BUILDINGS OVER 11M: THE JANUARY 2023 REGULATIONS (APPROVED DOCUMENT B, VOLUME 2, AMENDMENT 2023) REQUIRE ALL ELEMENTS OF AN EXTERNAL WALL SYSTEM ON BUILDINGS OVER 11M IN HEIGHT TO BE CLASS A2-S1,D0 OR BETTER — I.E., ESSENTIALLY NON-COMBUSTIBLE THROUGHOUT. COMBUSTIBLE TIMBER CLADDING IS NOT PERMITTED ON RESIDENTIAL BUILDINGS OVER 11M.

CAVITY FIRE BARRIERS MUST BE INSTALLED AT EVERY FLOOR LEVEL, AROUND EVERY WINDOW AND DOOR OPENING, AND AT EVERY JUNCTION WITH A COMPARTMENT WALL IN ANY MULTI-STOREY RESIDENTIAL BUILDING. CAVITY FIRE BARRIERS FOR TIMBER-FRAME CONSTRUCTION: ROCKWOOL FLEXIROCK OR KNAUF EARTHWOOL ACOUSTIC FIRE BARRIER (MINERAL WOOL, 50MM MINIMUM). FOR MASONRY CONSTRUCTION: PROPRIETARY INTUMESCENT CAVITY CLOSERS (FIRE AND SMOKE CAVITY BARRIER FSC 047 OR EQUIVALENT).

CLADDING SYSTEMS ARE FIXED TO THE STRUCTURE VIA A BATTEN AND COUNTER-BATTEN (OR VERTICAL BATTEN) FRAMEWORK. THE CORRECT SEQUENCE FOR A REAR EXTENSION CLADDING SYSTEM OVER A MASONRY OR TIMBER FRAME SUBSTRATE IS: BREATHER MEMBRANE + VERTICAL BATTENS + CLADDING PANELS.

BREATHER MEMBRANE: WRAPTITE SA (A. PROCTOR GROUP) OR TYVEK HOUSEWRAP (DU PONT) IS A VAPOUR-PERMEABLE, AIR-TIGHT BREATHER MEMBRANE THAT IS APPLIED DIRECTLY TO THE SUBSTRATE FACE (MASONRY OR TIMBER SHEATHING) BEFORE BATTENS ARE FIXED. IT PREVENTS WIND-DRIVEN RAIN AND DRAUGHTS FROM PENETRATING THE SUBSTRATE WHILE ALLOWING WATER VAPOUR TO DIFFUSE OUTWARD — CRITICAL FOR THE THERMAL PERFORMANCE OF THE WALL. THE MEMBRANE IS LAPPED HORIZONTAL JOINTS AT MINIMUM 150MM, VERTICAL JOINTS AT MINIMUM 150MM, AND TAPED WITH COMPATIBLE MEMBRANE TAPE AT ALL OVERLAPS.

VERTICAL BATTENS: 50X25MM (OR 50X38MM FOR THICKER CAVITY SYSTEMS) PRESSURE-TREATED SOFTWOOD (TANALITH E USE CLASS 3) FIXED AT 600MM CENTRES THROUGH THE BREATHER MEMBRANE INTO THE SUBSTRATE STRUCTURE (MASONRY VIA STAINLESS STEEL FRAME ANCHORS, TIMBER FRAME VIA 65MM RING SHANK NAILS OR SCREWS INTO STUDS). THE BATTENS CREATE THE VENTILATED CAVITY BEHIND THE CLADDING.

FOR FIBRE CEMENT CLADDING, SCREWS MUST BE STAINLESS STEEL A2 OR A4 GRADE — STANDARD ZINC-PLATED SCREWS WILL CORRODE WITHIN 5 TO 10 YEARS IN THE WET CAVITY ENVIRONMENT AND PRODUCE RUST STREAKS ON THE CLADDING FACE.

STARTER TRIM AND BASE DETAIL: A PROPRIETARY STARTER TRIM OR PERFORATED CAVITY CLOSER AT THE BASE OF THE CLADDING ALLOWS WATER TO DRAIN FROM THE CAVITY WHILE EXCLUDING INSECTS AND SMALL ANIMALS. WITHOUT A CORRECTLY DETAILED BASE, NESTING AND INSECT ENTRY INTO THE CAVITY ARE COMMON.

CLADDING COSTS FOR LONDON RESIDENTIAL PROJECTS DEPEND ON MATERIAL CHOICE, FIX COMPLEXITY, AND SCAFFOLD REQUIREMENT. INDICATIVE COSTS FOR SUPPLY AND INSTALLATION ARE AS FOLLOWS.

FIBRE CEMENT CLADDING (CEDRAL CLICK, INCLUDING BATTENS, BREATHER MEMBRANE, STARTER TRIM AND CORNER TRIM, FIXINGS): £65 TO £110 PER M' INSTALLED ON A STRAIGHTFORWARD SINGLE-STOREY REAR EXTENSION ELEVATION.

WESTERN RED CEDAR CLADDING (FEATHER-EDGE, INCLUDING BATTENS, MEMBRANE, STAINLESS STEEL FIXINGS, CEDAR OIL TREATMENT): £75 TO £130 PER M'.

ACCOYA CLADDING (SHADOWCLAD OR FEATHER-EDGE, FACTORY PRIMED, INCLUDING BATTENS, MEMBRANE): £110 TO £180 PER M'.

KEBONY CLADDING (VERTICAL TONGUE-AND-GROOVE, INCLUDING BATTENS): £100 TO £160 PER M'.

FOR A TYPICAL LONDON REAR EXTENSION WITH 30M' OF CLADDED ELEVATION, TOTAL INSTALLED COST RANGES FROM £1,950 TO £5,400 DEPENDING ON MATERIAL SPECIFICATION — SIGNIFICANTLY LESS THAN A FULL RENDER SYSTEM ON THE SAME AREA AND WITH LOWER ONGOING MAINTENANCE REQUIREMENTS.

SCAFFOLD IS TYPICALLY NOT REQUIRED FOR SINGLE-STOREY REAR EXTENSION CLADDING — A SCAFFOLD TOWER IS USUALLY SUFFICIENT. SCAFFOLD WOULD ADD £500 TO £1,500 IF REQUIRED FOR HIGHER ELEVATIONS.