External Render for London Homes: Silicone, Monocouche, Lime, and EWI Systems Guide

LONDON'S RESIDENTIAL PROPERTIES SPAN A WIDE RANGE OF CONSTRUCTION TYPES AND PERIODS, FROM GEORGIAN STUCCO-FRONTED TERRACES AND VICTORIAN BRICK TO 1930S PEBBLEDASH SEMIS AND POST-WAR CONCRETE. EACH PRESENTS DIFFERENT SUBSTRATE CHARACTERISTICS AND DIFFERENT FAILURE RISKS.

THE MOST COMMON FAILURE IN EXISTING RENDERED PROPERTIES IS CRACKING AND DELAMINATION OF HARD CEMENT RENDER APPLIED OVER SOFT MASONRY. VICTORIAN BRICKWORK USES SOFT, PERMEABLE BRICKS BONDED WITH WEAK LIME MORTAR — THIS SUBSTRATE IS WEAK AND SLIGHTLY FLEXIBLE. PORTLAND CEMENT RENDER (1:3 CEMENT:SAND) HAS HIGH COMPRESSIVE STRENGTH AND VIRTUALLY NO FLEXIBILITY. WHEN THE BUILDING MOVES SEASONALLY (ALL BUILDINGS DO, DUE TO THERMAL EXPANSION AND SUBSOIL MOISTURE MOVEMENT), THE RIGID RENDER CRACKS RATHER THAN ACCOMMODATING THE MOVEMENT, AND RAIN THEN ENTERS THE CRACKS AND BECOMES TRAPPED BEHIND THE IMPERMEABLE CEMENT SKIN. THIS LEADS TO FREEZE-THAW SPALLING, DAMP PENETRATION, AND EVENTUALLY DELAMINATION OF WHOLE RENDER PANELS.

THE SECOND COMMON FAILURE IS RENDER OVER MIXED SUBSTRATES WITHOUT DIFFERENTIAL MOVEMENT DETAILING. WHERE A RENDER SPANS ACROSS DIFFERENT BACKGROUND MATERIALS — BRICK, BLOCK, STEEL LINTEL, CONCRETE FRAME — EACH HAS A DIFFERENT COEFFICIENT OF THERMAL EXPANSION AND DIFFERENT MOISTURE ABSORPTION. WITHOUT A MOVEMENT JOINT AT THE INTERFACE, DIFFERENTIAL MOVEMENT CAUSES CRACKING AT THE TRANSITION.

THE THIRD FAILURE IS INAPPROPRIATE SPECIFICATION FOR LIME RENDER ON HERITAGE PROPERTIES. REPLACING ORIGINAL NHL LIME RENDER ON A PRE-1919 BUILDING WITH MODERN CEMENT RENDER IS EXTREMELY DAMAGING — THE IMPERMEABLE CEMENT TRAPS MOISTURE IN WALLS DESIGNED TO BREATHE, CAUSING SALT EFFLORESCENCE, MORTAR EROSION, AND SOMETIMES STRUCTURAL DAMAGE TO THE SOFT BRICKWORK BEHIND.

SILICONE THIN-COAT RENDER IS THE MOST POPULAR PREMIUM EXTERNAL RENDER FINISH FOR LONDON RESIDENTIAL PROJECTS. IT IS APPLIED AS A 1.5MM TO 3.0MM FINISH COAT OVER A REINFORCED BASE COAT AND COMBINES EXCELLENT WATER REPELLENCY (DUE TO SILICONE POLYMER CONTENT) WITH FULL VAPOUR PERMEABILITY — ALLOWING THE WALL TO BREATHE WHILE SHEDDING RAIN FROM THE SURFACE.

K-REND SILICONE TC15 IS THE LEADING SILICONE RENDER IN THE UK RESIDENTIAL MARKET. IT IS A PRE-MIXED, THROUGH-COLOURED SILICONE RENDER APPLIED AT 1.5MM SCRATCH-AND-FLOAT TEXTURE OVER A BASE COAT OF K-REND RENDER PRIMER OR K-REND BASE COAT (POLYMER-MODIFIED CEMENT BASE REINFORCED WITH ALKALINE-RESISTANT GLASS FIBRE MESH). K-REND SILICONE TC15 ACHIEVES A WATER ABSORPTION W VALUE OF <0.1 KG/(M'·H^0.5) (HYDROPHOBIC CATEGORY) AND A VAPOUR PERMEABILITY VALUE OF V2 (MEDIUM PERMEABILITY). IT IS AVAILABLE IN THE FULL K-REND COLOUR RANGE (200+ COLOURS) AND CARRIES A BBA CERTIFICATE (09/4665). TYPICAL COVERAGE AT 1.5MM TEXTURE: 12 TO 14 KG/M'.

WEBER.REND SILICONE TC (FORMERLY MONOCOUCHE SILICONE RENDER BY SAINT-GOBAIN) IS A DIRECT COMPETITOR WITH SIMILAR PERFORMANCE CHARACTERISTICS. WEBER ALSO OFFERS WEBER.PRAL M — A MONOCOUCHE ONE-COAT MINERAL RENDER WITH NO BASE COAT REQUIREMENT — FOR PROJECTS WHERE SIMPLICITY OF APPLICATION IS PRIORITISED OVER THE FINEST SURFACE FINISH.

PAREX MONOREX GM IS A MONOCOUCHE ONE-COAT RENDER USED EXTENSIVELY ON LONDON NEW-BUILD AND REFURBISHMENT PROJECTS. IT IS A PRE-MIXED, THROUGH-COLOURED POLYMER-MODIFIED CEMENTITIOUS RENDER APPLIED IN ONE 15MM TO 25MM COAT WITH A TYROLEAN (SCRATCH) TEXTURE. MONOREX GM ACHIEVES A GOOD BALANCE OF WATER REPELLENCY AND VAPOUR PERMEABILITY AND IS FASTER TO APPLY THAN A BASE-COAT-PLUS-SILICONE-FINISH SYSTEM.

FOR PRE-1919 PROPERTIES WITH SOLID BRICK OR STONE WALLS, LIME RENDER IS THE TECHNICALLY CORRECT AND BUILDING CONSERVATION AUTHORITY-APPROVED SPECIFICATION. LIME RENDER IS BREATHABLE, FLEXIBLE (ACCOMMODATING MOVEMENT IN SOFT HISTORIC MASONRY WITHOUT CRACKING), AND CHEMICALLY COMPATIBLE WITH TRADITIONAL LIME MORTARS AND SOFT BRICKS. PORTLAND CEMENT RENDER ON A HISTORIC SOLID WALL IS A CONSERVATION ERROR THAT CAN CAUSE IRREVERSIBLE DAMAGE.

ST ASTIER NATURAL HYDRAULIC LIME (NHL) 3.5 IS THE STANDARD SPECIFICATION FOR EXTERNAL RENDER ON LONDON VICTORIAN SOLID BRICK BUILDINGS. NHL 3.5 (NATURAL HYDRAULIC LIME WITH A HYDRAULIC STRENGTH OF 3.5 N/MM' AT 28 DAYS) PROVIDES SUFFICIENT STRENGTH FOR EXTERNAL EXPOSURE WHILE REMAINING SOFTER THAN THE MASONRY IT PROTECTS. THE TYPICAL SPECIFICATION IS A SCRATCH COAT OF NHL 3.5 + SHARP SAND AT 1:2.5 (4 TO 8MM, SCRATCHED), FLOATED COAT OF NHL 3.5 + SHARP SAND AT 1:2.5 (8 TO 12MM), AND A FINE FINISH COAT OF NHL 3.5 + FINE WASHED SAND AT 1:3 (3 TO 5MM). TOTAL BUILD: 15 TO 25MM.

FOR CONSERVATION AREA PROPERTIES OR LISTED BUILDINGS, THE LOCAL PLANNING AUTHORITY'S CONSERVATION OFFICER MAY SPECIFY THE EXACT MIX AND AGGREGATE SOURCE TO MATCH THE ORIGINAL RENDER PROFILE. SAMPLES ARE TYPICALLY REQUIRED FOR APPROVAL BEFORE FULL APPLICATION. LIME RENDER TAKES CONSIDERABLY LONGER TO CURE THAN CEMENT RENDER (28 TO 90 DAYS DEPENDING ON HYDRAULICITY AND WEATHER) AND MUST BE KEPT MOIST AND PROTECTED FROM FROST AND DIRECT SUN DURING CURING.

SCOTTISH LIME CENTRE, LIME GREEN PRODUCTS, AND ST ASTIER (UK DISTRIBUTED BY SMET BUILDING PRODUCTS) ARE THE MAIN SUPPLIERS OF QUALITY NHL LIME PRODUCTS FOR LONDON HERITAGE PROJECTS.

WHERE EXTERNAL RENDER IS APPLIED AS PART OF AN EXTERNAL WALL INSULATION SYSTEM (TO IMPROVE U-VALUE AND EPC RATING), THE RENDER IS NOT APPLIED DIRECT TO MASONRY BUT ONTO AN INSULATION BOARD (EPS, MINERAL WOOL, OR PIR) BONDED AND MECHANICALLY FIXED TO THE EXTERNAL WALL FACE.

THE TYPICAL EWI RENDER SYSTEM SPECIFICATION FOR A LONDON SOLID BRICK WALL UPGRADE IS AS FOLLOWS. PRIMER: PROPRIETARY PENETRATING PRIMER APPLIED TO CLEAN MASONRY. ADHESIVE: PERMAROCK ADHESIVE MORTAR OR WEBER.THERM ADHESIVE APPLIED TO BACK OF INSULATION BOARDS IN VERTICAL DABS OR NOTCHED-TROWEL FULL COVERAGE. INSULATION: 90MM OR 120MM GRAPHITE EPS BOARD (PERMAROCK EWI BOARD, SWISSPEARL EPS BOARD, OR WEBERTHERM EPS) FOR CAVITY-WALL ALTERNATIVE, OR 60MM ROCKWOOL RS MINERAL WOOL BOARD WHERE NON-COMBUSTIBLE EWI IS SPECIFIED (REQUIRED FOR BUILDINGS OVER 11M — SEE BUILDING REGULATIONS PART B). MECHANICAL FIXINGS: MINIMUM 6 FIXINGS PER M' OF EPS OR 8 PER M' OF MINERAL WOOL BOARD IN EXPOSED CONDITIONS, USING EJOTHERM STR U FIXINGS (PLASTIC-HEADED, NO THERMAL BRIDGE). BASE COAT: PERMAROCK BASE COAT OR WEBER.THERM CLASSIC BASE COAT, WITH ALKALINE-RESISTANT MESH EMBEDDED IN WET BASE COAT. FINISH COAT: K-REND SILICONE TC15 OR WEBER.REND SILICONE TC APPLIED AT 1.5MM TO 3.0MM OVER THE CURED BASE COAT.

CRITICAL BUILDING REGULATIONS POINT: PART B (FIRE SAFETY) REQUIRES NON-COMBUSTIBLE EXTERNAL WALL SYSTEMS (CLASS A2-S1,D0 OR BETTER) FOR ALL RESIDENTIAL BUILDINGS OVER 11M HEIGHT. EPS INSULATION IS CLASSIFIED AS CLASS E (COMBUSTIBLE) AND IS NOT PERMITTED ON BUILDINGS OVER 11M WITHOUT MINERAL WOOL CAVITY BARRIERS. FOR HOUSES (TYPICALLY UNDER 11M), EPS IS PERMITTED. ALWAYS CONFIRM BUILDING HEIGHT BEFORE SPECIFYING EPS EWI.

RENDER WITHOUT MOVEMENT JOINTS WILL CRACK. THIS IS A FUNDAMENTAL PRINCIPLE OF RENDER DESIGN THAT IS FREQUENTLY IGNORED IN RESIDENTIAL PROJECTS, WITH EXPENSIVE CONSEQUENCES.

MOVEMENT JOINTS ARE REQUIRED AT THE FOLLOWING LOCATIONS. AT CHANGES OF SUBSTRATE: WHERE RENDER SPANS FROM BRICK TO BLOCKWORK, FROM BLOCKWORK TO STEEL LINTEL, OR FROM MASONRY TO TIMBER FRAME. AT OPENINGS: CORNER BEADS AT WINDOW AND DOOR REVEALS MUST INCLUDE A STOP BEAD WITH 10MM MOVEMENT JOINT BETWEEN THE RENDER FACE AND THE WINDOW FRAME, SEALED WITH POLYURETHANE SEALANT (SIKAFLEX 11FC+ OR TREMCO SPECTREM 2). AT HORIZONTAL LEVELS WHERE STRUCTURAL MOVEMENT IS CONCENTRATED: TYPICALLY AT FIRST FLOOR CEILING LEVEL IN OLDER PROPERTIES WHERE DIFFERENTIAL SETTLEMENT MAY OCCUR. AT PANELS EXCEEDING 5M IN HORIZONTAL DIRECTION: A VERTICAL MOVEMENT JOINT AT MAX 5M CENTRES IS REQUIRED FOR LARGE RENDER PANELS TO ACCOMMODATE THERMAL EXPANSION.

FOR EWI SYSTEMS, THE EWI MANUFACTURER'S DETAILS MUST BE FOLLOWED FOR ALL MOVEMENT JOINTS AND WINDOW REVEALS — PROPRIETARY STARTER TRACK, CORNER BEAD, AND WINDOW SILL TRIM PROFILES ARE PART OF THE CERTIFIED SYSTEM AND MUST NOT BE SUBSTITUTED.

REVEALS AND SOFFITS: WHERE RENDER WRAPS INTO A WINDOW OR DOOR REVEAL, THE CORNER IS VULNERABLE TO IMPACT DAMAGE. PROPRIETARY STAINLESS STEEL OR PVC CORNER BEADS (RENDERPLAS OR BEADALON PROFILES) EMBEDDED IN THE BASE COAT PROVIDE CRISP ARRISES AND PROTECT CORNERS FROM CHIPPING.

EXTERNAL RENDER COSTS IN LONDON VARY WIDELY BY SYSTEM TYPE, SCAFFOLD REQUIREMENT, SURFACE AREA, AND SUBSTRATE CONDITION. INDICATIVE COSTS FOR A TYPICAL LONDON SEMI-DETACHED OR TERRACED HOUSE (FRONT AND SIDE ELEVATIONS, APPROXIMATELY 60 TO 100M' TOTAL RENDER AREA) ARE AS FOLLOWS.

SCOTCH CEMENT RENDER (OLD-FASHIONED OPC RENDER — NOT RECOMMENDED FOR NEW WORK): £1,500 TO £3,000.

MONOCOUCHE ONE-COAT RENDER (PAREX MONOREX GM OR WEBER PRAL M, BASE COAT NOT REQUIRED): £3,500 TO £6,500 FOR 60 TO 100M'. FASTER TO APPLY THAN TWO-COAT SYSTEMS.

SILICONE THIN-COAT RENDER (BASE COAT + MESH + K-REND SILICONE TC15 OR WEBER SILICONE TC): £4,500 TO £9,000 FOR 60 TO 100M'. THE HIGHER COST REFLECTS SUPERIOR DURABILITY AND FINISH QUALITY.

LIME RENDER (ST ASTIER NHL 3.5, THREE-COAT SYSTEM): £5,500 TO £11,000 FOR 60 TO 100M'. SLOWER APPLICATION AND CURING INCREASES LABOUR COSTS. SPECIALIST LIME APPLICATORS TYPICALLY CHARGE A PREMIUM.

EWI SYSTEM (90MM EPS + BASE COAT + K-REND SILICONE TC15): £8,000 TO £18,000 FOR A TYPICAL TERRACE DEPENDING ON SURFACE AREA AND INSULATION THICKNESS. SCAFFOLD IS ALWAYS REQUIRED FOR EWI AND IS TYPICALLY INCLUDED IN THE SYSTEM CONTRACTOR'S PRICE.

SCAFFOLD: TYPICALLY £800 TO £2,500 DEPENDING ON HEIGHT AND CONFIGURATION — USUALLY INCLUDED IN THE RENDER CONTRACTOR'S PRICE FOR THE FRONT ELEVATION BUT MAY BE ADDITIONAL FOR REAR/SIDE.