Bay Window Repair and Renovation in London Victorian Homes: Structural, Roof, and Glazing Guide

A GROUND-FLOOR BAY WINDOW IN A VICTORIAN LONDON TERRACE CARRIES A SIGNIFICANT STRUCTURAL LOAD. THE FIRST-FLOOR WALL ABOVE THE BAY — TYPICALLY A FULL MASONRY WALL INCORPORATING A FIRST-FLOOR BAY (TWO STOREYS OF BAY) OR A FLAT BRICKWORK FACE PARAPET — IS SUPPORTED ON THE ROOF STRUCTURE OF THE BAY AND ULTIMATELY BY THE LINTELS OR RELIEVING ARCHES SPANNING THE BAY WINDOW OPENINGS.

IN VICTORIAN BRICKWORK CONSTRUCTION, THE OPENINGS WITHIN THE BAY ARE SPANNED BY ONE OF FOUR STRUCTURAL SYSTEMS: BRICK FLAT ARCH (SOLDIER COURSE ARCH WITH TAPERED JOINTS, COMMON IN EARLY VICTORIAN); BRICK SEGMENTAL ARCH (CURVED ARCH RISING TO A CROWN, COMMON IN LATE VICTORIAN); WROUGHT IRON OR STEEL FLAT BAR LINTEL (FROM APPROXIMATELY 1880 ONWARD); OR CAST IRON LINTEL (PARTICULARLY IN HIGHER-SPECIFICATION PROPERTIES). AFTER THE 1950S, CONCRETE LINTELS BECAME STANDARD FOR REPAIR AND REPLACEMENT WORK.

COMMON FAILURE MODES: IRON AND STEEL FLAT BAR LINTELS RUST — THE RUST PRODUCT HAS A VOLUME APPROXIMATELY THREE TIMES THE ORIGINAL IRON, CAUSING BRICKWORK DIRECTLY ABOVE TO SPLIT AND CRACK (KNOWN AS RUSTING LINTEL SYNDROME). THIS IS ONE OF THE MOST COMMON STRUCTURAL DEFECTS IN INNER LONDON VICTORIAN TERRACES AND CAN BE IDENTIFIED BY CHARACTERISTIC HORIZONTAL CRACKING IN THE BRICKWORK IMMEDIATELY ABOVE THE WINDOW OPENING. BRICK ARCHES CAN FAIL DUE TO MORTAR EROSION, DIFFERENTIAL SETTLEMENT, OR PREVIOUS INCOMPETENT REPAIR (E.G., CUTTING OUT THE ARCH TO FIT A MODERN FLAT LINTEL).

THE STANDARD REPAIR FOR A FAILED IRON LINTEL IS: TEMPORARY PROPPING OF THE STRUCTURE ABOVE, REMOVAL OF THE FAILED LINTEL, INSTALLATION OF A PRECAST CONCRETE PRESTRESSED LINTEL (CATNIC HEAVY DUTY CG90/100 OR IGS S-BEAM S/L 140) OR STRUCTURAL STEEL ANGLE LINTEL ON A PADSTONE, AND MAKING GOOD THE BRICKWORK. A STRUCTURAL ENGINEER SHOULD SPECIFY THE LINTEL FOR ANY BAY WINDOW OPENING ABOVE 1.5M SPAN.

THE FLAT ROOF OF A GROUND-FLOOR BAY WINDOW IS STATISTICALLY THE MOST COMMON SOURCE OF WATER INGRESS INTO A VICTORIAN LONDON TERRACE. THIS IS BECAUSE THE BAY ROOF: IS EXPOSED ON THREE SIDES (FRONT AND BOTH RETURNS); COLLECTS DEBRIS (MOSS, LEAVES, BIRD DETRITUS) THAT HOLDS MOISTURE; HAS COMPLEX FLASHINGS AT THE JUNCTION WITH THE MAIN HOUSE WALL; OFTEN RETAINS ORIGINAL BITUMEN FELT OR LEAD COVERING THAT IS 30 TO 80 YEARS OLD; AND IS FREQUENTLY WALKED ON BY MAINTENANCE WORKERS ACCESSING THE FIRST-FLOOR WINDOWS.

ORIGINAL LEAD COVERING ON A VICTORIAN BAY ROOF IS THE IDEAL MATERIAL: CODE 5 OR CODE 6 MILLED LEAD SHEETING WITH PROPERLY FORMED DRIPS, WELTED SEAMS, AND LEAD FLASHING DRESSED INTO THE MASONRY CHASE ABOVE. WELL-MAINTAINED ORIGINAL LEAD CAN LAST 60 TO 80 YEARS AND BEYOND. WHERE THE LEAD IS IN SOUND CONDITION (NO SPLITS, PUNCTURES, OR OPEN JOINTS), IT SHOULD BE RETAINED AND MAINTAINED RATHER THAN REPLACED.

WHERE THE ORIGINAL COVERING IS FAILED BITUMEN FELT OR DETERIORATED LEAD, REPLACEMENT OPTIONS ARE: NEW LEAD COVERING (CODE 5 OR 6 MILLED LEAD, £80 TO £120 PER M' INSTALLED BY A SKILLED PLUMBER); FIRESTONE EPDM RUBBERCOVER (1.14MM SINGLE-PLY MEMBRANE, FULLY ADHERED WITH BONDING ADHESIVE, WITH FIRESTONE PRESSURE-SENSITIVE FLASHING TAPE AT ALL UPSTANDS AND JUNCTION WITH WALL): £45 TO £75 PER M' FOR SMALLER AREAS; SARNAFIL G410-15EL PVC MEMBRANE (HEAT-WELDED LAPS, SARNAFIL DETAIL MEMBRANE AT UPSTANDS): £65 TO £110 PER M'; OR IKO ARMOURPLAN PVC MEMBRANE (SIMILAR TO SARNAFIL BUT LOWER COST, FULLY WELDED LAPS).

FOR BAY WINDOW FLAT ROOFS, THE CRITICAL DETAIL IS THE UPSTAND HEIGHT AT THE JUNCTION WITH THE MAIN HOUSE WALL. BUILDING REGULATIONS APPROVED DOCUMENT C REQUIRES A MINIMUM 150MM UPSTAND ABOVE THE FINISHED ROOF SURFACE AT ALL UPSTAND JUNCTIONS — FAILURE TO ACHIEVE THIS IS ONE OF THE MOST COMMON CAUSES OF WATER INGRESS AT THIS LOCATION. THE FLASHING AT THE JUNCTION WITH THE HOUSE WALL MUST BE DRESSED INTO A MORTAR CHASE OR BEDDED UNDER A LEAD COVER FLASHING — SIMPLY BEDDING FLASHING TO THE WALL SURFACE WITH SEALANT IS NOT ADEQUATE AND WILL FAIL WITHIN 2 TO 5 YEARS.

THE TIMBERS MOST VULNERABLE TO ROT IN A BAY WINDOW ASSEMBLY ARE: THE SILL (HORIZONTAL MEMBER AT THE BASE OF EACH WINDOW OPENING); THE OUTER CASING AND BOTTOM RAIL OF THE SASHES (THE MOST EXPOSED SURFACES); THE EXTERNAL FASCIA AND CORNICE BOARDS (PARTICULARLY AT JOINTS AND WHERE PAINT HAS FAILED); AND THE STRUCTURAL TIMBER WITHIN THE BAY ROOF DECK (TYPICALLY 50X200MM OR 50X150MM JOISTS) IF THE COVERING HAS FAILED AND WATER HAS ENTERED THE DECK.

TIMBER CONDITION ASSESSMENT IS PERFORMED WITH A SHARP PROBE (POINTED AWL OR SCREWDRIVER): SOUND TIMBER RESISTS THE PROBE; ROTTEN TIMBER ALLOWS EASY PENETRATION. MOISTURE METER READINGS ABOVE 20% MOISTURE CONTENT IN TIMBER INDICATE ACTIVE MOISTURE CONDITIONS THAT WILL SUPPORT ROT PROGRESSION.

FOR MINOR TO MODERATE TIMBER DECAY IN SILLS, FRAMES, AND FASCIA BOARDS, EPOXY WOOD REPAIR SYSTEMS PROVIDE AN EFFECTIVE STRUCTURAL REPAIR WITHOUT FULL TIMBER REPLACEMENT. THE STANDARD SYSTEMS ARE: REPAIR CARE DRY FLEX (DRY FLEX CONSOLIDANT + DRY FLEX FINE FILLER) — A TWO-PART EPOXY CONSOLIDANT AND FILLER SYSTEM APPROVED FOR HISTORIC WINDOW REPAIR. ABATRON WOODEPOX (EPOXITE LIQUID + WOODEPOX FILLER) — A US SYSTEM USED BY SPECIALIST JOINERY RESTORERS. FOR EPOXY REPAIR TO BE DURABLE, ALL DECAYED MATERIAL MUST BE CUT BACK TO SOUND TIMBER, THE CONSOLIDANT MUST BE APPLIED TO CONSOLIDATE ANY FRIABLE REMAINING WOOD, AND THE FILLER APPLIED IN LAYERS BEFORE THE FINAL EPOXY COAT AND MICROPOROUS PAINT SYSTEM.

FOR SEVERELY DECAYED STRUCTURAL ELEMENTS (SILL SECTIONS, LOWER RAILS OF SASHES, FASCIA BOARDS), FULL REPLACEMENT IN ACCOYA (ACETYLATED RADIATA PINE — 50-YEAR ABOVE-GROUND WARRANTY) OR TREATED SOFTWOOD WITH DOUBLE VACUUM PRESSURE TREATMENT (TANALITH E TREATMENT TO USE CLASS 3) IS THE APPROPRIATE SPECIFICATION.

LEAD FLASHINGS AT THE JUNCTION BETWEEN THE BAY WINDOW ROOF AND THE MAIN HOUSE WALL ARE ESSENTIAL TO WATERPROOFING AND ARE A COMMON FAILURE POINT IN LONDON PROPERTIES. THE STANDARD SPECIFICATION FOR A NEW LEAD FLASHING IS CODE 4 OR CODE 5 MILLED LEAD SHEET, STEPPED TO FOLLOW THE BRICKWORK COURSE AT 150MM STEPS, WITH THE TOP EDGE DRESSED INTO A RAKED-OUT MORTAR JOINT AT MINIMUM 25MM DEPTH AND POINTED WITH MORTAR (NOT MASTIC — WHICH DRIES, SHRINKS, AND FAILS).

WHERE THE JUNCTION IS AT AN ABUTMENT (FLAT ROOF MEETING A VERTICAL WALL), THE FLASHING MUST HAVE A MINIMUM 150MM UPSTAND ON THE VERTICAL FACE AND A MINIMUM 75MM LAP ONTO THE ROOF MEMBRANE HORIZONTAL SURFACE. FOR AN EPDM OR PVC MEMBRANE ROOF, THE LEAD FLASHING IS BONDED TO THE MEMBRANE WITH THE MEMBRANE MANUFACTURER'S BONDING ADHESIVE AND THEN MECHANICALLY FIXED AT THE TOP EDGE INTO THE MORTAR JOINT.

IN CONSERVATION AREAS, SOME LOCAL AUTHORITIES SPECIFY THAT ORIGINAL LEAD FLASHINGS MUST BE REPAIRED RATHER THAN REPLACED WHERE THEY ARE IN REPAIRABLE CONDITION. THE SOCIETY OF MASTER SADDLERS' LEAD BURN WELDING TECHNIQUE (PATINATION OIL APPLICATION TO PREVENT STREAKING) IS THE TRADITIONAL LEAD JOINING METHOD USED BY SPECIALIST PLUMBERS.

BAY WINDOWS IN VICTORIAN TERRACES TYPICALLY CONTAIN THREE WINDOW UNITS — THE FRONT-FACING CENTRE WINDOW AND TWO ANGLED RETURN WINDOWS AT THE BAY ANGLES. IN ORIGINAL CONSTRUCTION, THESE ARE PAIRED SASH WINDOWS (TWO SLIDING SASHES PER UNIT) OR CASEMENTS. THE GLAZING IN UNRESTORED PROPERTIES IS ALMOST ALWAYS SINGLE-PANE.

THE FULL RANGE OF THERMAL UPGRADE OPTIONS FOR BAY WINDOW GLAZING FOLLOWS THE SAME HIERARCHY AS FOR STANDARD SASH WINDOWS: DRAUGHT PROOFING FIRST (£180 TO £350 PER WINDOW), THEN SLIMLINE DOUBLE GLAZING (12MM UNITS, £280 TO £500 PER WINDOW INCLUDING REGLAZING LABOUR), THEN FINEO VACUUM GLAZING (6.7MM, 0.7 W/M'K, £600 TO £1,200 PER WINDOW).

FOR BAY WINDOWS IN CONSERVATION AREAS, THE SAME PLANNING RULES APPLY: ORIGINAL TIMBER SASH CONFIGURATION MUST BE RETAINED; UPVC OR ALUMINIUM REPLACEMENT WINDOWS REQUIRE PLANNING PERMISSION AND ARE ROUTINELY REFUSED; FINEO VACUUM GLAZING IN THE EXISTING FRAMES IS THE TECHNICALLY AND PLANNING-APPROVED OPTIMAL SOLUTION.

SECONDARY GLAZING IS AN ALTERNATIVE WORTH CONSIDERING IN CONSERVATION AREA PROPERTIES. A SECONDARY GLAZING SYSTEM (SELECTAGLAZE SERIES 10 HORIZONTAL SLIDING, CLEARVIEW SECONDARY GLAZING, OR SASHGLAZE) IS FITTED INSIDE THE WINDOW REVEAL, BEHIND THE EXISTING SINGLE-GLAZED SASH, CREATING A 50MM TO 100MM AIR GAP. THIS SYSTEM ACHIEVES WHOLE-WINDOW U-VALUES OF 1.8 TO 2.5 W/M'K AND PROVIDES EXCEPTIONAL ACOUSTIC INSULATION (SIGNIFICANTLY BETTER THAN DOUBLE GLAZING FOR TRAFFIC NOISE). IT DOES NOT AFFECT THE EXTERNAL APPEARANCE AND IS ALWAYS PLANNING-COMPLIANT. TYPICAL COST: £200 TO £400 PER WINDOW.

A COMPLETE BAY WINDOW RENOVATION IN A LONDON VICTORIAN TERRACE — COVERING STRUCTURAL LINTEL ASSESSMENT AND REPAIR, BAY ROOF REPLACEMENT, TIMBER RESTORATION, LEAD FLASHINGS, REPOINTING, AND GLAZING UPGRADE — IS A MULTI-TRADE PROJECT REQUIRING COORDINATION BETWEEN STRUCTURAL ENGINEER, BRICKLAYER, ROOFER/PLUMBER, JOINER, AND DECORATOR.

TYPICAL COST RANGE FOR A FULL GROUND-FLOOR BAY WINDOW RENOVATION (2M WIDE, SINGLE-STOREY BAY, NOT INCLUDING MAJOR STRUCTURAL WORKS TO MAIN HOUSE): STRUCTURAL ENGINEER FEE (IF REQUIRED FOR LINTEL ASSESSMENT): £350 TO £800. LINTEL REPLACEMENT (PROPPING, REMOVAL, CATNIC HEAVY DUTY OR STEEL ANGLE, REINSTATEMENT): £1,200 TO £3,500 PER LINTEL. BAY ROOF REPLACEMENT (FIRESTONE EPDM OR SARNAFIL, INCLUDING UPSTANDS, LEAD FLASHINGS, NEW OSB DECK IF REQUIRED): £800 TO £2,500 FOR A STANDARD BAY ROOF OF 2 TO 4M'. TIMBER RESTORATION (EPOXY REPAIR TO SILLS AND FRAMES, NEW SASH CORDS, DRAUGHT PROOFING, REDECORATION): £1,200 TO £3,500 PER BAY. FINEO VACUUM GLAZING TO THREE WINDOW UNITS: £1,800 TO £3,600. REPOINTING AND EXTERNAL DECORATION: £800 TO £2,000.

TOTAL FOR A FULL RENOVATION OF ONE GROUND-FLOOR BAY WITHOUT MAJOR STRUCTURAL ISSUES: £4,000 TO £11,600. WHERE LINTEL REPLACEMENT IS NEEDED, ADD £1,200 TO £3,500 PER OPENING.