Steel Frame Construction for London Victorian Terrace Extensions: Portal Frame, SFS, and Structural Steel

STRUCTURAL STEEL IS USED IN LONDON VICTORIAN TERRACE EXTENSIONS IN FOUR MAIN CONFIGURATIONS: (1) STRUCTURAL STEEL BEAMS OVER LARGE REAR WALL OPENINGS (THE MOST COMMON STRUCTURAL STEEL APPLICATION IN LONDON VICTORIAN TERRACE EXTENSIONS — A STEEL BEAM (TYPICALLY A UB — UNIVERSAL BEAM SECTION — OR RSJ — ROLLED STEEL JOIST — THOUGH RSJ IS AN OLDER TERM AND CURRENT SPECIFICATION USES UB OR UC (UNIVERSAL COLUMN) SECTIONS AS APPROPRIATE) IS INSTALLED OVER A NEW OPENING FORMED IN THE LOAD-BEARING REAR WALL OF THE EXISTING PROPERTY — TRANSFERRING THE STRUCTURAL LOAD ABOVE THE OPENING (FLOOR ABOVE, ROOF LOAD, WALL LOAD) TO THE PIERS ON EITHER SIDE OF THE OPENING — ENABLING AN OPEN CONNECTION BETWEEN THE ORIGINAL REAR ROOM AND THE NEW EXTENSION WITHOUT INTERMEDIATE COLUMNS); (2) PORTAL FRAME (A STRUCTURAL FRAME SYSTEM USING STEEL COLUMNS AND A STEEL ROOF BEAM CONNECTED AT THE COLUMN HEAD BY A RIGID (MOMENT) CONNECTION — CREATES A SELF-SUPPORTING FRAME CAPABLE OF SPANNING LARGE WIDTHS WITHOUT INTERMEDIATE SUPPORTS — USED IN WIDE-SPAN SINGLE-STOREY REAR EXTENSIONS WHERE THE STRUCTURAL DEPTH OF A CONVENTIONAL TIMBER OR CONCRETE ROOF BEAM WOULD BE EXCESSIVE OR WHERE THE HEAD HEIGHT IS CRITICAL); (3) SFS (LIGHT GAUGE STEEL FRAMING SYSTEM — ALSO CALLED LIGHT STEEL FRAME OR LSF): COLD-FORMED GALVANISED LIGHT STEEL SECTIONS (TYPICALLY C-SECTIONS 70-150MM DEEP — 1.2-2.0MM THICKNESS — GALVANISED Z275 COATING) USED AS WALL STUDS, FLOOR JOISTS, AND RAFTERS — AN ALTERNATIVE TO TIMBER STUD FRAMING — USED IN EXTENSIONS WHERE GREATER DIMENSIONAL ACCURACY, HIGHER FIRE PERFORMANCE, OR A SPECIFIC STRUCTURAL SOLUTION IS NEEDED; (4) EXPOSED STRUCTURAL STEEL (AESTHETIC ELEMENT — WHERE THE STEEL IS LEFT EXPOSED AS AN ARCHITECTURAL FEATURE OF THE INTERIOR SPACE — TYPICALLY HOT-ROLLED STEEL COLUMNS AND BEAMS PAINTED IN BLACK (OFTEN RAL 9005 JET BLACK OR A DARK GREY — DULUX TRADE SMOOTH STRUCTURAL STEEL OR ZINGA COLD ZINC GALVANISING PRIMER FOLLOWED BY EPOXY TOP COAT) — A COMMON DESIGN FEATURE IN CONTEMPORARY LONDON EXTENSION INTERIORS).

THE STRUCTURAL STEEL BEAM OVER A LARGE REAR WALL OPENING IS THE MOST COMMON STRUCTURAL STEEL ELEMENT IN A LONDON VICTORIAN TERRACE EXTENSION — THE CORRECT BEAM SIZE IS CALCULATED BY A CHARTERED STRUCTURAL ENGINEER BASED ON THE SPAN (WIDTH OF THE OPENING), THE LOAD ABOVE THE BEAM (FLOOR LOAD, ROOF LOAD, NUMBER OF STOREYS ABOVE, AREA SUPPORTED), AND THE BEARING AT EACH END (THE LENGTH OF BEARING ON THE SUPPORT PIERS OR PADSTONES).

TYPICAL UB SIZES FOR LONDON VICTORIAN TERRACE REAR WALL OPENINGS (ILLUSTRATIVE ONLY — MUST BE CONFIRMED BY STRUCTURAL ENGINEER FOR EACH PROJECT): 1.5M TO 2.5M SPAN (SMALL OPENING): UB 152X89X16 S275 OR UB 178X102X19 S275; 2.5M TO 3.5M SPAN (MEDIUM OPENING — THE MOST COMMON REAR WALL OPENING WIDTH IN A LONDON VICTORIAN MID-TERRACE): UB 203X133X25 S275 OR UB 254X102X25 S275 OR UB 254X146X31 S275; 3.5M TO 4.5M SPAN (LARGE OPENING — FULL WIDTH REAR ELEVATION OPENING): UB 305X127X37 S275 OR UB 305X165X40 S275 OR UB 356X127X33 S275; OVER 4.5M SPAN (VERY LARGE OPENING OR COMBINED REAR AND SIDE WALL REMOVAL FOR WRAP-AROUND EXTENSION): UB 356X171X45 S275 OR UB 406X140X39 S275 OR POTENTIALLY TWIN PARALLEL BEAMS BOLTED TOGETHER; NOTE: THE UB DESIGNATION USES THE FORMAT DEPTH MM X FLANGE WIDTH MM X WEIGHT KG/M — FOR EXAMPLE UB 254X146X31 = 254MM DEEP X 146MM WIDE FLANGE X 31 KG/M; S275 = STRUCTURAL STEEL GRADE 275 N/MM² MINIMUM YIELD STRENGTH (THE MOST COMMON GRADE FOR RESIDENTIAL STRUCTURAL STEEL IN THE UK — S355 IS HIGHER STRENGTH AND USED WHERE A SMALLER SECTION IS NEEDED TO FIT A CONSTRAINED SPACE); PADSTONES: ALL STRUCTURAL STEEL BEAMS MUST BEAR ON PADSTONES AT EACH END — THE PADSTONE DISTRIBUTES THE CONCENTRATED POINT LOAD FROM THE BEAM END INTO THE SUPPORTING MASONRY — TYPICAL PADSTONE: 215X140X65MM ENGINEERING BLUE BRICK (CLASS B ENGINEERING BRICK — MINIMUM 75 N/MM² COMPRESSIVE STRENGTH) OR PRE-CAST CONCRETE PADSTONE (300X150X100MM FOR MEDIUM-SPAN BEAMS) — SIZED AND SPECIFIED BY STRUCTURAL ENGINEER; TEMPORARY SUPPORT: DURING CONSTRUCTION, THE MASONRY ABOVE THE PROPOSED OPENING MUST BE TEMPORARILY SUPPORTED WITH ACROW PROPS AND STEEL SPREADER BEAMS (STRONGBOYS OR RSJ SPREADERS) BEFORE ANY MASONRY IS REMOVED — THIS IS A CRITICAL HEALTH AND SAFETY REQUIREMENT AND MUST BE DONE BY EXPERIENCED STRUCTURAL STEELWORK OPERATIVES.

A PORTAL FRAME IS A RIGID STRUCTURAL FRAME COMPRISING TWO COLUMNS CONNECTED TO A RAFTER (ROOF BEAM) AT THE COLUMN HEAD BY MOMENT-RESISTING (RIGID) CONNECTIONS — THE RIGIDITY OF THE COLUMN-TO-RAFTER CONNECTION PROVIDES FRAME STABILITY AGAINST LATERAL (HORIZONTAL) FORCES (WIND, NOTIONAL HORIZONTAL FORCES) WITHOUT DIAGONAL BRACING IN THE PLANE OF THE FRAME — ENABLING THE INTERNAL SPACE TO BE UNOBSTRUCTED BY DIAGONAL BRACING.

WHEN PORTAL FRAMES ARE USED IN LONDON VICTORIAN TERRACE REAR EXTENSIONS: WIDE-SPAN FLAT ROOF EXTENSIONS (SPAN GREATER THAN APPROXIMATELY 4-5M) WHERE THE STRUCTURAL DEPTH OF A CONVENTIONAL SIMPLY-SUPPORTED FLAT ROOF BEAM (UB OR URS) WOULD BE EXCESSIVE FOR THE DESIRED HEAD HEIGHT; BUTTERFLY ROOF OR PITCHED ROOF EXTENSIONS WHERE THE ROOF STRUCTURE NEEDS TO SPAN THE FULL WIDTH WITHOUT INTERNAL COLUMNS; GROUND-FLOOR EXTENSIONS WHERE THE REAR WALL OF THE EXISTING PROPERTY IS FULLY OPENED AND THE ROOF STRUCTURE NEEDS TO BEAR AT THE EXISTING PROPERTY JUNCTION AND THE REAR OF THE EXTENSION ONLY — THE PORTAL FRAME COLUMN AT THE REAR OF THE EXTENSION TRANSFERS THE ROOF LOAD TO THE FOUNDATIONS WITHOUT REQUIRING A LOAD-BEARING BRICK REAR WALL; CONNECTION TYPES IN PORTAL FRAME: (A) WELDED MOMENT CONNECTION (COLUMN TO RAFTER — THE PREFERRED STRUCTURAL ENGINEERING SOLUTION — WELDED BY A CODED WELDER TO AWS D1.1 / BS EN 1011 STANDARDS — PRODUCES A FULLY RIGID MOMENT CONNECTION — MUST BE FABRICATED IN A STEELWORK FABRICATION SHOP — NOT SITE-WELDED); (B) BOLTED MOMENT CONNECTION (COLUMN TO RAFTER — EXTENDED END PLATE CONNECTION — BOLTED WITH M20 OR M24 GRADE 8.8 HIGH-STRENGTH BOLTS — DESIGNED BY STRUCTURAL ENGINEER — MORE PRACTICAL FOR SITE ASSEMBLY); UNIVERSAL COLUMN SECTIONS (UC) ARE OFTEN USED FOR PORTAL FRAME COLUMNS (AS OPPOSED TO UB SECTIONS USED FOR BEAMS) BECAUSE THE MORE EQUAL-DEPTH CROSS-SECTION OF A UC PROVIDES GREATER BENDING RESISTANCE IN BOTH AXES — TYPICAL: UC 152X152X23 S275 (LIGHT COLUMN — SMALL EXTENSION) TO UC 203X203X46 S275 (HEAVIER COLUMN — LARGER SPAN) — SPECIFIED BY STRUCTURAL ENGINEER.

SFS (LIGHT GAUGE STEEL FRAMING SYSTEM — ALSO CALLED LIGHT STEEL FRAME (LSF) OR STEEL STUD FRAMING — THE SYSTEM USES COLD-FORMED GALVANISED STEEL SECTIONS (C-SECTION STUDS, C-SECTION TRACK, Z-SECTION BRIDGING — TYPICALLY ZINTEC OR HOT-DIP GALVANISED Z275 COATING TO BS EN 10346) IN THE SAME STRUCTURAL ARRANGEMENT AS TIMBER STUD FRAMING — STUDS AT 400MM OR 600MM CENTRES — BUT USING DIMENSIONALLY STABLE STEEL SECTIONS RATHER THAN TIMBER) AS AN ALTERNATIVE TO TRADITIONAL TIMBER STUD FRAMING FOR THE WALLS AND ROOF OF A REAR EXTENSION ON A LONDON VICTORIAN TERRACE.

ADVANTAGES OF SFS OVER TIMBER STUD FRAMING FOR LONDON EXTENSIONS: DIMENSIONAL ACCURACY (STEEL SECTIONS DO NOT WARP, SHRINK, OR SWELL WITH MOISTURE CONTENT CHANGES — CRITICAL FOR ACHIEVING PRECISE INTERNAL DIMENSIONS AND FLAT WALL FACES IN EXTENSIONS WITH LARGE GLAZED WALL SYSTEMS); NON-COMBUSTIBLE (STEEL SECTIONS ARE NON-COMBUSTIBLE — IMPORTANT FOR COMPLIANCE WITH PART B FIRE SAFETY REQUIREMENTS IN EXTENSIONS CLOSE TO BOUNDARIES — ALTHOUGH THE OVERALL WALL BUILD-UP INCLUDING INSULATION AND INTERNAL BOARD MUST ALSO COMPLY); PEST AND ROT RESISTANCE (STEEL DOES NOT PROVIDE FOOD OR HABITAT FOR INSECTS OR ROT FUNGI — PARTICULARLY RELEVANT IN LONDON BASEMENTS AND DAMP ENVIRONMENTS); MAIN DISADVANTAGES: THERMAL BRIDGING (STEEL IS A HIGHLY CONDUCTIVE MATERIAL — WITHOUT CAREFUL DETAILING OF A CONTINUOUS EXTERNAL INSULATION LAYER (EWI — EXTERNAL WALL INSULATION) OR AN INTERNAL INSULATED COMPOSITE PANEL, STEEL STUDS CONDUCT HEAT THROUGH THE WALL BUILD-UP FAR MORE EFFICIENTLY THAN TIMBER STUDS — SIGNIFICANTLY INCREASING THE U-VALUE OF THE WALL — SOLUTION: USE AN EXTERNAL CONTINUOUS INSULATION LAYER (KINGSPAN KOOLTHERM K5 OR INSULATED COMPOSITE PANELS) TO BREAK THE THERMAL BRIDGE AT THE STUD); SPECIALIST TRADE (SFS INSTALLATION IS A SPECIALIST TRADE — NOT WITHIN THE SKILL SET OF A GENERAL BUILDER — SPECIALIST SFS CONTRACTORS INCLUDE THOSE WORKING IN THE DRY LINING TRADE — INTERIORS / DRY LINING / METAL STUD PARTITIONING BACKGROUND); COMMON SFS SECTION SIZES FOR LONDON EXTENSION WALLS: 70MM C-SECTION STUD (LIGHT PARTITION OR INFILL WALL), 100MM C-SECTION STUD (EXTERNAL INFILL WALL WITH INSULATION), 150MM C-SECTION STUD (EXTERNAL INFILL WALL WHERE THICKER INSULATION IS REQUIRED TO MEET PART L THERMAL PERFORMANCE).

STRUCTURAL STEEL COSTS FOR LONDON VICTORIAN TERRACE EXTENSIONS (LONDON 2025 — APPROXIMATE RANGE — SUPPLY AND FIT — STRUCTURAL STEEL CONTRACTOR — ALL PRICES EXCLUSIVE OF STRUCTURAL ENGINEER FEES AND BUILDING CONTROL FEES).

STRUCTURAL STEEL BEAM SUPPLY AND FIT (SINGLE BEAM OVER REAR WALL OPENING — INCLUDING TEMPORARY SUPPORT WITH ACROW PROPS AND SPREADER BEAMS, MASONRY REMOVAL, BEAM INSTALLATION, PADSTONE INSTALLATION, MAKING GOOD AROUND BEAM ENDS): APPROXIMATELY £1,200-2,500 (SMALL TO MEDIUM BEAM — UB 203X133X25 TO UB 254X146X31 — 2.5-3.5M SPAN); APPROXIMATELY £1,800-4,000 (LARGE BEAM — UB 305X165X40 TO UB 356X171X45 — 3.5-5.0M SPAN); TWIN BEAMS (WHERE THE STRUCTURAL ENGINEER SPECIFIES TWO PARALLEL BEAMS BOLTED TOGETHER — FOR VERY WIDE SPANS OR HEAVILY LOADED CONDITIONS): ADD 50-80% TO SINGLE BEAM COSTS; PORTAL FRAME (FABRICATED STEEL PORTAL FRAME — COLUMNS AND RAFTER — DESIGNED BY STRUCTURAL ENGINEER — SUPPLIED BY SPECIALIST STEELWORK FABRICATOR — ERECTED AND CONNECTED ON SITE): APPROXIMATELY £3,000-8,000 (SMALL SINGLE-SPAN PORTAL — 4-6M SPAN — SINGLE-STOREY REAR EXTENSION); APPROXIMATELY £6,000-15,000 (LARGER OR MORE COMPLEX PORTAL FRAME — WIDER SPAN, HIGHER COLUMNS, MORE COMPLEX CONNECTIONS); STRUCTURAL ENGINEER FEES (STRUCTURAL CALCULATIONS FOR EXTENSION BEAMS AND CONNECTIONS — RESIDENTIAL LONDON VICTORIAN TERRACE EXTENSION): APPROXIMATELY £600-1,500 (CALCULATION PACKAGE FOR BUILDING CONTROL SUBMISSION — SINGLE REAR WALL OPENING AND EXTENSION STRUCTURE); BUILDING CONTROL NOTICE OR FULL PLANS APPLICATION FEE (STRUCTURAL ALTERATIONS AND EXTENSION): APPROXIMATELY £300-800 (LOCAL AUTHORITY BUILDING CONTROL) — APPROXIMATELY £250-700 (APPROVED INSPECTOR — PRIVATE BUILDING CONTROL — E.G. ASSURE, BUTLER AND YOUNG GROUP, STROMA CERTIFICATION — FASTER TURNAROUND THAN LA BUILDING CONTROL IN MANY CASES); NOTE ON BUILDING CONTROL FOR STRUCTURAL STEEL: ALL STRUCTURAL STEEL INSTALLATION IN RESIDENTIAL EXTENSIONS IS NOTIFIABLE WORK UNDER THE BUILDING REGULATIONS — BUILDING CONTROL MUST INSPECT THE STRUCTURAL STEEL INSTALLATION AT KEY STAGES — BUILDING CONTROL OFFICER (BCO) OR APPROVED INSPECTOR MUST INSPECT: AFTER TEMPORARY SUPPORT INSTALLED, BEFORE MASONRY REMOVAL; BEAM INSTALLATION (INSPECTION OF BEARING LENGTHS, PADSTONES, CONNECTIONS, LEVEL); OVERALL STRUCTURAL FRAME (WHERE A PORTAL FRAME OR SFS SYSTEM IS USED).