Foul Drainage for London Victorian Terraces: Extensions, Soil Stacks, and Drainage Layouts

THE ABOVE-GROUND FOUL DRAINAGE SYSTEM OF A LONDON VICTORIAN TERRACE CONSISTS OF: THE SOIL STACK (SVP — SOIL VENT PIPE): THE PRIMARY VERTICAL PIPE THAT COLLECTS WC, BATH, SHOWER, BASIN, AND OTHER WASTE CONNECTIONS AND DISCHARGES TO THE BELOW-GROUND DRAINAGE SYSTEM. IN ORIGINAL LONDON VICTORIAN TERRACE CONSTRUCTION: THE EXTERNAL SOIL STACK WAS CAST IRON (110MM DIAMETER — NOW TYPICALLY REPLACED WITH UPVC AS PART OF RENOVATION OR EXTENSION WORK). UPVC REPLACEMENT SPECIFICATION: POLYPIPE 110MM SOIL SERIES BS EN 1329 GREY (FOR EXTERNAL SOIL STACKS — UV-STABLE) OR OSMA TERRAIN UPVC 110MM GREY (EQUIVALENT). MINIMUM STACK HEIGHT: THE TOP OF THE SOIL STACK (SVP TERMINAL) MUST BE: MINIMUM 900MM ABOVE ANY OPENING WINDOW WITHIN 3M HORIZONTAL DISTANCE (PART H1 BUILDING REGULATIONS 2010 — APPROVED DOCUMENT H).

TRAP MINIMUM DEPTHS (APPROVED DOCUMENT H — PART H1): TO PREVENT FOUL AIR ENTERING THE BUILDING, ALL SANITARY APPLIANCES MUST BE FITTED WITH A TRAP OF MINIMUM DEPTH: WC: INTEGRAL TRAP (P-TRAP IN THE PAN ITSELF — TYPICALLY 50MM SEAL ON MODERN CLOSE-COUPLED WCS). WASHBASIN: 75MM MINIMUM WATER SEAL (BOTTLE TRAP OR P-TRAP — HEPWORTH DUROPLAST 32MM P-TRAP). BATH AND SHOWER TRAY: 50MM MINIMUM WATER SEAL (BATH TRAP — IDEAL STANDARD BATH TRAP 40MM — OR SHOWER TRAY TRAP — MCALPINE 90MM OR 50MM MINIMUM SEAL SHOWER TRAP). KITCHEN SINK: 75MM MINIMUM WATER SEAL (WASTE TRAP — MCALPINE WP6 TUBULAR SWIVEL P-TRAP 38MM/40MM).

WASTE PIPE FALL REQUIREMENTS (APPROVED DOCUMENT H — PART H1): TO ACHIEVE SELF-CLEANSING FLOW AND PREVENT BLOCKAGES, WASTE PIPES MUST BE LAID TO MINIMUM FALLS: 32MM WASTE PIPE (BASIN, BIDET): MINIMUM FALL 1:12 (1 IN 12) — MAXIMUM LENGTH 1.7M TO STACK WITHOUT AIR ADMITTANCE VALVE (AAV). 40MM WASTE PIPE (BATH, SHOWER, KITCHEN SINK): MINIMUM FALL 1:40 TO 1:100 — MAXIMUM LENGTH 3M TO STACK. 50MM WASTE PIPE (MINIMUM FOR KITCHEN SINK IF LONGER THAN 3M): MINIMUM FALL 1:40. 110MM SOIL PIPE (WC DIRECT CONNECTION TO STACK): MINIMUM FALL 1:80 TO STACK, MAXIMUM DISTANCE FROM TRAP TO STACK: 6M.

THE BELOW-GROUND FOUL DRAINAGE SYSTEM OF A LONDON VICTORIAN TERRACE RUNS FROM THE BASE OF THE SOIL STACK (OR FROM THE GULLY AT THE REAR OF THE PROPERTY) TO THE LATERAL CONNECTION INTO THE PUBLIC SEWER — WHICH IN LONDON RUN BENEATH THE STREET OR SOMETIMES BENEATH THE REAR GARDEN.

PIPE MATERIALS — LONDON VICTORIAN TERRACE BELOW-GROUND DRAINAGE: ORIGINAL CONSTRUCTION: VITRIFIED CLAY SOCKET-AND-SPIGOT PIPE (100MM DIAMETER, LEAD-CAULKED JOINTS) — STILL IN PLACE BENEATH MANY LONDON VICTORIAN TERRACES. IF SOUND (CCTV SURVEY OR DRAIN ROD CHECK — NO CRACKING, NO ROOT INGRESS, NO DISPLACED JOINTS): CAN BE RETAINED. IF CRACKED, COLLAPSED, ROOT-INFESTED, OR DISPLACED: MUST BE REPLACED. REPLACEMENT SPECIFICATION: HEPWORTH SUPERSLEVE VITRIFIED CLAY (100MM — PUSH-FIT FLEXIBLE JOINT — BS EN 295) — THE GOLD STANDARD FOR LONDON VICTORIAN TERRACE BELOW-GROUND DRAINAGE REPLACEMENT. ALTERNATIVELY: POLYPIPE TERRAIN 110MM UPVC UNDERGROUND (ORANGE — BS EN 1401 — SOLID WALL SDR 34) — ACCEPTABLE FOR BELOW-GROUND USE BUT CLAY IS PREFERRED BY LONDON BOROUGH BUILDING CONTROL INSPECTORS FOR ITS LONG-TERM DURABILITY. GRADIENT: MINIMUM 1:40 (2.5%) FOR 100MM DRAIN RUN — PREFERRED 1:80 IS ACCEPTABLE FOR SELF-CLEANSING AT WC FLOW RATES BUT BORDERLINE FOR MIXED FLOWS. MAXIMUM GRADIENT FOR 100MM: 1:20 (FASTER GRADIENT = LIQUID RACES AHEAD OF SOLIDS — POTENTIAL BLOCKAGE).

ACCESS POINTS: RODDING EYE OR INSPECTION CHAMBER: ACCESS POINTS MUST BE PROVIDED: AT EVERY CHANGE OF DIRECTION (MORE THAN 45°). AT THE HEAD OF THE DRAIN RUN (ENTRY FROM SOIL STACK). AT INTERVALS NOT EXCEEDING 22M ON STRAIGHT RUNS (100MM PIPE). INSPECTION CHAMBER (IC) SPECIFICATION: PRE-CAST CONCRETE IC (SHALLOW DEPTH — LESS THAN 1000MM DEEP): POLYPIPE WAVIN HEPWORTH OR FLOPLAST PVC IC BASE — 150MM CHANNEL INVERT; PRECAST CONCRETE RINGS (300MM OR 450MM DIAMETER); PRECAST CONCRETE COVER SLAB + CAST IRON DOUBLE-SEALED COVER (GRADE A — LIGHT TRAFFIC) OR POLYPIPE PLASTIC IC COVER (300X300MM — GARDEN TRAFFIC ONLY).

ONE OF THE MOST SIGNIFICANT PLANNING AND CONSTRUCTION CONSTRAINTS FOR LONDON VICTORIAN TERRACE EXTENSIONS IS THE SEWER BUILD-OVER REQUIREMENT — MANAGED BY THAMES WATER (THE STATUTORY SEWERAGE UNDERTAKER FOR MOST OF LONDON).

WHICH SEWERS REQUIRE THAMES WATER BUILD-OVER APPROVAL: SEWERS UP TO 160MM DIAMETER (THE MOST COMMON SEWER DIAMETER BENEATH LONDON VICTORIAN TERRACE REAR GARDENS — THESE WERE ADOPTED BY THAMES WATER UNDER THE WATER INDUSTRY ACT 1991 AND LATER UNDER THE 2011 PRIVATE SEWERS TRANSFER): A BUILD-OVER AGREEMENT IS REQUIRED IF THE PROPOSED EXTENSION FOUNDATION OR STRUCTURE FALLS WITHIN THE EXCLUSION ZONE (WHICH IN LONDON FOR A 100-150MM DIAMETER PUBLIC SEWER IS: 3M EITHER SIDE OF THE PIPE CENTRELINE FOR SEWERS OVER 1000MM DEEP — WITHIN 1M OF THE PIPE CROWN FOR SHALLOWER SEWERS).

THAMES WATER BUILD-OVER AGREEMENT (FORMAL AGREEMENT): REQUIRED WHERE: THE PROPOSED STRUCTURE IS WITHIN THE EXCLUSION ZONE. THE SEWER IS MORE THAN 150MM DIAMETER. THE SEWER IS MORE THAN 3M DEEP (DEEP SEWERS HAVE MORE STRINGENT REQUIREMENTS). APPLICATION TO THAMES WATER BUILD-OVER TEAM: SUBMIT: DRAINAGE DRAWINGS (AS-BUILT INVERT LEVELS, COVER LEVELS, DIRECTION OF FLOW, PIPE SIZES), STRUCTURAL DRAWINGS (FOUNDATIONS — MUST SPAN THE SEWER WITH ADEQUATE STRUCTURAL CLEARANCE — TYPICALLY REINFORCED CONCRETE RAFT OR RING BEAM), LOAD CALCULATIONS. THAMES WATER WILL REQUIRE: ACCESS MAINTENANCE DISTANCE MAINTAINED (MINIMUM 1000MM CLEAR BETWEEN PIPE OUTER AND FOUNDATION FACE — THAMES WATER GUIDANCE). CCTV SURVEY OF THE SEWER PRE-CONSTRUCTION AND POST-CONSTRUCTION. NO PERMANENT STRUCTURE DIRECTLY OVER THE SEWER CROWN (RAFT FOUNDATION WITH VOID FORMER OVER SEWER IS THE TYPICAL SOLUTION FOR BUILD-OVER COMPLIANCE IN LONDON — CREATING A STRUCTURAL ARCH OVER THE SEWER).

SEWER DIVERSION (ALTERNATIVE TO BUILD-OVER): IF BUILD-OVER IS NOT FEASIBLE (SEWER TOO LARGE, TOO SHALLOW, OR THAMES WATER REFUSES APPROVAL): THE SEWER MUST BE DIVERTED AWAY FROM THE EXTENSION FOOTPRINT. COST: £5,000 TO £20,000+ FOR A SEWER DIVERSION IN LONDON (DEPENDING ON SEWER DIAMETER, DEPTH, LENGTH OF DIVERSION, AND WHETHER DISRUPTION TO TRAFFIC OR ADJACENT PROPERTIES IS REQUIRED). THAMES WATER CONTRACTOR: THE DIVERSION MUST BE CARRIED OUT BY A THAMES WATER APPROVED CONTRACTOR (FROM THAMES WATER FRAMEWORK LIST).

ADDING A NEW BATHROOM, KITCHEN, OR UTILITY ROOM IN A LONDON VICTORIAN TERRACE EXTENSION REQUIRES CAREFUL DRAINAGE PLANNING — PARTICULARLY FOR WC AND SHOWER POSITIONS WHERE WASTE PIPE FALLS AND DISTANCES TO THE SOIL STACK ARE THE CRITICAL CONSTRAINTS.

NEW BATHROOM IN A REAR EXTENSION — DRAINAGE PLANNING CONSIDERATIONS: WC POSITION: THE MOST IMPORTANT FACTOR IS THE HORIZONTAL DISTANCE FROM THE WC PAN OUTLET TO THE SOIL STACK (MAXIMUM 6M FOR A 110MM SOIL PIPE — BUT CLOSER IS BETTER — A WC DIRECTLY BEHIND THE SOIL STACK WITH LESS THAN 1M RUN IS IDEAL). IF THE PROPOSED WC IS FURTHER THAN 6M FROM THE EXISTING STACK: A NEW SOIL STACK POSITION IN THE EXTENSION MAY BE REQUIRED. ALTERNATIVELY: A MACERATOR WC SYSTEM (SANIFLO STA12 OR SANIFLO SANITOP MACERATOR — PUMPED WASTE SYSTEM) CAN OVERCOME THE DISTANCE LIMITATION BY PUMPING MACEATED WASTE THROUGH A 32MM PIPE FOR UP TO 100M HORIZONTAL + 5M VERTICAL — HOWEVER THIS IS NOT PERMITTED BY SOME LONDON BOROUGH BUILDING CONTROL AUTHORITIES AS THE PRIMARY WC (PREFER GRAVITY DRAINAGE FROM AT LEAST ONE WC).

SHOWER TRAP IN AN EXTENSION GROUND FLOOR: A SHOWER IN A GROUND-FLOOR EXTENSION IN A LONDON VICTORIAN TERRACE PRESENTS ONE OF THE MOST CHALLENGING DRAINAGE PLANNING ISSUES: THE FLOOR SLAB (TYPICALLY 100-150MM CONCRETE OVER TYPE 1 SUB-BASE) HAS LIMITED DEPTH FOR THE SHOWER TRAP — A STANDARD SHOWER TRAY TRAP NEEDS 50MM MINIMUM DEPTH BELOW THE TRAY AND THE TRAP CONNECTS TO A 40MM WASTE PIPE WHICH MUST FALL AT 1:40-1:100. OPTIONS: LINEAR DRAIN (ACO SHOWER LINE 600MM — CONNECTS TO 50MM OR 40MM HORIZONTAL WASTE) IN A FLUSH TILE SHOWER — MINIMISES TRAP DEPTH REQUIREMENT. RECESSED TRAY FORMER (WEDI FUNDO PRIMO RECESSED — THE SHOWER TRAY FORMER IS BUILT INTO THE SCREED, REDUCING THE FLOOR LEVEL REQUIREMENT). TANKED WETROOM (WEDI BOARD OR SCHLÜTER DITRA DRAIN — THE ENTIRE FLOOR SURFACE FALLS TO A CENTRAL OR LINEAR DRAIN INLET).

BEFORE DESIGNING AND PRICING A REAR EXTENSION OR RENOVATION INVOLVING DRAINAGE CHANGES ON A LONDON VICTORIAN TERRACE, A DRAINAGE SURVEY IS STRONGLY RECOMMENDED — AND IN SOME CASES IS REQUIRED BY BUILDING CONTROL OR THAMES WATER BEFORE APPROVAL IS GRANTED.

CCTV DRAINAGE SURVEY (THE MOST IMPORTANT PRE-DESIGN INVESTIGATION FOR FOUL DRAINAGE ON A LONDON VICTORIAN TERRACE EXTENSION): A CCTV DRAINAGE SURVEY USES A CRAWLER CAMERA (RADIODETECTION RD8100 OR PEARPOINT FLEXITRAX P350 — THE STANDARD INSPECTION CAMERAS USED BY LONDON DRAINAGE CONTRACTORS) PASSED THROUGH THE DRAIN RUN FROM THE SOIL STACK BASE TO THE LATERAL CONNECTION, TO PROVIDE: VIDEO FOOTAGE OF THE INTERNAL CONDITION OF THE DRAIN (CRACKS, FRACTURES, ROOT INGRESS, DISPLACED JOINTS, SILT DEPOSITS, PARTIAL COLLAPSE). PIPE INVERT LEVELS AND DEPTHS (TAKEN FROM THE CAMERA DEPTH SENSOR). PIPE MATERIAL AND DIAMETER. DIRECTION AND APPROXIMATE POSITION OF THE DRAIN RUN UNDER THE GARDEN. THIS INFORMATION IS CRITICAL FOR: ESTABLISHING WHETHER THE DRAIN FALLS WITHIN THE PROPOSED EXTENSION FOOTPRINT (BUILD-OVER RISK). IDENTIFYING ANY PRE-EXISTING DEFECTS (WHICH BUILDING CONTROL MAY REQUIRE TO BE REPAIRED AS PART OF THE EXTENSION WORKS). PROVIDING ACCURATE INVERT LEVELS FOR DRAINAGE DESIGN.

COST OF CCTV DRAINAGE SURVEY IN LONDON: £200 TO £450 FOR A STANDARD LONDON VICTORIAN TERRACE (SINGLE-STACK DRAINAGE, 1-2 INSPECTION CHAMBERS, UP TO 20M DRAIN RUN). A WRITTEN REPORT WITH DVD/USB VIDEO IS TYPICALLY INCLUDED.

DRAINAGE DESIGN FOR BUILDING CONTROL: PART H DRAINAGE DRAWINGS REQUIRED FOR BUILDING CONTROL APPLICATION (FULL PLANS ROUTE): DRAINAGE LAYOUT PLAN (1:50 OR 1:100 — SHOWING ALL PIPE RUNS, INVERT LEVELS AT EACH JUNCTION AND IC, PIPE SIZES AND GRADIENTS, TRAP SPECIFICATIONS, SOIL STACK POSITION AND HEIGHT). DRAINAGE SECTION (SHOWING DEPTHS AND FALLS FROM EACH APPLIANCE TO THE LATERAL CONNECTION). APPROVED DOCUMENT H COMPLIANCE STATEMENTS.

FOUL DRAINAGE COSTS FOR LONDON VICTORIAN TERRACE EXTENSION AND RENOVATION PROJECTS RANGE FROM A SIMPLE WASTE-ONLY SINGLE BATHROOM ADDITION (£800 TO £2,000) TO A COMPLEX SEWER BUILD-OVER + NEW SOIL STACK + DRAINAGE REDESIGN (£5,000 TO £20,000+).

SIMPLE BATHROOM ADDITION (WASTE ONLY — NO NEW WC — BASIN + SHOWER — EXISTING SOIL STACK): ABOVE-GROUND PIPEWORK (40MM UPVC WASTE, TRAPS, BOSSES TO STACK — LABOUR AND MATERIALS): £300 TO £600. TOTAL (NO BELOW-GROUND WORK REQUIRED): £300 TO £600.

FULL BATHROOM ADDITION (WC + BASIN + BATH/SHOWER — SOIL PIPE CONNECTION TO EXISTING STACK — WITHIN 6M): ABOVE-GROUND SOIL AND WASTE PIPEWORK (110MM SOIL PIPE, 40MM WASTE PIPES, TRAPS, BOSS FITTINGS — LABOUR AND MATERIALS): £600 TO £1,200. TOTAL (NO BELOW-GROUND WORK REQUIRED): £600 TO £1,200.

NEW SOIL STACK (IN EXTENSION — 110MM UPVC — EXTERNAL — 3-4M HEIGHT — CONNECTED TO BELOW-GROUND DRAIN AT BASE): MATERIALS (POLYPIPE 110MM SOIL SYSTEM — STACK, BENDS, BRACKETS): £150 TO £350. LABOUR (PLUMBER — 1-2 DAYS): £400 TO £700. BELOW-GROUND CONNECTION (JUNCTION INTO EXISTING CLAY DRAIN — NEW CLAY BRANCH — EXCAVATION): £300 TO £800. TOTAL NEW SOIL STACK: £850 TO £1,850.

BELOW-GROUND DRAINAGE WORKS (NEW DRAIN RUN IN EXTENSION — 3-5M LENGTH — CLAY 100MM — TO EXISTING IC): EXCAVATION (HAND DIG — LONDON CLAY — 600-900MM DEPTH): £200 TO £500. PIPE SUPPLY AND LAY (HEPWORTH SUPERSLEVE CLAY, CONCRETE HAUNCH AND BED): £250 TO £600. BACKFILL AND REINSTATE: £100 TO £300. TOTAL NEW DRAIN RUN: £550 TO £1,400.

THAMES WATER BUILD-OVER AGREEMENT (ADMINISTRATION AND CONSULTANT FEE — EXCLUDES STRUCTURAL COSTS): £500 TO £1,500 (STRUCTURAL ENGINEER DRAINAGE DRAWINGS AND THAMES WATER SUBMISSION FEE).