Green Roofs for London Victorian Terrace Extensions and Garden Rooms: Sedum, Wildflower, and Biodiverse

GREEN ROOF SYSTEMS ARE BROADLY CLASSIFIED AS EXTENSIVE (SHALLOW SUBSTRATE, LOW-MAINTENANCE, LOW-WEIGHT — THE DOMINANT TYPE FOR LONDON TERRACE EXTENSION ROOFS), SEMI-INTENSIVE (MEDIUM SUBSTRATE — MORE PLANT VARIETY), OR INTENSIVE (DEEP SUBSTRATE — ALLOWS TREES AND SHRUBS — DESIGNED FOR ROOF TERRACES AND LARGER STRUCTURAL ROOFS).

EXTENSIVE GREEN ROOF (SUBSTRATE DEPTH 60-150MM — THE STANDARD GREEN ROOF FOR LONDON SINGLE-STOREY EXTENSION OR GARDEN ROOM FLAT ROOF): SEDUM BLANKET (THE MOST COMMON EXTENSIVE GREEN ROOF FOR LONDON TERRACE EXTENSIONS): PRE-GROWN SEDUM BLANKET (ZINCO SEDUM BLANKET, BAUDER GREENROOF SEDUM BLANKET, OPTIGRÜN SEDUM SYSTEM FKL 50) — A CARPET OF LOW-GROWING SUCCULENT SEDUM SPECIES (SEDUM ALBUM, SEDUM ACRE, SEDUM REFLEXUM, SEDUM SEXANGULARE — ALL DROUGHT-TOLERANT, FROST-HARDY SPECIES WELL-SUITED TO LONDON'S CLIMATE). SUBSTRATE DEPTH: 50-80MM (LIGHTWEIGHT EXPANDED CLAY OR LECA-BASED GROWING MEDIUM — ZINCO FLORADRAIN FD 25 DRAINAGE LAYER + ZINCO SYSTEMERDE E SUBSTRATE). WEIGHT: 50-150 KG/M² WHEN SATURATED (DEAD LOAD — MUST BE CONSIDERED IN THE FLAT ROOF STRUCTURAL DESIGN OF THE EXTENSION). MAINTENANCE: LOW — SEDUM ROOFS REQUIRE NO REGULAR IRRIGATION (SEDUM IS DROUGHT-TOLERANT), OCCASIONAL WEEDING (2X PER YEAR), AND PERIODIC EDGE INSPECTION.

WILDFLOWER MEADOW GREEN ROOF (BIODIVERSE — LONDON BIODIVERSITY NET GAIN COMPLIANT): A WILDFLOWER MEADOW GREEN ROOF (BAUDER BIOSOLAR SUBSTRATE + WILDFLOWER MIX — PICTORIAL MEADOWS LONDON MIX INCLUDING CORN POPPY, CORNFLOWER, OXEYE DAISY, BIRD'S-FOOT TREFOIL, COMMON VETCH) PROVIDES HIGHER BIODIVERSITY VALUE THAN A PLAIN SEDUM BLANKET — CRITICAL FOR DEVELOPMENTS SUBJECT TO THE MANDATORY BIODIVERSITY NET GAIN (BNG) REQUIREMENT INTRODUCED IN ENGLAND IN FEBRUARY 2024 (BNG ≥ 10% NET GAIN REQUIRED ABOVE BASELINE). SUBSTRATE DEPTH: 80-120MM. MAINTENANCE: 1X ANNUAL CUT (LATE SUMMER — ALLOWING SEED HEADS TO FORM AND DROP BEFORE CUTTING).

BIODIVERSE BROWN ROOF (CRUSHED BRICK AND RUBBLE SUBSTRATE — REPLICATING LONDON'S LOST BROWNFIELD HABITAT): THE BIODIVERSE BROWN ROOF USES A VARIED SUBSTRATE OF CRUSHED BRICK, TOPSOIL BLENDS, AND RUBBLE (OBTAINED FROM THE DEMOLITION OF THE EXISTING LONDON TERRACE OUTBUILDINGS WHERE RELEVANT — TRULY CIRCULAR) TO CREATE A HABITAT THAT REPLICATES THE BARE BROWNFIELD GROUND THAT BLACK REDSTARTS (A PROTECTED LONDON BIRD — S41 BIODIVERSITY ACTION PLAN SPECIES) AND GROUND-NESTING SOLITARY BEES USE.

A GREEN ROOF SYSTEM IS INSTALLED OVER AN EXISTING FLAT ROOF WATERPROOFING MEMBRANE — THE WATERPROOFING IS THE MOST CRITICAL ELEMENT, AS THE SATURATED GROWING MEDIUM AND ROOT ACTIVITY OF THE PLANTS PRESENT SIGNIFICANT RISKS TO AN INADEQUATE OR DAMAGED WATERPROOFING LAYER.

GREEN ROOF-COMPATIBLE WATERPROOFING MEMBRANES: SIKA SARNAFIL G410-15EL PVC MEMBRANE WITH SARNAFIL G410/S327 ROOT BARRIER OVERLAY (OPTIONAL SEPARATE ROOT-RESISTANT LAYER): SARNAFIL G410 IS NATURALLY ROOT-RESISTANT (PVC IS RESISTANT TO ROOT PENETRATION) BUT WHERE ADDITIONAL CERTAINTY IS NEEDED (INTENSIVELY PLANTED GREEN ROOFS WITH TREES OR SHRUBS), A SEPARATE POLYPROPYLENE OR HDPE ROOT-BARRIER LAYER IS ADDED ABOVE THE PVC. FIRESTONE RUBBERCOVER EPDM (1.14MM FULLY BONDED EPDM — ALSO NATURALLY ROOT-RESISTANT): FIRESTONE RUBBERCOVER IS ROOT-RESISTANT AND IS ACCEPTED BY LEADING GREEN ROOF SYSTEM MANUFACTURERS (ZINCO, BAUDER, OPTIGRÜN) FOR USE UNDER THEIR EXTENSIVE GREEN ROOF SYSTEMS. BAUDER THERMOFOL U15 (ROOT-RESISTANT PVC SINGLE-PLY — BAUDER'S OWN WATERPROOFING MEMBRANE DESIGNED FOR BAUDER GREEN ROOF SYSTEMS). WATERPROOFING THAT IS NOT SUITABLE UNDER A GREEN ROOF: GRP FIBREGLASS: NOT ACCEPTED BY MOST GREEN ROOF SYSTEM MANUFACTURERS FOR USE UNDER A LIVING GREEN ROOF — GRP CAN BE VULNERABLE TO ROOT PENETRATION AT JOINTS OVER TIME AND IS NOT ON ZINCO, BAUDER, OR OPTIGRÜN APPROVED SUBSTRATE LISTS. ASPHALT AND FELT: SIMILARLY NOT RECOMMENDED UNDER A GREEN ROOF SYSTEM.

DRAINAGE LAYERS: GREEN ROOF SYSTEM DRAINAGE LAYER (ZINCO FLORADRAIN FD 25 OR FD 40 — THE STANDARD DRAINAGE LAYER FOR LONDON EXTENSIVE GREEN ROOFS): A DIMPLED POLYPROPYLENE RETENTION AND DRAINAGE MAT — RETAINS WATER FOR PLANT USE WHILE ALLOWING EXCESS RAINFALL TO DRAIN TO THE ROOF OUTLETS. INSTALLED BETWEEN THE WATERPROOFING MEMBRANE AND THE GROWING SUBSTRATE. ROOF OUTLETS (SCUPPERS AND DRAINS): THE ORIGINAL FLAT ROOF DRAINAGE OUTLETS MUST REMAIN ACCESSIBLE THROUGH THE GREEN ROOF SYSTEM — ZINCO FLORADRAIN INCLUDES INSPECTION CHAMBERS AROUND DRAIN POSITIONS.

THE ADDITIONAL WEIGHT OF A GREEN ROOF SYSTEM — GROWING MEDIUM, DRAINAGE LAYER, VEGETATION, AND RETAINED RAINWATER — MUST BE ACCOUNTED FOR IN THE STRUCTURAL DESIGN OF THE FLAT ROOF FRAMING OF THE LONDON VICTORIAN TERRACE EXTENSION.

GREEN ROOF DEAD LOADS FOR STRUCTURAL DESIGN: EXTENSIVE SEDUM GREEN ROOF (50MM SUBSTRATE): SATURATED DEAD LOAD: APPROXIMATELY 80-120 KG/M². EXTENSIVE WILDFLOWER GREEN ROOF (80MM SUBSTRATE): SATURATED DEAD LOAD: APPROXIMATELY 120-180 KG/M². SEMI-INTENSIVE (150MM SUBSTRATE): SATURATED DEAD LOAD: APPROXIMATELY 200-250 KG/M². INTENSIVE (500MM SUBSTRATE — TREES AND SHRUBS): SATURATED DEAD LOAD: 500-1000 KG/M² — REQUIRES REINFORCED CONCRETE OR STEEL FRAME ROOF STRUCTURE.

STRUCTURAL IMPLICATIONS FOR A TYPICAL LONDON TERRACE SINGLE-STOREY FLAT ROOF EXTENSION: A STANDARD FLAT ROOF TIMBER STRUCTURE (220X57MM C24 JOISTS AT 400MM CENTRES SPANNING 3-4M FROM HOUSE WALL TO STEEL BEAM) IS TYPICALLY DESIGNED FOR A DEAD LOAD OF 1.0-1.5 KN/M² (APPROXIMATELY 100-150 KG/M²) — WHICH IS AT THE LIMIT OF THE CAPACITY FOR A SEDUM GREEN ROOF AT 50MM SUBSTRATE. IF AN EXTENSIVE GREEN ROOF IS PLANNED FROM THE START OF THE EXTENSION DESIGN, THE STRUCTURAL ENGINEER SHOULD SIZE THE ROOF JOISTS AND STEEL BEAM FOR THE ADDITIONAL GREEN ROOF DEAD LOAD: INCREASE JOIST SIZE FROM 220X57MM TO 250X63MM C24 OR ADD INTERMEDIATE JOIST SUPPORT (MID-SPAN BEAM).

ROOF SLOPE (FALL): A FLAT ROOF USED FOR A GREEN ROOF SYSTEM SHOULD HAVE A MINIMUM FALL OF 1:80 (1.25%) — GREEN ROOF SYSTEMS CAN TOLERATE EVEN LOWER FALLS THAN STANDARD FLAT ROOFS BECAUSE THE DRAINAGE LAYER MANAGES WATER DISTRIBUTION — BUT COMPLETELY FLAT (NO FALL) IS NOT RECOMMENDED AS STANDING WATER IN THE SUBSTRATE PROMOTES ANAEROBIC CONDITIONS UNFAVOURABLE TO THE PLANTS.

GREEN ROOFS ARE INCREASINGLY SUPPORTED — AND IN SOME CASES REQUIRED — BY LONDON PLANNING POLICY AS A MEANS OF DELIVERING SUSTAINABLE URBAN DRAINAGE (SUDs), BIODIVERSITY NET GAIN (BNG), AND URBAN HEAT ISLAND MITIGATION.

LONDON PLAN POLICY (GLA — GREATER LONDON AUTHORITY): THE LONDON PLAN (2021) POLICY G5 (URBAN GREENING) AND POLICY SI13 (SUSTAINABLE DRAINAGE) TOGETHER ENCOURAGE GREEN ROOFS (AND OTHER GREEN INFRASTRUCTURE) IN LONDON PLANNING APPLICATIONS. WHILE A HOUSEHOLDER PLANNING APPLICATION FOR A SINGLE-STOREY REAR EXTENSION IS UNLIKELY TO TRIGGER A FORMAL GREEN ROOF REQUIREMENT FROM THE LONDON BOROUGH PLANNER, GREEN ROOFS CAN IMPROVE THE PLANNING CASE FOR A LARGER EXTENSION IN A CONSERVATION AREA (BY DEMONSTRATING ENVIRONMENTAL BENEFIT) OR REDUCE SURFACE WATER RUNOFF CALCULATIONS IN SCHEMES WHERE DRAINAGE IS A PLANNING CONDITION.

BIODIVERSITY NET GAIN (BNG — MANDATORY IN ENGLAND FROM FEBRUARY 2024 FOR MOST PLANNING APPLICATIONS): FOR PLANNING APPLICATIONS REQUIRING BNG COMPLIANCE (MOST MAJOR APPLICATIONS FROM FEBRUARY 2024), A GREEN ROOF WITH WILDFLOWER MEADOW OR BROWN ROOF (CRUSHED BRICK BIODIVERSE SUBSTRATE) CAN CONTRIBUTE POSITIVE BIODIVERSITY UNIT VALUES IN THE BNG CALCULATION. A SEDUM-ONLY ROOF SCORES LOWER IN THE BNG METRIC THAN A WILDFLOWER OR BROWN ROOF (BECAUSE SEDUM MONOCULTURE IS LESS BIODIVERSE THAN A MIXED WILDFLOWER OR RUBBLE SUBSTRATE HABITAT). GREEN ROOF AND BLACK REDSTART: BLACK REDSTARTS (PHOENICURUS OCHRUROS) ARE A SCHEDULE 1-PROTECTED SPECIES THAT BREED IN LONDON ON BROWNFIELD SITES AND ROOFTOPS — THE RSPB AND LONDON WILDLIFE TRUST SUPPORT BIODIVERSE BROWN ROOFS ON NEW LONDON EXTENSIONS AS COMPENSATORY HABITAT FOR BLACK REDSTART BREEDING HABITAT LOST TO DEVELOPMENT. SUDS BENEFIT: A GREEN ROOF RETAINS APPROXIMATELY 50-80% OF ANNUAL RAINFALL (FOR AN EXTENSIVE SEDUM SYSTEM IN LONDON — RAINFALL APPROXIMATELY 600MM/YEAR) AND ATTENUATES STORM PEAKS — REDUCING SURFACE WATER RUNOFF TO THE LONDON COMBINED SEWER SYSTEM.

GREEN ROOF SYSTEM COSTS FOR LONDON VICTORIAN TERRACE SINGLE-STOREY EXTENSIONS AND GARDEN ROOMS VARY BY SYSTEM TYPE AND SUBSTRATE DEPTH.

GREEN ROOF MATERIAL COSTS (SUPPLY ONLY — PER M² OF ROOF AREA): ZINCO SEDUM BLANKET EXTENSIVE SYSTEM (FLORADRAIN FD 25 DRAINAGE MAT + SYSTEMERDE E SUBSTRATE 50MM + SEDUM BLANKET — FULLY PRE-VEGETATED): £35 TO £60 PER M² SUPPLY. BAUDER WILDFLOWER EXTENSIVE SYSTEM (BIOSOLAR SUBSTRATE 80MM + WILDFLOWER SEEDING): £40 TO £70 PER M² SUPPLY. BIODIVERSE BROWN ROOF (CRUSHED BRICK SUBSTRATE 80-100MM + TOPSOIL BAYS): £25 TO £45 PER M² MATERIALS.

GREEN ROOF INSTALLATION LABOUR (PER M²): EXTENSIVE SEDUM BLANKET (DELIVERY TO SITE + INSTALLATION LABOUR — TYPICALLY A 2-PERSON TEAM, 20-30 M² PER DAY): £15 TO £30 PER M².

TOTAL GREEN ROOF SUPPLY AND INSTALL (PER M² — EXTENSIVE SEDUM): £50 TO £90 PER M².

TYPICAL LONDON TERRACE SINGLE-STOREY REAR EXTENSION GREEN ROOF (15 M² FLAT ROOF — SEDUM EXTENSIVE — ZINCO SYSTEM): WATERPROOFING (IF NEW — SARNAFIL 1.5MM OVER OSB SUBSTRATE): £20 TO £35 PER M² = £300 TO £525. GREEN ROOF SYSTEM SUPPLY AND INSTALL (ZINCO SEDUM): £50 TO £90 PER M² = £750 TO £1,350. STRUCTURAL UPGRADE (IF JOISTS NEED UPSIZING): £500 TO £1,500. TOTAL (15 M²): £1,550 TO £3,375 (EXCLUDING WATERPROOFING IF ALREADY SPECIFIED AND INSTALLED AS PART OF EXTENSION WORKS).

COMPARED TO STANDARD PAVING ON A TERRACE OR PLAIN MEMBRANE ROOF: A GREEN ROOF ADDS APPROXIMATELY £30 TO £55 PER M² ABOVE THE COST OF A PLAIN SINGLE-PLY MEMBRANE FLAT ROOF FINISH — A RELATIVELY MODEST PREMIUM FOR A 15-20 M² LONDON TERRACE EXTENSION ROOF (ADDITIONAL COST: £450 TO £1,100 OVER A PLAIN ROOF).

EXTENSIVE GREEN ROOFS (SEDUM AND WILDFLOWER) ON LONDON VICTORIAN TERRACE EXTENSIONS ARE INHERENTLY LOW-MAINTENANCE ONCE ESTABLISHED — BUT THEY ARE NOT ENTIRELY MAINTENANCE-FREE, AND HOMEOWNERS SHOULD UNDERSTAND THE MAINTENANCE REQUIREMENTS WHEN COMMISSIONING A GREEN ROOF.

ESTABLISHMENT PERIOD (FIRST 2 YEARS): SEDUM BLANKETS ARRIVE PRE-ESTABLISHED (GROWN FOR 12+ MONTHS BEFORE DELIVERY) AND DO NOT REQUIRE AN ESTABLISHMENT PERIOD IN THE SAME WAY AS SEEDED GREEN ROOFS. HOWEVER, IN THE FIRST SUMMER AFTER INSTALLATION (PARTICULARLY IN A DROUGHT YEAR — LONDON HAS EXPERIENCED SEVERAL DRY SUMMERS IN RECENT YEARS, INCLUDING 2022 AND 2023), THE GREEN ROOF MAY NEED SUPPLEMENTARY IRRIGATION (ONCE OR TWICE DURING AN EXTREME DROUGHT) TO PREVENT SEDUM DIE-BACK. WILDFLOWER SEEDED GREEN ROOFS REQUIRE AN ESTABLISHMENT YEAR OF MORE CARE — WEEDING TO PREVENT VIGOROUS PIONEER SPECIES (ROSEBAY WILLOWHERB, RAGWORT) FROM SMOTHERING THE SLOWER-ESTABLISHING NATIVE WILDFLOWERS.

ONGOING MAINTENANCE (PER YEAR — ESTABLISHED SEDUM GREEN ROOF): VISUAL INSPECTION (ONCE PER YEAR — CHECK DRAINAGE OUTLETS, LOOK FOR LARGE WEED GROWTH, CHECK MEMBRANE AT PERIMETER AND UPSTAND): 1 HOUR HOMEOWNER. WEEDING (REMOVE ANY TREE SEEDLINGS AND INVASIVE WEEDS — PARTICULARLY WILLOW AND BIRCH SEEDLINGS WHICH WILL ESTABLISH IN THE SUBSTRATE AND THEIR ROOTS CAN EVENTUALLY DAMAGE THE MEMBRANE): 2-4 HOURS PER YEAR. DRAINAGE OUTLET CLEARANCE (REMOVE DEBRIS FROM INSPECTION CHAMBERS AROUND ROOF DRAINS): 30 MINUTES PER YEAR. MEMBRANE INSPECTION (VISUAL CHECK OF ALL EXPOSED EDGES, UPSTANDS, DRAINS, AND PENETRATIONS FOR DAMAGE OR DETERIORATION): BY A WATERPROOFING SPECIALIST EVERY 5 YEARS (COST: £200-400 — TYPICALLY INCLUDED IN MANUFACTURER WARRANTY INSPECTION).

LIFESPAN OF GREEN ROOF COMPONENTS: WATERPROOFING MEMBRANE (SARNAFIL PVC OR FIRESTONE EPDM): 20-25 YEAR MANUFACTURER GUARANTEE; EXPECTED LIFESPAN 40+ YEARS (PROTECTED BY GREEN ROOF SUBSTRATE FROM UV AND THERMAL CYCLING). DRAINAGE MAT (ZINCO FLORADRAIN POLYPROPYLENE): EXPECTED LIFESPAN 40+ YEARS. SUBSTRATE: PERMANENT — DOES NOT DEGRADE. VEGETATION: PERENNIAL — SEDUM BLANKETS DO NOT NEED REPLACING; WILDFLOWER MEADOW PATCHES OUT OVER TIME BUT SELF-SEEDS.