Roof Terraces & Roof Gardens in London: Planning, Waterproofing & Costs

ROOF TERRACES ARE NOT PERMITTED DEVELOPMENT IN MOST CIRCUMSTANCES IN LONDON. UNLIKE A SIMPLE FLAT ROOF CONVERSION (ADDING INSULATION AND REROOFING), CREATING A ROOF TERRACE — A USABLE OUTDOOR AMENITY SPACE ON A FLAT ROOF WITH ACCESS FROM THE PROPERTY — TYPICALLY REQUIRES PLANNING PERMISSION BECAUSE IT:

1. CHANGES THE USE OF THE ROOF FROM MAINTENANCE ACCESS TO HABITABLE OUTDOOR SPACE 2. INTRODUCES THE POTENTIAL FOR OVERLOOKING OF NEIGHBOURING PROPERTIES 3. MAY REQUIRE A BALUSTRADE OR PARAPET THAT INCREASES THE APPARENT HEIGHT OF THE BUILDING 4. GENERATES NOISE AND ACTIVITY THAT NEIGHBOURING PROPERTIES WERE NOT EXPECTING

PLANNING POLICY IN LONDON: THE LONDON PLAN DOES NOT SPECIFICALLY RESTRICT ROOF TERRACES, BUT INDIVIDUAL BOROUGHS APPLY POLICIES ON: — PRIVACY AND OVERLOOKING: MOST BOROUGHS REQUIRE THAT A ROOF TERRACE DOES NOT CREATE DIRECT OVERLOOKING INTO NEIGHBOURING HABITABLE ROOMS OR GARDENS. PLANNING CONDITIONS OFTEN RESTRICT GLAZED BALUSTRADES ON SIDES OVERLOOKING NEIGHBOURS, SPECIFY SOLID OPAQUE BALUSTRADES ON PARTY WALL SIDES, OR RESTRICT OCCUPATION TO CERTAIN AREAS OF THE ROOF. — NOISE AND DISTURBANCE: IN HIGH-DENSITY LONDON TERRACE AND FLAT ENVIRONMENTS, PLANNING OFFICERS ASSESS WHETHER A ROOF TERRACE WOULD CREATE UNACCEPTABLE NOISE AND DISTURBANCE FOR NEIGHBOURS. APPLICATIONS ON UPPER FLOORS IN TIGHTLY PACKED STREETS FACE STRONG SCRUTINY. — HERITAGE IMPACT: IN CONSERVATION AREAS, ROOF TERRACES THAT ARE VISIBLE FROM THE PUBLIC HIGHWAY OR FROM ADJOINING PROPERTIES FACE THE HIGHEST PLANNING BARRIERS.

SUCCESSFUL APPLICATIONS TYPICALLY: — LIMIT TERRACE AREA TO PART OF THE ROOF (NOT THE FULL ROOF AREA) — PROVIDE SOLID OPAQUE SCREENING ON SIDES OVERLOOKING NEIGHBOURS — LIMIT TERRACE TO REAR ONLY (NOT FRONT OF PROPERTY) — INCLUDE PLANNING CONDITIONS RESTRICTING NOISE-GENERATING ACTIVITIES

PRACTICAL ADVICE: CONSULT A PLANNING CONSULTANT BEFORE COMMISSIONING ARCHITECTURAL DRAWINGS. PRE-APPLICATION ADVICE FROM THE LPA (£300-£600) IS STRONGLY RECOMMENDED FOR ROOF TERRACE APPLICATIONS TO ESTABLISH WHETHER THE PROPOSAL IS ACCEPTABLE IN PRINCIPLE.

CONVERTING A FLAT ROOF TO A USABLE TERRACE IMPOSES SIGNIFICANTLY HIGHER LOADS ON THE ROOF STRUCTURE THAN A STANDARD FLAT ROOF:

LOADING COMPARISON: — STANDARD FLAT ROOF (MAINTENANCE ACCESS ONLY): DEAD LOAD (STRUCTURE + INSULATION + MEMBRANE) TYPICALLY 1.5-3.0 KN/M2 + SNOW LOAD 0.75 KN/M2 — ROOF TERRACE (HABITABLE USE): IMPOSED LIVE LOAD OF 1.5 KN/M2 (DOMESTIC OCCUPANCY UNDER BS EN 1991-1-1, TABLE 6.2) + DEAD LOAD OF DECKING SYSTEM (TYPICALLY 0.3-1.2 KN/M2 DEPENDING ON MATERIAL) + PLANTERS (CAN BE 3-10 KN/M2 FOR HEAVY PLANTED AREAS) — BALUSTRADE: HORIZONTAL LOADING OF 0.74 KN/M LINEAR FOR DOMESTIC BALUSTRADES (BS 6399-1)

A STRUCTURAL ENGINEER MUST ASSESS: 1. EXISTING ROOF JOIST OR FLAT ROOF BEAM CAPACITY (TYPICALLY 200MM SOFTWOOD JOISTS AT 400MM CENTRES — THESE MAY ADEQUATELY CARRY 1.5 KN/M2 IMPOSED LOAD OVER MODERATE SPANS BUT MUST BE CHECKED) 2. LOAD PATH FROM ROOF STRUCTURE TO SUPPORTING WALLS AND FOUNDATIONS 3. BALUSTRADE POST FIXINGS (BOLTED THROUGH TO STRUCTURAL ELEMENTS — NOT SURFACE-FIXED INTO THE ROOF MEMBRANE) 4. PLANTER LOCATIONS (CONCENTRATE HEAVY PLANTERS OVER STRUCTURAL BEAMS OR WALLS)

COST OF STRUCTURAL ASSESSMENT AND DESIGN: £500-£1,500 DEPENDING ON COMPLEXITY.

STRUCTURAL UPGRADES: WHERE EXISTING ROOF STRUCTURE IS INADEQUATE, OPTIONS INCLUDE: — ADDITIONAL BLOCKING BETWEEN JOISTS — DOUBLING OR SISTERING EXISTING JOISTS — INSTALLING NEW STRUCTURAL BEAMS BELOW EXISTING ROOF — COMPLETE STRUCTURAL FLAT ROOF REPLACEMENT WITH HIGHER-CAPACITY MEMBERS COST: £2,000-£8,000 DEPENDING ON EXTENT OF STRUCTURAL UPGRADE REQUIRED.

THE WATERPROOFING MEMBRANE IS THE MOST CRITICAL ELEMENT OF A ROOF TERRACE — AND THE ELEMENT MOST COMMONLY UNDER-SPECIFIED. A STANDARD FELT OR BASIC EPDM MEMBRANE DESIGNED FOR MAINTENANCE-ACCESS FLAT ROOFS IS NOT APPROPRIATE FOR A TERRACE. THE ADDITIONAL FOOT TRAFFIC, DECKING FIXING LOADS, AND PLANTER DRAINAGE LOADS REQUIRE AN UPGRADED WATERPROOFING SPECIFICATION.

RECOMMENDED SYSTEMS FOR ROOF TERRACES:

1. SARNAFIL G410-15EL (MECHANICALLY FIXED SINGLE-PLY PVC MEMBRANE): THE MOST WIDELY SPECIFIED FLAT ROOF MEMBRANE FOR LONDON TERRACE APPLICATIONS. 1.5MM THICKNESS, REINFORCED POLYESTER SCRIM, HOT-AIR WELDED SEAMS. WALKABLE GRADE (G410-15EL) IS SPECIFIED FOR TERRACE USE. OVERLAPPED BY BALLAST LAYER, DECKING, OR PAVING.

2. FIRESTONE RUBBERCOVER EPDM (1.14MM): SINGLE-PLY RUBBER MEMBRANE, FULLY BONDED USING FIRESTONE BONDING ADHESIVE. HIGHLY RESISTANT TO UV, OZONE, AND THERMAL CYCLING. WALKABLE WITH PROTECTIVE LAYER. FOR LARGE TERRACE AREAS, EPDM SHEETING IN SINGLE PIECES ELIMINATES SEAMS AND REDUCES LEAK RISK.

3. BAUDER BITUMEN FELT (3-LAYER SBS): TORCH-APPLIED BITUMEN FELT SYSTEM WITH MINERAL-SURFACED CAP SHEET (BAUDER TOPTHERM 5.0 OR SOPREMA ELASTOPHENE FLAM PLUS). HEAVIER AND MORE DURABLE THAN SINGLE-PLY SYSTEMS. SUITABLE FOR TERRACES WHERE A BALLAST LAYER (GRAVEL OR PAVING SLABS) IS USED.

4. LIQUID WATERPROOFING (SIKA SIKALASTIC, TREMCO TREMPROOF, REMMERS PU 400): COLD-APPLIED LIQUID COATINGS THAT CURE TO A SEAMLESS MEMBRANE. IDEAL FOR RETROFIT APPLICATIONS WHERE EXISTING MEMBRANE IS SOUND BUT REQUIRES UPGRADE FOR TERRACE USE. SUITABLE FOR COMPLEX ROOF GEOMETRIES WITH MULTIPLE UPSTANDS, OUTLETS, AND PENETRATIONS.

WATERPROOFING SYSTEM COST: £80-£150/M2 INSTALLED (DEPENDING ON SYSTEM AND COMPLEXITY). A 25M2 EXTENSION ROOF TERRACE: £2,000-£3,750 FOR WATERPROOFING ALONE.

THE TERRACE FINISH LAYER SITS ABOVE THE WATERPROOFING SYSTEM ON A SEPARATION LAYER AND/OR DRAINAGE LAYER.

PEDESTAL-MOUNTED PORCELAIN OR STONE PAVING: HARDROCK SUPPLY LTD LARGE-FORMAT PORCELAIN (600X600MM OR 600X1200MM, 20MM THICKNESS), MARSHALLS SYMPHONY PORCELAIN (600X600MM, 20MM PORCELAIN PAVIOURS), OR NATURAL STONE (SLATE, YORKSHIRE SANDSTONE, KANDLA GREY INDIAN SANDSTONE) MOUNTED ON ADJUSTABLE PLASTIC PEDESTALS (BUZON PEDESTALS, ETERNO IVICA SUPASTEP, OR PAVIFIX DPH). PEDESTALS ALLOW LEVEL ADJUSTMENT TO ACHIEVE A FLAT SURFACE REGARDLESS OF DRAINAGE FALLS, AND CREATE AN AIR GAP THAT PROTECTS THE MEMBRANE FROM UV AND ALLOWS DRAINAGE. COST: £80-£180/M2 INCLUDING PEDESTALS AND PAVING.

TIMBER DECKING (IPE, CUMARU, COMPOSITE): HARDWOOD DECKING BOARDS ON ALUMINIUM OR STAINLESS STEEL JOIST FRAMEWORK ABOVE THE MEMBRANE. IPE (BRAZILIAN WALNUT — MOHS HARDNESS 3.5-4, OUTSTANDING DURABILITY, 25+ YEAR OUTDOOR LIFE WITHOUT TREATMENT). CUMARU (SIMILAR PROPERTIES TO IPE, SLIGHTLY LOWER COST). COMPOSITE DECKING (TREX ENHANCE, MILLBOARD ENHANCED GRAIN — WOOD-PLASTIC COMPOSITE WITH MINIMAL MAINTENANCE). ALL MUST BE INSTALLED ON A FRAMEWORK THAT DISTRIBUTES POINT LOADS TO AVOID MEMBRANE PUNCTURE — NEVER FIX DIRECTLY THROUGH A SINGLE-PLY MEMBRANE. COST: COMPOSITE DECKING ON FRAMEWORK £120-£200/M2; IPE HARDWOOD £150-£250/M2.

GREEN ROOF (SEDUM OR BIODIVERSE): IF THE TERRACE IS NOT PRIMARILY INTENDED FOR HUMAN OCCUPATION BUT FOR BIODIVERSITY AND THERMAL/ACOUSTIC BENEFITS, A GREEN ROOF SYSTEM IS APPROPRIATE. BAUDER WILDFLOWER GREEN ROOF, ENVIROMAT PRE-GROWN SEDUM BLANKET, OR ZIN ZINCO FLORADRAIN BIODIVERSE SUBSTRATE SYSTEMS. GREEN ROOFS REDUCE SURFACE WATER RUNOFF BY 50-90%, IMPROVE THERMAL PERFORMANCE, AND ARE ENCOURAGED BY THE LONDON PLAN AS PART OF GREEN INFRASTRUCTURE POLICY. SOME LONDON BOROUGHS (E.G. LEWISHAM, SOUTHWARK) ACTIVELY SUPPORT GREEN ROOF APPLICATIONS AS AN ALTERNATIVE TO TERRACE APPLICATIONS. COST: £80-£180/M2 FOR A SEDUM GREEN ROOF SYSTEM INSTALLED.

BALUSTRADE (GUARD RAIL): A ROOF TERRACE REQUIRES A BALUSTRADE TO AT LEAST 1.1M HEIGHT ON ALL EXPOSED EDGES UNDER BUILDING REGULATIONS PART K (PROTECTION FROM FALLING). IN PRACTICE, 1.1M IS THE MINIMUM — TALLER BALUSTRADES (1.2-1.5M) ARE INCREASINGLY COMMON IN LONDON FOR ADDITIONAL PRIVACY AND SECURITY.

BALUSTRADE OPTIONS: — GLASS BALUSTRADE (FRAMELESS OR FRAMED): 21.5MM TOUGHENED LAMINATED SAFETY GLASS PANELS ON STAINLESS STEEL OR POWDER-COATED ALUMINIUM POSTS. PROVIDES UNOBSTRUCTED VIEWS. NOTE: MAY BE RESTRICTED BY PLANNING CONDITIONS WHERE OVERLOOKING IS A CONCERN. — STEEL POWDER-COATED RAILINGS: GALVANISED STEEL FLAT BAR OR HOLLOW SECTION, POWDER-COATED ANTHRACITE GREY (RAL 7016) OR BLACK. OPEN RAILING STYLE (100MM MAX OPENING BETWEEN RAILS UNDER PART K). — SOLID PARAPET EXTENSION: MASONRY, TIMBER, OR CEMENT BOARD SOLID PARAPET BUILT UP FROM THE EXISTING WALL OR UPSTAND. MOST PRIVACY, MOST WIND PROTECTION, SUITABLE FOR CONSERVATION AREA APPLICATIONS WHERE PRIVACY IS A PLANNING CONDITION.

COST: GLASS BALUSTRADE £350-£700/LM; STEEL RAILING £200-£400/LM; SOLID PARAPET £200-£500/LM (DEPENDING ON MATERIAL AND FINISH).

ACCESS: STEEL EXTERNAL STAIRCASE (GALVANISED + POWDER-COATED) FROM FIRST-FLOOR WINDOW OR DOORWAY: £1,500-£4,000. LOFT HATCH-STYLE ROOF ACCESS HATCH (VELUX INTEGRA ELECTRIC ROOF HATCH OR COXDOME UPVC ROOF ACCESS HATCH): £600-£1,500 SUPPLY AND FIT.

DRAINAGE: RESIDUAL SURFACE WATER DRAINAGE FROM THE TERRACE MUST BE CHANNELLED TO DOWNPIPES THROUGH THE EXISTING ROOF OUTLETS OR NEW ACO LINEAR CHANNEL DRAINS. NEVER ALLOW STANDING WATER ON A TERRACE — OVERLOADS MEMBRANE AND ACCELERATES DETERIORATION.

TOTAL COST GUIDE FOR A 25M2 LONDON ROOF TERRACE: PLANNING: £1,500-£3,000 (CONSULTANT + APPLICATION) STRUCTURAL ENGINEER + BUILDING REGS: £1,500-£2,500 ROOFING AND WATERPROOFING: £2,000-£4,000 DECKING OR PAVING ON PEDESTALS: £2,000-£4,500 BALUSTRADE (15LM): £3,000-£10,500 (GLASS TO STEEL) ACCESS HATCH OR EXTERNAL STAIR: £600-£4,000 TOTAL: £10,600-£28,500