Structural feasibility first
Existing flat roofs are rarely designed for occupied loads. Domestic flat roof design load is typically 0.6 kN/m² (snow only); roof terraces need 1.5 kN/m² minimum (occupied), more for planters and furniture. Structural engineer survey first to assess existing joists, deck and supports. Reinforcement options: doubling up joists, new steel beams, new deck buildup. Allow £3,500–£12,000 for structural works. Older buildings may need full deck replacement.
Waterproofing and falls
Roof terrace warm-roof buildup: existing structure → vapour barrier → PIR insulation (typically 120–150mm) → liquid-applied or sheet membrane (Sika Liquid Plastics, IKO, single-ply TPO/EPDM) → protection layer → pedestals or screed → decking. Falls must shed water away from upstands and any access doors (1:80 minimum). Door thresholds need careful detailing — accessible threshold without ponding risk. Typical waterproofing buildup cost £180–£280 per m².
Planning, party wall and balustrade
Most roof terraces need full planning permission — they are not PD because they affect privacy and amenity. Refusal common where overlooking neighbours' gardens at distances under 21m. Party Wall etc. Act 1996 notices required for any work to party walls or boundary parapets. Balustrade options: frameless structural glass (£480–£780 per linear m), powder-coated steel (£280–£440 per linear m), brick parapet (rebuilt £600–£900 per linear m). Minimum 1100mm height to deck level (Approved Document K).
