Roof Structure Types for London Home Extensions: Flat, Pitched, and Hybrid

Modern flat roofs are nearly always constructed as warm-deck roofs — the insulation sits above the structural roof deck (typically an 18mm or 22mm OSB or plywood deck on timber joists or steel beams), and the waterproofing layer sits above the insulation. This places the insulation on the warm side of the structure, preventing condensation within the insulation zone. Warm-deck waterproofing systems used on London extensions: GRP (fibreglass) roofing: Glass-reinforced polyester resin, laid in two coats over a fibreglass chopped strand mat. Very common on domestic London extensions. Seamless, durable (25+ year lifespan if correctly installed), available in any colour. Requires careful application — installer quality matters significantly. Single-ply membrane (TPO, EPDM, PVC): Factory-manufactured flat sheets of polymer membrane, mechanically fixed or adhered to the deck and joined at seams. EPDM (synthetic rubber) is widely used on domestic extensions in the UK — highly durable (30–50 year lifespan), resistant to UV and weathering. Cold deck roofs: The traditional approach where insulation sits between the joists below the deck, with a ventilated void above. Cold decks are now disfavoured because the ventilated void is prone to condensation problems. Any existing cold deck flat roof discovered during works should be replaced with a warm deck. Inverted (upside-down) warm deck: The insulation sits above the waterproofing layer. Used where foot traffic is expected on the roof (roof terraces, planted roofs) — the insulation is protected from mechanical damage. Requires a drainage layer above the insulation.

Pitched roof extensions are more complex and expensive to construct than flat roofs but may be more appropriate in some planning contexts — particularly in conservation areas, on properties where the rear elevation is visible from the street, or where the homeowner wants to use the roof space above the extension for storage or additional accommodation. Mono-pitch (lean-to) construction: A single roof plane pitching from the eaves (at the outer wall of the extension) up to the ridge (at the rear house wall). The structural roof is typically formed from rafters spanning from a wall plate on the outer extension wall to a wall plate bolted to the house, with the pitch angle determined by the desired internal ceiling height. A mono-pitch roof at a pitch of 15 degrees or more can be finished with standard clay or concrete roof tiles (matching the existing house), giving a more traditional appearance. At shallow pitches (below 10 degrees), a waterproofed membrane rather than tiles must be used. Gable (dual-pitch) construction: A pitched roof with a triangular gable at one or both ends. Less common on London rear extensions but used where the extension extends across the full width of the house, or where an A-frame roof shape is desired. Parapet roof: Not a roof structure type as such, but a design treatment where the flat roof is surrounded by a raised parapet wall (a low wall at the edge of the roof). Parapet-enclosed flat roofs give a contemporary clean-line appearance on London extensions and hide gutters and rooflights from the rear garden view. They are common in London new-build and conversion projects.

The roof type affects the planning implications of a London extension in several ways: Permitted development height limits: Under Schedule 2, Part 1, Class A of the GPDO, the maximum height of a single-storey rear extension under permitted development is 4m (and the eaves height must not exceed 3m within 2m of the boundary). This applies to both flat and pitched roofs — the 4m limit is to the highest point. For extensions with pitched roofs, the ridge height may be the critical dimension; for flat roofs, the roof surface height is. Height visible from a highway: In conservation areas and other Article 1(5) land, the roof design of an extension may require planning permission even where the extension would otherwise be permitted development, if the roof covering is different from the original house or if the roof design creates a visual impact visible from the street. Neighbour impact (overlooking, overshadowing): Planning authorities in London frequently ask whether the roof type affects the overlooking or overshadowing of neighbouring properties. A mono-pitch lean-to with the high point adjacent to the house may have a different overshadowing profile from a flat roof extension of the same footprint. Rooflights: Rooflights are permitted development subject to limits (they must not protrude more than 150mm above the roof plane and must be at least 1,800mm from the edge of the roof if the edge is adjacent to a highway). Conservation area rooflights visible from the street may require planning permission.