The roof specification for a rear extension is one of the most consequential design decisions — it affects natural light, headroom, thermal performance, appearance, maintenance requirements, and cost.
Flat Roof
A flat roof (more accurately a very low-pitched roof — typically 1–3°) is the most common choice for single-storey rear extensions in London. Modern flat roofs use one of three primary membrane systems:
EPDM (rubber): Flexible, UV-stable synthetic rubber sheet laid over insulation. Very durable (25–30 year life expectancy), good flexibility around outlets and upstands, cost-effective. The standard choice for most residential extensions.
GRP (fibreglass/GRP): Glass-reinforced polyester resin laid in situ. Very rigid, seamless, can be formed around complex details. Popular finish — looks similar to mineral felt when finished with topcoat but with far greater durability. Common on extensions needing a neat visual finish visible from first-floor windows.
Felt (SBS modified bitumen): More traditional system. Can perform well but less durable than EPDM or GRP on complex details. Less common on new extensions.
Advantages of flat roof:
- Lower cost than pitched roof (simpler structure, lower material cost)
- Suitable for roof lanterns, large flat rooflights, walk-on decking (with appropriate build-up)
- Maximum internal headroom for a given external height
- Clean contemporary aesthetic
Disadvantages:
- Requires proper drainage falls — poor falls cause ponding, membrane deterioration
- Maintenance — outlets must be kept clear of debris
- Some clients perceive as lower quality than pitched (incorrectly if specified correctly)
Pitched Roof
A pitched roof on a rear extension can take several forms:
- Mono-pitch (lean-to): Single slope from the rear wall of the house down to the outer wall. Simple to build, suits permitted development.
- Dual-pitch (hipped or gabled): Two slopes meeting at a ridge. More traditional appearance.
- Matching the existing house pitch: Most appropriate for extensions visible from the street or in conservation areas.
Advantages of pitched roof:
- More traditional appearance — suits period properties
- Generally lower long-term maintenance (no flat membrane to check)
- May be required in conservation areas or on certain planning conditions
Disadvantages:
- Higher cost than flat roof (more complex structure, more materials)
- Reduced headroom at the eaves
- Reduced scope for rooflights compared to flat roof
Roof Lanterns
A roof lantern is a glazed roof structure sitting on a flat or low-pitched roof — typically a glazed ridge or square lantern. In kitchen extensions, a roof lantern is the single most popular way to flood the rear of the house with daylight.
Advantages:
- Exceptional natural light quality — sun tracks across the lantern through the day
- Dramatic focal point in open-plan kitchen-dining spaces
- No side wall interference — the light comes from above
Disadvantages:
- Cost — a quality aluminium roof lantern (2m × 2m or 3m × 1.5m) costs £3,000–£8,000 supply only
- Heat gain in summer (mitigated with self-cleaning solar-control glass)
- Cleaning — the glazing accumulates debris and needs periodic cleaning
Typical sizes: 1.5m × 1.5m (suitable for small flat-roof extensions), 3m × 1.5m (standard), 4m × 2m (large kitchen extension).
rcbGroup
rcbGroup advises on roof specification as part of extension design — specifying the system appropriate for each project's requirements, planning context, and budget.