The Roof Type Decision
The roof of a rear or side extension is one of the most significant design choices — affecting cost, planning, maintenance, internal light, and the visual relationship with the main house.
In London, flat roofs dominate the extension market, but this is not simply preference. There are practical reasons for the choice, and some situations where a pitched or mono-pitch roof is the stronger option.
Flat Roofs
What "Flat" Means in Practice
No functional flat roof is truly flat. All flat roofs are built with a minimum 1:80 fall (roughly 1.25%) to allow rainwater to drain. Most are specified at 1:40 to ensure drainage even if the structure deflects slightly over time.
Flat Roof Construction Methods
Modern flat roofs are almost always warm roof construction — insulation above the structural deck, with a waterproofing membrane on top. The three main systems are:
- Single-ply membrane (TPO/PVC/EPDM): Fast to install, reliable, and the most commonly specified system for residential extensions. Typical lifespan 20–30 years.
- Built-up felt (BUR): An older system with multiple layers of bitumen felt. Less common in new work.
- Green/sedum roof: A growing membrane with a shallow growing medium planted with sedum or wildflowers. Adds biodiversity value, improves drainage management, and can aid planning acceptance.
Flat Roof Glazing Options
A flat roof extension can incorporate:
- Roof lanterns: A raised glazed structure sitting above the main roof plane — excellent for light and visual drama
- Flat rooflights: Flush-fitting glazed units that sit at low pitch within the flat roof — sleeker appearance
- Glass roofs: A full or partial glazed roof — maximum light, higher cost, more thermal management required
Advantages of Flat Roofs
- Lower build cost than equivalent pitched roof
- Faster to build
- More planning-friendly (lower overall height, less visual impact on neighbours)
- Allows roof glazing options that pitched roofs cannot easily accommodate
- Allows a parapet detail for a clean, contemporary elevation
Disadvantages of Flat Roofs
- Require regular inspection and maintenance of the membrane and drainage
- Lifespan of the membrane (20–30 years) is shorter than a tiled pitched roof
- More susceptible to failure from poor workmanship or debris build-up over drains
Pitched Roofs
A pitched roof on an extension matches the pitch and tile of the existing house, creating a more traditional appearance.
When a Pitched Roof is the Right Choice
- The local planning authority or Conservation Area guidance requires a pitched roof to match the host building
- The property is terraced and a flat roof would look out of character with the street scene
- The extension is a side extension where the roof needs to integrate with the main roof structure
- The client prefers a more traditional aesthetic
Pitched Roof Disadvantages
- Higher build cost (more complex structure, tile laying, ridge detailing)
- Reduced internal ceiling height at the edges (the eaves reduce usable headroom)
- Smaller glazing options (roof windows rather than large rooflights or lanterns)
Mono-Pitch (Lean-to) Roofs
A mono-pitch roof slopes in one direction only — typically from the back of the main house down to the eaves of the extension. It is common in:
- Permitted development extensions where total height must remain under 4 metres
- Contemporary designs where a single slope suits the architecture
- Narrow extensions where a dual-pitch would not work structurally
It is cheaper than a full pitched roof and offers a clean line from the rear garden perspective.
Planning Considerations
Planning authorities in London generally accept flat roofs for rear extensions, particularly where the extension is not visible from the street. For front extensions or properties in Conservation Areas, a matching pitched roof is more likely to be required.
Neighbours occasionally object to flat roof extensions on overlooking grounds if the extension roof is close to a boundary and creates a raised platform — though this is more of a planning consideration than a structural one.
Cost Comparison
| Roof Type | Typical Cost Premium vs. Basic Flat Roof |
|---|---|
| Flat roof (single-ply membrane) | Baseline |
| Flat roof with rooflight | + £1,500–£3,500 per rooflight |
| Flat roof with roof lantern | + £3,000–£8,000 |
| Mono-pitch (lean-to) | + 5–10% |
| Pitched roof (dual pitch, tiled) | + 15–30% |
RCB Design & Build
RCB advises clients on roof type as part of the early design discussion, taking account of planning context, budget, the existing house architecture, and the client's priorities for light and finish.