Roofing in London: Flat Roofs, Pitched Roofs and Rooflights

Flat roofs are the most common roof system for single-storey rear extensions, side return extensions, and garage conversions in London — they maximise internal ceiling height, are typically more cost-effective than pitched roofs, and allow rooflights to be installed for natural light.

• **GRP (Glass Reinforced Plastic / fibreglass) flat roofing:**
• The most common system for domestic London extensions:
• •Seamless, one-piece membrane with no joints
• •Typical lifespan: 25–35 years with minimal maintenance
• •Cold flat roof application: GRP over insulated decking
• •Cost for a single-storey extension roof (mid size, 20–30m²): £4,000–£8,000 supply and install
• •Not suitable for very large continuous areas (over approximately 500m²) but ideal for domestic extensions

• **EPDM (Ethylene Propylene Diene Monomer) flat roofing:**
• A rubber membrane popular for its flexibility and long lifespan:
• •Highly resistant to UV degradation and ozone
• •Suitable for both cold deck and warm deck (inverted) applications
• •Typical lifespan: 40–50 years
• •Available in large single sheets, reducing joints
• •Cost: typically similar to or slightly higher than GRP — £4,500–£9,000 for a domestic extension roof

• **Liquid applied flat roofing:**
• A cold-applied liquid membrane that cures to a flexible, seamless waterproof layer:
• •Can be applied to complex shapes and details that are difficult to waterproof with sheet systems
• •Used for terraces, balconies, and complex roof details
• •Typical lifespan: 20–30 years depending on specification
• •Cost: £5,000–£10,000 for a domestic extension roof

• **Cold deck vs. warm deck (inverted) flat roof:**
• •**Cold deck:** Insulation between the joists, vapour control layer below, ventilated void above — traditional approach, lower cost, requires adequate ventilation to prevent interstitial condensation
• •**Warm deck:** Insulation above the deck, membrane on top — all construction is above the dew point, no ventilation void required, thermally more reliable but slightly higher cost
• For new London extensions, warm deck (or inverted) construction is standard specification under current Building Regulations (Part L thermal performance requirements).

The pitched roofs of London Victorian and Edwardian terraces are predominantly original Welsh slate (or concrete tile replacements fitted in the 20th century). Roof replacement or repair is one of the highest-value maintenance investments for these properties.

• **When roof replacement is needed:**
• •Concrete tiles added in the 1960s–1990s are approaching end of life — concrete tiles typically last 40–60 years and many London replacements from this period are now failing
• •Original Welsh slate may still be serviceable if the battens are sound — slate lasts 80–150 years
• •Signs requiring action: water ingress through the ceiling, slipped or missing slates, failing lead flashing around chimneys and valleys, visible day-light through the roof structure

• **Roof replacement options:**
• •**Natural slate:** The premium and most period-appropriate option for Victorian and Edwardian London properties. Welsh or Spanish slate, £80–£140/m² supply and install. Longest lifespan.
• •**Reconstituted slate:** Fibre cement or composite slate products that replicate the appearance of natural slate at lower cost. £50–£90/m².
• •**Plain clay tile:** Traditional for earlier Victorian properties (pre-1870 era) — clay tiles are long-lasting and contextually appropriate. £70–£120/m².
• •**Concrete interlocking tile:** The lowest-cost option, £35–£70/m². Most common replacement tile used in the mid-to-late 20th century.

**Lead flashing:** All London pitched roofs with chimney stacks, hips, valleys, and flat roof junctions require lead flashing. Lead work should be Code 4 (at minimum) or Code 5 lead to the appropriate detail. Lead flashing has a 40–80 year lifespan when correctly specified and installed.

**Roof structure:** Where roof structure timbers (rafters, ridge board, wall plates) are in poor condition, partial or full rafter replacement may be needed alongside re-roofing. This should be confirmed from a loft inspection before committing to the roofing specification.

Rooflights and dormers are the primary way of introducing natural light into rear extensions (via flat roof rooflights) and loft conversions (via pitched roof rooflights or dormers).

**Flat roof rooflights:** For single-storey extensions, flat roof rooflights are the most effective way to bring natural light deep into the plan — particularly important for extensions that extend the full width of the house and block light that previously entered through windows.

• •**Standard fixed rooflights:** Double or triple glazed, thermally broken aluminium frame. Popular sizes: 1000×1500mm to 2000×3000mm. Cost: £800–£3,000 per unit supply only; installation additional.
• •**Panoramic / walk-on rooflights:** Structural glass rated for pedestrian loading — used for terraces, garden rooms, and roof decks. Cost: £3,000–£8,000+ per unit.
• •**Opening rooflights:** Electric or manual opening for ventilation. Cost: £1,500–£4,000 per unit supply only.

• **Pitched roof rooflights (Velux / equivalent):**
• For loft conversions where a rear dormer is not used, or to supplement dormer light:
• •Velux or equivalent in-roof rooflight: £400–£800 supply only; installation £300–£600 per unit
• •Centre-pivot or top-hung opening for ventilation
• •Roof windows in loft bedrooms must comply with escape window requirements (minimum 0.33m² opening area, minimum 450mm clear height and 450mm clear width, sill height no more than 1,100mm from floor) where the bedroom does not have another means of escape

**Dormers:** Dormers add significantly more floor area and headroom to a loft conversion than rooflights alone. See the loft conversion planning guide for detail on dormer planning and costs.