Introduction
When homeowners compare contractor quotes for a loft conversion or extension, the most visible elements — glazing, kitchen fit-out, bathroom specification — tend to attract the most attention. What gets less scrutiny is the roof. This is a mistake. The roof is one of the most consequential elements of any building project. A properly built roof is watertight for decades, energy-efficient, and maintenance-free. A poorly built roof leaks, deteriorates prematurely, or fails thermal performance standards — problems that are expensive and disruptive to fix after the fact.
The Roofing Specification
A detailed quote for a loft conversion should specify the roofing system — not just say "new roof." Key elements include the underlay (breathable membrane) as the primary waterproofing layer beneath tiles — modern builds use a breathable membrane rather than older non-breathable felt, which can cause condensation issues. The tile or slate type and grade affects both appearance and longevity — on Victorian and Edwardian properties in conservation areas, matching the existing tile type and profile matters for both planning reasons and visual coherence. Timber battens should be graded, treated timber of the right dimensions; counter-battens create the necessary air gap for ventilation and drainage. For Velux or similar roof windows, the installation kit matters as much as the window itself — a properly specified flashing kit, correctly installed for the tile type and pitch, is essential for weatherproofing. Installations without the correct flashing kit are a common source of roof leaks.
Insulation in the Roof Plane
For a habitable loft room, the roof needs to be well insulated. Building Regulations require a specified U-value (typically 0.18 W/m²K for new roofs in a loft conversion), achieved through insulation of appropriate thickness between rafters and either above or below the rafter depth. The sequence of installation and how insulation integrates with the breathable membrane and vapour control layer matters — a poorly executed roof insulation installation can allow interstitial condensation, causing timber decay over time. Ask your contractor specifically: what U-value will the completed roof achieve, and how will it be insulated?
Flat Roof Construction
For extensions with flat or low-pitch roofs, the specification is particularly important because flat roofs have historically been a source of problems. Modern flat roofing uses higher-quality systems — GRP fibreglass, EPDM rubber membrane, or single-ply membranes with manufacturer warranties of 20–25+ years. Any flat roof on a new extension should use a modern system with a proper warranty, not a basic felt installation. Drainage falls are also critical — a minimum fall of 1:40 is required to prevent standing water, and this needs to be designed in and verified during construction.
Firewalls, Parapets, and Upstands
Where a flat roof terminates against a wall — a parapet, a flank wall, or an adjoining structure — the waterproofing upstand detail is one of the most common sources of failure. Upstands must be taken sufficiently high and properly terminated. This is a detail where cheap work fails early.
What to Ask When Comparing Quotes
Ask specifically: what roofing system and product are you specifying; what U-value will the roof achieve and how; what warranty is provided on the flat roof membrane; are Velux installations being carried out using the manufacturer's specified flashing kit; and is the roofing contractor working to any particular trade association standard. Differences in these answers often explain differences in quoted prices. Contact RCB Design & Build to discuss the roofing approach for your loft conversion or extension.