Why Roof Quality Matters More Than Most Homeowners Realise
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.
This guide explains what proper roof quality looks like on a loft conversion or extension, and what to look for when comparing contractors.
The Roofing Specification
A detailed quote for a loft conversion should specify the roofing system — not just say "new roof." Key elements include:
Underlay (breathable membrane). The primary waterproofing layer beneath tiles or slates. Modern builds use a breathable membrane rather than the older-style non-breathable felt, which can cause condensation issues. The product specified matters.
Tile or slate type and grade. The specification of tiles or slates affects both appearance and longevity. Concrete interlocking tiles, clay plain tiles, natural slate, and reconstituted slate all behave differently over time. For loft conversions on Victorian and Edwardian properties in conservation areas, matching the existing tile type and profile matters — both for planning reasons and for visual coherence.
Battens and counter-battens. The timber battens to which tiles are fixed should be graded, treated timber of the right dimensions. Counter-battens create the necessary air gap beneath the tile for ventilation and drainage.
Roof windows. For Velux or similar roof windows, the installation kit matters as much as the window. A properly specified flashing kit, correctly installed for the tile type and pitch, is essential for weatherproofing. Installations that don't use 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. The Building Regulations require a specified U-value (typically 0.18 W/m²K for new roofs in an extension or loft conversion). Achieving this requires insulation of appropriate thickness and the right specification — typically a combination of insulation between rafters and insulation either above or below the rafter depth.
The sequence in which insulation is installed, and how it integrates with the breathable membrane and vapour control layer, matters. A poorly executed roof insulation installation can allow interstitial condensation — moisture forming within the structure — which causes timber decay over time.
Ask your contractor specifically: what U-value will the completed roof achieve? How will it be insulated? This is a question worth asking.
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 — particularly older built-up felt systems that had a limited lifespan.
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 on flat roofs. A minimum fall of 1:40 is required to prevent standing water. This needs to be designed in and verified during construction — not discovered as a problem after the roof is on.
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 (if applicable)? - Are Velux installations being carried out using the manufacturer's specified flashing kit? - Is the roofing contractor working to any particular trade association standard?
Differences in these answers often explain differences in quoted prices.
Want a roofing specification you can rely on?
Contact RCB Design & Build to discuss the roofing approach for your loft conversion or extension.
📞 07359 872594 | contact@rcbgroup.co.uk | www.rcbgroup.co.uk