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Technical Warm Deck Flat Roofing Build-up
Part L Compliant Thermal Specification

Warm Deck Insulation
& Thermal Rebuilds

Modern thermal management in South Devon requires a transition to definitive 'Warm' deck construction. We specialise in high-performance 120mm PIR insulation rebuilds designed to eliminate condensation and structural rafter decay permanently. Our thermal rebuilds are fully Part L compliant and include third-party inspections for certified sign-off.

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The Definitive 6-Stage Spec

Achieving a legal U-Value of 0.18 W/m²K requires a factual, multi-stage layering process that handles the extreme maritime humidity of Torbay.

At Raven Roofing Services, our warm deck protocol begins with a structural 18mm OSB3 conditioned deck. We then install a heavyweight Vapour Control Layer (VCL) bonded directly to the deck to arrest moisture migration. The core thermal layer is 120mm PIR insulation, which is structurally supported at the perimeter by technical hard edging grounds.

This hard edging is a critical structural component that many roofers omit; it provides a definitive fixing point for edge trims and maintains the integrity of the insulation stack. The build-up is completed with a high-tensile carrier underlay and a final weathering layer of 20-year spec mineral SBS bitumen felt. To ensure complete regulatory peace of mind, every installation is engineered for Part L compliance and undergoes an independent third-party inspection for technical sign-off.

Technical Build-up

01

18mm OSB3 Deck

The structural foundation board providing high-stability and level falls.

02

Heavyweight VCL

High-bond Vapour Control Layer to protect the structure from internal humidity.

03

120mm PIR Insulation

Rigid polyisocyanurate thermal core to move the dew point outside the structure.

04

Structural Hard Edging

Timber perimeter grounds to support the insulation stack and provide trim fixings.

05

HP SBS 20yr System

High-performance underlay capped with 20-year spec mineral-surfaced bitumen felt.

Aperture Integration

While structural thermal layers are the priority, the integration of roof windows requires technical foresight. In a warm deck scenario, the original height of the skylight aperture is raised by 120mm.

We construct specialized technical upstands (kerbs) for Velux installations to ensure the window unit remains proud of the new insulation. This allows for a definitive watertight seal between the window frame and the 3-layer SBS system.

Thermal Continuity Note

"Installing a Velux window during a warm deck rebuild is the optimal time for fitting. This ensures complete thermal continuity and prevents cold spots around the window frame that otherwise lead to internal mould growth."

Thermal Performance Metrics

System Metric Legacy Cold Roof Raven Warm Deck Spec
Thermal Layer Mineral Wool (Internal) 120mm PIR Rigid Board
Dew Point Position Internal Timber Rafters External (Above Structural Deck)
Perimeter Support None (Fascia Only) Technical Hard Edging Grounds
Compliance & Sign-off Non-Compliant Part L + 3rd Party Inspected

Factual data based on BS 6229:2018 and Part L Building Regulations.

Modernise your property's thermal layer.

07460 250171

Technical Warm Deck Contractors • Serving TQ1 - TQ12

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