What U-Value Targets Mean in Practice
Current Part L Building Regulations require a flat roof U-value of 0.18 W/m²K for new build and significant renovation. For context: an uninsulated concrete deck flat roof from the 1990s might have a U-value of 3.5–5.0 W/m²K — losing 20–30 times more heat than a well-insulated equivalent. The difference is visible in heating bills and condensation behaviour.
Most 1990s–2000s Pewsham and Chippenham estate extensions were built with 50–75mm of insulation to then-current Building Regulations, producing U-values around 0.35–0.45 W/m²K. When these flat roofs are replaced in GRP, adding insulation to meet current targets is straightforward — and the opportunity doesn't come again cheaply for another 25–40 years.
Warm Deck vs Cold Deck vs Inverted
| System | Position | Pros | Cons |
|---|---|---|---|
| Cold deck | Below membrane | Simple, low cost | Ventilation critical, condensation risk |
| Warm deck | Above structure, below membrane | Most common, good performance | Membrane exposed to thermal stress |
| Inverted | Above membrane | Membrane protected, long life | Higher material cost, drainage needs care |
For most Chippenham domestic extensions, a warm deck construction using PIR insulation boards is the standard and correct specification. We advise on build-up during the flat roof survey based on the existing construction and target U-value.
Upgrading Insulation During GRP Replacement
When we replace a flat roof in GRP, the existing membrane is stripped and the deck exposed. This is the optimal point to assess and upgrade insulation — the deck is accessible, the build-up can be adjusted before the new membrane is applied, and the cost of adding insulation at this stage is modest compared to the improvement in thermal performance.
A standard upgrade from 75mm PIR to 120mm PIR on a 20m² Pewsham extension adds approximately £300–£500 to the GRP replacement cost and brings the roof to current Building Regulations standard. For new build or material renovation, current Part L compliance is required. Call 01249 247 076 for a combined insulation and flat roof survey.