Worked Example · Flat Roof (warm)
Flat Roof (Warm) U-Value Worked Example
A warm flat roof build-up with insulation above the structural deck, checked against the Approved Document L (2021) limit of 0.11 W/m²K for flat roofs.
Build-up layers
| # | Layer | Thickness (mm) | λ (W/m·K) | R (m²K/W) |
|---|---|---|---|---|
| — | External surface resistance (Rse) | — | — | 0.040 |
| 1 | Waterproofing membrane | 3 | 0.200 | 0.015 |
| 2 | PIR insulation | 200 | 0.022 | 9.091 |
| 3 | Plywood deck | 18 | 0.130 | 0.138 |
| 4 | Plasterboard ceiling | 12.5 | 0.250 | 0.050 |
| — | Internal surface resistance (Rsi) | — | — | 0.100 |
Calculated U-value
0.106 W/m²K
Total resistance R = 9.434 m²K/W
ADL 2021 limit
0.11 W/m²K
England — Approved Document L (2021), new dwellings
PASS
The calculated U-value of 0.106 W/m²K is at or below the 0.11 W/m²K limit — compliant with ADL 2021.
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