What is a warm roof?
A warm roof is a roof build-up in which the insulation is placed above the structural deck, so the deck stays on the warm, inside side. There is no ventilated air gap between insulation and waterproofing. It is the standard build-up for new flat roofs and is also used on pitched roofs.
Build-up of a warm flat roof (inside to outside)
- Structure: concrete slab, timber joists with deck boarding, or profiled steel.
- Vapour control layer on the warm side, to stop moist indoor air condensing in the insulation.
- Insulation: rigid boards, for example PIR, EPS or mineral wool roof boards.
- Waterproofing: bitumen membrane, EPDM or single-ply membrane.
- Optional ballast or a green roof.
A variant is the inverted roof: moisture-resistant insulation (XPS) goes above the waterproofing, held down by ballast.
Warm roof versus cold roof
In a cold roof the insulation sits between or below the joists with a ventilated void above. On flat roofs this carries a higher risk of interstitial condensation and timber decay if vapour control is not perfect. In a warm roof the structure stays warm and dry.
| Warm roof | Insulation above the structural deck, no ventilated air gap between insulation and waterproofing; the structure stays warm and dry |
|---|---|
| Inverted roof | Moisture-resistant insulation (XPS) above the waterproofing, held down by ballast |
| Cold roof | Insulation between or below the joists with a ventilated void above; on flat roofs a higher risk of interstitial condensation and timber decay if vapour control is not perfect |
Tips
- Insulation thickness: follow your national building regulations for new work and renovation, such as Part L in England.
- Continuous vapour control: tape all laps and connections to upstands carefully; a gap in the vapour control layer is the most common cause of wet insulation.
- Falls: provide falls to outlets, for example with tapered insulation, so water does not pond.
- Accessible roofs: choose insulation with enough compressive strength.
- Thermal bridges: insulate parapets and upstands too.
- Converting a cold roof: preferably remove the insulation between the joists and put the whole layer on top. Hybrid solutions need a condensation risk analysis.
Related terms
- Cold roof
- Flat roof
- Thermal insulation
- Thermal bridge
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