What Is the Difference Between a Warm Deck and a Cold Deck?

What is a warm deck roof?

In a warm deck, or warm roof, construction the principal insulation layer is installed above the structural roof deck and beneath the waterproofing system.

This keeps the roof deck and much of the supporting structure on the warm side of the insulation. A suitable air and vapour control layer is normally incorporated as part of the roof build-up beneath the insulation where required by the design. This is consistent with current industry guidance for warm flat roofs. LRWA

Because the structural deck remains relatively warm, a correctly designed warm roof generally presents a more straightforward way of controlling interstitial condensation than a traditional cold-roof arrangement.

A warm roof does not normally rely on a ventilated void between the insulation and roof deck. That should not be confused with ventilation of the room below, which remains a separate consideration.

What is a cold deck roof?

In a cold deck, or cold roof, construction the insulation is positioned below the roof deck, typically between or beneath the supporting joists.

The structural deck therefore remains on the colder side of the insulation.

Warm, moisture-laden air from inside the building can create a risk of condensation if it reaches these colder parts of the roof structure. Conventional cold flat-roof construction therefore requires particular attention to vapour control, airtightness and ventilation of the void above the insulation where the design requires it.

NHBC specifically identifies cold roof constructions with insulation between rafters as being more vulnerable to interstitial condensation because of the difficulty of maintaining an effective vapour-control layer and providing sufficient ventilation above the insulation. NHBC

This does not mean that a cold roof is inherently defective. It means that the moisture-control strategy and detailing need to be properly designed and constructed.

Warm roof vs cold roof: the practical difference

The fundamental difference is the position of the insulation.

With a warm roof, the structural deck sits within the insulated envelope.

With a cold roof, the structural deck is outside the principal insulation layer and is therefore exposed to colder conditions.

Both approaches can achieve a required U-value. Thermal performance depends on the complete construction — including insulation type and thickness, continuity of insulation, junction detailing and thermal bridging — rather than the label “warm” or “cold” alone.

For modern flat roofs, current good-practice guidance is contained in BS 6229:2025, Flat roofs with continuously supported flexible waterproof coverings – Code of practice. The latest edition was published in December 2025 and covers the design, construction, care and maintenance of flat roofs, including thermal performance and moisture-related considerations. BSI Knowledge

Condensation and moisture control

Controlling moisture within the roof construction is just as important as achieving the required insulation value.

BS 5250:2021, Management of moisture in buildings – Code of practice, provides the principal British Standard guidance for assessing and managing moisture risks within building construction. It considers the movement of moisture through the building fabric and the potential for surface and interstitial condensation. BSI Knowledge

Approved Document C also deals with resistance to moisture in roofs and includes guidance on damp proofing and ventilation. GOV.UK

Whichever roof construction is chosen, insulation, airtightness, vapour control and any required ventilation should therefore be considered together rather than as separate details.

What does this mean for a roof lantern?

A roof lantern creates an opening through the insulated flat-roof construction, making the junction between the roof, kerb or upstand and lantern particularly important.

The insulation should remain as continuous as reasonably practicable around the opening, while the kerb needs to provide a suitable structural base for securely fixing the roof lantern.

Poor detailing around this junction can create localised thermal bridging, allowing the internal surface temperature to fall and increasing the possibility of surface condensation.

This does not mean that a roof lantern is inherently more suited to a warm roof than a cold roof. The important consideration is that the lantern, supporting kerb and surrounding roof build-up are designed as one coordinated detail.

The kerb itself should be square, level and structurally suitable for fixing the roof lantern, while the insulation, vapour-control layer and waterproofing should be correctly integrated with it.

Read our roof lantern kerb and upstand guide →

Building Regulations and U-values

For domestic extensions in England, the current Approved Document L applicable in October 2026 gives a maximum U-value of 0.15 W/m²K for a new roof element in an existing dwelling. This includes new roof elements constructed as part of an extension. GOV.UK

That figure applies to the roof construction, not to the roof lantern glazing itself. Approved Document L treats rooflights separately and gives separate limiting values for them. GOV.UK

The previous JRL article referred to 0.18 W/m²K for the roof. I would remove that figure.

It is also worth noting that a new 2026 edition of Approved Document L has already been published. For ordinary building work in England, however, it does not take effect until 24 March 2027, subject to the relevant transitional provisions. Earlier standards continue to apply to work falling under the previous regulatory regime. GOV.UK

Read our Building Regulations and glazed extensions guide →

Is a warm roof better for a roof lantern?

For many new flat-roof extensions, a warm-roof construction offers practical advantages because the structural deck remains within the insulated envelope and the roof does not normally depend on ventilation immediately below the deck.

However, I would not describe a warm roof as automatically “more energy efficient” than a cold roof. Either construction can achieve the required U-value when properly designed.

The more significant difference is the way each construction manages temperature and moisture within the roof build-up. Cold roofs require especially careful attention to the continuity of vapour control and to any necessary ventilation, while warm roofs require correct installation of the insulation, vapour-control layer and waterproofing system.

The appropriate construction should therefore be determined by the architect, roof designer, contractor or other suitably competent professional for the particular project.

Which roof construction should you choose?

For a new extension, the roof construction should ideally be established before the roof lantern and its kerb are finally specified.

The roof designer needs to consider structural requirements, insulation, moisture control, waterproofing, drainage and the way the lantern opening is incorporated into the roof.

Just Roof Lanterns can provide the dimensions and upstand information required for the selected roof lantern, but the design and construction of the supporting roof and kerb form part of the wider building works.

You can view our roof lantern kerb and upstand guidance, read our Building Regulations guide, or browse our full range of roof lanterns when planning your project.