Flat roof – warm or cold & why!

In construction many flat roofs are detailed as either warm roofs or cold roofs. Both roof types have their place, however there are differences to know about. This article explores the differences between the two roof constructions.

Firstly, a flat roof is any roof construction with a pitch of 10 degrees or less. In flat roofing, the term ‘warm’ or ‘cold’ applies to the structural substrate, typically a plywood, OSB, profiled steel or concrete roof deck.

Warm roof

When the insulation layer is laid on top of the roof substrate or deck we call this a warm roof. This is because the roof structure and substrate are on the warm side of the insulation.

Cold Roof

When the insulation is fitted between the rafters / structure and the roof substrate or deck is fitted above the insulation it makes the roof a cold roof structure because the substrate is on the cold side of the insulation.

Some of the pros and cons of both constructions;

Roof build up depth:

  • Cold roof: As the majority of the insulation is between the rafters, the overall build-up of the roof is often less.
  • Warm roof: The insulation is laid above the substrate layer so the overall roof construction is generally deeper.

Vapour Control Layer (VCL):

  • Cold roof: The VCL is punctured where the internal surfaces are fixed meaning that there may be a greater possibility of moisture vapour permeating through from the internal environment.
  • Warm roof: The VCL is bonded on top of the substrate layer. Where the insulation and waterproofing layer are bonded it preserves the integrity of the VCL. Where the waterproofing layer is mechanically fixed the VCL is penetrated.


  • Cold roof: Cold roofs should be ventilated in accordance with BS5250.
  • Warm roof: Warm roofs are unventilated.


  • Cold roof: Can be used with all roof coverings.
  • Warm roof: Can be used with most roof coverings apart from fully supported metal roofing.

Thermal Performance:

  • Cold roof: The insulation is within the structure which means the structural elements can become a source of thermal bridging. Some insulation may be required internally to mitigate this risk.
  • Warm roof: The structure is on the warm side of the insulation meaning that there is less chance of thermal bridging.

Moisture/interstitial condensation in the roof construction:

  • Cold roof: The VCL is normally penetrated when fixing the internal finishes. This means cold roofs should be ventilated in accordance with BS5250. Potential moisture/interstitial condensation is dealt with via ventilation.
  • Warm roof: Theoretically there should not be any moisture / interstitial condensation in a warm roof, hence warm roof constructions are unventilated. In the case of poor or incorrectly installed VCL and water ingress due to inclement weather during roof construction, there is no means of escape for the moisture which can lead to degradation of insulation and loss of U-Value.

Bearing load of roof surface:

  • Cold roof: The roof covering is supported by a substrate directly below the roof covering material. This provides a good supporting layer for plant, decking or pedestrian access
  • Warm roof: The roof covering material is supported by insulation. Insulation is susceptible to compression under load. In some situations this can lead to damage of the waterproofing membrane.

Obviously, both roof constructions are useful but providing the ventilation to a cold flat roof can be challenging. AIRTRAK Roof Void Ventilation System can help provide the correct detailing of cold roof constructions ensuring a long roof service life.

roof void ventilation system

AIRTRAK® is a proprietary range of ventilators designed specifically for the ventilation of roof voids. AIRTRAK® assists specifiers, builders and specialist contractors in helping them to meet the requirements of the Building Regulations in relation to the important issue of roof void ventilation and the control of condensation.

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