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Guidance - Flat Roof - Condensation

Flat Roof - Condensation


Condensation within a flat roof mainly occurs during cold weather when moisture vapour in the air which has been generated within the heated building rises from the room below into the cold roof void above the ceiling.

When the temperature of the vapour falls to or below its dew point the water vapour condenses on cold surfaces.

The warmer than air the more water vapour it can contain and the higher the moisture content in the air (relative humidity; RH) the lower the dew point temperature will be.

Condensation is a particular problem in roofs above rooms which generate a lot of moisture such as kitchens and bathrooms.

A flat roof should be designed to minimise condensation and a condensation risk analysis should be undertaken taking into account positioning of insulating materials, vapour control layers, ventilation, thermal insulation and the choice of materials. This can be calculated using computer programmes.

Surface Condensation

Another type of condensation is surface condensation which is visible on surfaces within the building and occurs when the temperature of the surface is at or below the dew point of the moist air.

This type of condensation is often identifiable by black mould on the walls, windows, ceilings etc.

Interstitial Condensation

Condensation which occurs within the roof structure is called interstitial condensation. It is particularly dangerous because it can cause unseen decay in roof timbers and fixings.

Condensation in a cold roof

Interstitial condensation is a particular problem in cold deck roofs where the insulation is placed in-between the joists in the void above the ceiling.

The position of the insulation means that the roof deck and most of its structure has no protection from low temperatures during the winter.

These elements then become much colder than the interior of building, and moisture vapour which has made its way up from the room below is then liable to condense on the timber structure possibly leading to decay.

Cold deck roofs are not recommended for new work, and actually banned in Scotland.


To help disperse the moisture vapour, the building regulations require cross ventilation to be provided in the form of a 50mm air gap between the deck and the insulation and continuous gap of about 25mm at the eaves.

This can be difficult to achieve where roofs abut external walls and propriety mushroom vents to provided the equivalent 25mm continuous ventilation are available.

Vapour control layer for a cold roof

However the cross ventilation does not completely remove the moisture vapour in the ceiling void and a vapour control layer sealed at joints and penetrations is required under the insulation and over the plasterboard to provide a barrier against moisture vapour rising up from the room below.

Alternatively metalized polyester lined plasterboard can be used.

Vapour control layer for a warm roof

A vapour layer should be positioned under the insulation or, in some situations, immediately below the roof covering to minimise water vapour condensing beneath the membrane.

Vapour control layer may be formed using any of the following:

  • A polythene sheet membrane loose laid and restrained by mechanical fasteners or nailed to the deck (timber decks) all laps should be sealed with an appropriate adhesive.
  • A reinforced bitumen sheet. - one layer of BS 747 Type 5U felt, fully bonded or mechanically nailed to the deck.
  • Two layers of Type 5U felt fully bonded in hot bitumen. check - all laps must be sealed with bitumen.
  • For single-ply membranes, the VCL should be either polythene or reinforced aluminium foil.

Designers often specify readymade ‘composite’ decking that combines together plywood, insulation, vapour control layer and felt covering into one product.

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