Water vapour and heat constantly flow into and out of any construction and its materials, including the roof. This movement is driven by the differences between the internal and external environments. In the majority of buildings warm moisture laden air will rise where it is generally cooled, which makes it denser and less able to carry water vapour. As a result of the cooling of the air, any water vapour present is likely to be squeezed out as liquid water, or more precisely condensation, at some point. This is usually referred to as the ‘dew point’. This dew point can occur within the roof construction itself or within the thickness of a material layer. This is referred to as interstitial condensation.


Most roof constructions and materials will be able to tolerate a certain amount of condensate build-up in the ‘winter period’, however, it should be noted that different materials have different tolerance levels. The build-up of condensate in the ‘winter’ must not reach a level that it will not be cleared away by the conditions appertaining to the ‘summer period’. It is these two factors that dictate the need for a vapour control layer. A vapour control layer is used as a means of reducing, or considerably reducing, depending on the level of risk, the flow of moisture into the roof construction.

Calculations in accordance with the provisions of BS 5250 will prove the required amount of vapour transmission reduction to limit damaging interstitial condensation. The Services Engineers or the manufacturers of the insulation material usually provide these calculations.

The greater the difference between the internal environmental factors of relative humidity and temperature and the external ones, the greater possibility there is of creating condensation.

Ideally, the vapour control layer (vcl) should always be installed on the ‘warm’ side of any insulation to ensure it is kept at as high a temperature as possible. However, some forms of construction utilising aerated concrete or any other ‘insulating’ materials mean that the vapour control layer has to be placed higher in the insulation sandwich. In these cases the calculation is of even greater importance as condensation occurring on the underside of the vcl becomes more likely because it is ‘colder’.

The varying permeability of the different layers in the roof build-up can have an impact on how the roof will perform. It is of great importance that the outer layer of the roof offers as little resistance to the passage of water vapour as possible; this allows the roof to breathe.

Sika-Trocal plasticised PVC waterproofing sheets, in building terms, offer a comparatively low resistance to the passage of water vapour and this combined with the ‘open topped’ detailing to the upstands provides a roof that breathes easily. Conversely materials, such as many ‘rubbers’ and other different plastics that offer a much higher resistance tend to suffocate a roof by preventing it from breathing, leading to a more rapid build-up of condensate.

‘Breather vents’ have in the past been associated with the construction of certain types of flat roofing, they are, however, neither intended, nor suitable for use with Sika-Trocal membranes and should never be installed.


NOTE: For ballasted roof applications, it should be noted that the frequent presence of a film of water across the surface of the membrane acts as a very effective vapour control layer and will inhibit the permeability of the membrane. Where Sika-Trocal membranes are to be laid over a roof deck containing construction water, that is, insitu concrete and/or screeds, it is recommended in BS 6229 that a minimum vapour control layer, such as the S-VAP 500E, be laid to prevent too much of this entrapped water drying out upwards into the roof build up. This requirement is still necessary even if the calculation demonstrates that a vapour control layer is not necessary in ‘service’, because the calculation process takes no account of the entrapped water.


As highlighted in BS 6229, the excess construction water present in insitu concrete slabs and/or screeds must dry out downwards. In order to aid that process, there are recommendations about forming temporary drainage holes at the points of lowest sag of the slab.

Refer to the standard for the full recommendations.

Where metal decking is used as permanent shuttering, BS 6229 states that even with the drainage holes formed as the previous paragraph, the concrete will not be able to fully dry out downwards. Therefore this should be taken into account at design stage.

If an un-bonded wet screed is to be used to create falls, then it should be a minimum of 65mm thickness and no greater than 200mm in accordance with BS 6229. Screeds of all types must only be laid below the vapour control layer.

When a vapour control layer is installed beneath insulation it is considered good practice to ‘wrap’ it around the ends of the boards and ‘trap’ it under the Sika-Trocal Metal edge profiles.