Cookies disclaimer

I agree Our site saves small pieces of text information (cookies) on your device in order to deliver better content and for statistical purposes. You can disable the usage of cookies by changing the settings of your browser. By browsing our website without changing the browser settings you grant us permission to store that information on your device.

What Is A Flat Roof Vapour Control Layer?

Vapour control layers are used in warm roof construction to reduce the spread of moisture into the insulation. The spread of water vapour in building materials is normally very slow. And the use of a vapour control layer only becomes necessary as humidity inside the building increases. As humidity and temperature rise a more efficient vapour control layer is required. In practice, to save excessive complexity, only two categories of efficiency are considered when analysing vapour control layers. These categories are:

1) A vapour check to control low levels of humidity.

2) A vapour barrier to control high humidity.

Is A Vapour Control Layer Needed For A Flat Roof?

The need for a vapour control layer; whether a vapour barrier or vapour check is dependent on the way in which the building in question is to be used and the type of flat roof construction it has. For instance, flat roofs which have a cold roof construction, do not have a necessity for vapour control. Below we will look at different building types and discuss how their typical conditions impact the need of a vapour control layer for a flat roof.


Most houses will have a satisfactory degree of ventilation to allow the steam and water vapour generated in day to day living to clear away. However, a small percentage of houses do not have acceptable ventilation. For this reason for construction purposes the worst case scenario should be assumed. This means housing should be thought of as a building which is subject to relatively high humidity. The periods of high humidity in housing will be irregular which means a vapour check would normally be enough. In situations where heating is used continuously with poor ventilation more may be required.

Swimming Pools

The temperature inside swimming pools is typically 30° C with a relative humidity of 60%. These means conditions with warm moist air rising are normally continuous. This means it is difficult to find any weaknesses in vapour control layers when a problem arises. In all circumstances a swimming pool will need a high quality vapour barrier. Additionally, the type of insulation used must be able to withstand leaking water vapour. The reason for this is even with a highly efficient two layer vapour control system, the local leaking of water vapour is possible. Proven types of insulation for a swimming pool include cork and a cork/polyurethane composite.

The reason for this is both types of insulation are moisture resistant and can be installed fully bonded between the vapour barrier and the waterproofing. This leaves no air pockets or layers in the roofing system which are partly bonded. The benefit of this is moisture will only leak into such a system because of diffusion. If there were air pockets the problem would be can circulate in between layers in the system, which could result in condensation absorbing into the insulation.

Baths, Toilets, Kitchen Accommodation, Sports Halls and Communal Showers

These buildings produce high humidity but only for intermittent periods. This means warm moist air will only be rising at certain times of the day. These buildings are normally designed with a lot of ventilation, although it is important to ensure the management of these facilities makes correct use of the ventilation systems. An analysis of these buildings will normally show a vapour check is needed. However, a vapour control will not always be necessary. Despite this if there is any doubt as to whether a vapour control is needed, it is best to use one as a precaution.

Commercial Buildings

Commercial buildings such as offices, shopping centres and individual shops will vary greatly in their humidity levels. In most cases only a vapour check will be needed, however each building must be assessed on an individual basis.

Warehouses and Factories With Normal Dry Processes

Most of these buildings operating under normal dry processes will not need any vapour control layers. However, shower rooms and kitchens for commercial catering in constant use, will normally need a vapour barrier. Toilets and small kitchen units in all warehouses and factories will need to have a good level of ventilation.


During the school day humidity can reach high levels in classrooms due to the number of people in a confined space. This is especially true if no windows are opened. However, a large amount of schools have successfully been built without vapour control layers over metal decks covering classrooms, corridors and halls.

Changing The Use Of A Building

Moisture gain analysis will help guide those who want to make allowances for a building which is changing its purpose to be something involving high humidity conditions. Installing an efficient vapour barrier is an expensive precaution, but in most conditions a vapour check is unlikely to be adequate. When planning for a change to high humidity conditions, the worst conditions should be allowed for.

Installing Vapour Control Layers

Vapour Barriers

An efficient vapour barrier needs to have bonded overlaps and penetrations should be effectively sealed. Precautions should be taken to make sure the vapour barrier is isolated from the buildings movements to ensure its integrity remains throughout it's surface life. Sometimes this will mean a partially-bonded two-layer barrier is required.

Vapour Checks

A vapour check is constructed from a single layer. With a vapour check, partial bonding is not required. The reason for this is a small amount of damage from building movement will make no difference to the function of the vapour check. This means limited damage is acceptable, as well as unsealed overlaps and penetrations.

Choosing A Vapour Control Layer

The material used to form the vapour check or barrier, and their method of attachment will depend upon the substrate which they are to be used on. The types of decks are as follows:

Concrete Decks

For concrete decks a vapour barrier is best provided either by a two layer system or a single layer which has a specialised metal lined vapour barrier material. If a two layer system is used the first layer can be polyester or 5U. With the second layer being a glass based material which seals the vapour control system. This second layer also provides lap security.

If a single layer with a specialised metal lined vapour barrier material is used it is normally fully bonded. In the case of mastic asphalt a 13mm layer of mastic asphalt on glass tissue can be used as a vapour control layer.

For concrete decks a vapour check can be provided by bitumen bonding a layer of glass based roofing BS 747 type 3B or similar to the roof deck. Polyester based roofing is also often used.

Metal Decks

Most high humidity buildings will have metal decks. The vapour control barrier is normally bonded straight onto the deck. As a metal deck has open troughs it is hard to ensure lap security. For this reason a vapour control system on a metal deck is normally two layers. The first layer can be polyester or 5U. With the second layer being a glass based material which seals the vapour control system. This second layer also provides lap security.

Although some manufacturers recommend a single vapour control layer with a specialised metal lined vapour barrier material, in practice it is hard to ensure lap security due to the troughs of a metal roof deck. For this reason, all longitudinal roof laps should be 100mm and  formed over the continuous top flats of the deck. The edges of the vapour barrier and all end laps should be fully supported using 150mm wide sheet metal strips to match the material of the deck and support the vapour barrier. With such an approach the standard of workmanship must be extremely high to ensure lap security.

If a building is extremely humid and a specialised metal lined vapour barrier material is being used, it is recommended to add another layer to create a two layer system. The second part of the layer can be a bonded layer of BS 747 type 3B material.

Timber Decks

Normal timber roofs should not be used above buildings with highly humid conditions. For such circumstances only high grade structural timber with natural resistance to moisture is suitable. When possible, a two layer vapour barrier should be chosen, with a nailed first layer of polyester or a material with similar strength. A second layer should be bonded to the first layer using hot bitumen. This second layer may be a high performance layer or BS 747 type 3B.

A vapour check for a timber deck can be a single nailed layer of polyester base roofing or a similar high strength material. The insulation is bonded to the nailed vapour check using hot bitumen.

Particleboard Decks

Particleboard is not suitable to use as a structural deck if highly humid conditions are anticipated. This means a vapour control layer should not be used on a particleboard deck as the deck is not suitable.

Plywood and Woodwool

Both plywood and woodwool structural decks are likely to incur a significant amount of movements around the joints. The first layer in such as system should be perforated glass base BS 747 type 3G followed by polyester based roofing.

In the case of a metal lined vapour barrier, manufacturers may advise part bonding on decks which are liable to suffer joint movement. If this is the case a first layer of type 3G is the best method of achieving a part bonding.

For a vapour check on plywood and woodwool structural decks there are two main options. The first is polyester based roofing. And the other is achieved by using glass based roofing BS 747 or similar and binding it to the deck using bitumen.

Want To Know More About Vapour Control Layers?

If you would like RJ Evans to provide any information related to vapour control layers, please contact us or call us now on 01277 375 511. One of our friendly flat roofing specialists will be delighted to help.