The Best Membranes Found in Garden Room Roofs

Welcome to the final part of our series of three articles in which we explore the membranes used in the construction of the garden rooms of today. Our first article covered the types of membranes used in the foundation, also known as the base or concrete base, of a garden room and the second looked at the membranes used in the walls of a garden room. This third article will look at the use of membranes in the roof of a garden room, we will examine why they are used and how they are installed.

These articles are intended to be a helpful tool for those amongst us who are taking their first tentative steps towards the ownership of a garden room, or for those who have never even heard of a garden room and are looking to find out what all the fuss is about. We hope they can also be a source of reference for anyone seeking to retrofit membranes to their garden room, perhaps in an effort to upgrade an existing garden building and turn it into something that can be used all year round.

The use of membranes in the roofs of buildings has greatly increased over recent years, mostly because it is so lightweight compared to the more traditional roofing felt. There are two types of membranes used in garden room roofs, breathable and non-breathable, both are widely used depending upon the type of roof system and both perform the same function of preventing the ingress of water.

Vapour permeable breather membranes are primarily used in roofs to prevent the ingress of water but they also give some protection against wind and insects. Breather membranes are installed to the outer, or cold, side of the insulation, beneath the support of the tiles, and their function is to help prevent damp, mildew and mould by allowing vapour and condensation to escape through the roof.

Non-breathable membranes fulfil the same function as breathable membranes, in that they prevent the ingress of water and help to prevent damp, mildew and mould. This type of membrane is not air permeable so it will not allow moisture to escape, this means it is only suitable in cold, well ventilated roofs.

Modern buildings are so well insulated that the lack of ventilation can cause issues with condensation, it is not pleasant to keep a window open for ventilation when it is in the depths of winter and absolutely freezing outside, so most of us don’t, this can often result in black mould on walls and ceilings, rotting wooden window frames and sills, and so on.


When warm, moist air comes into contact with a cold surface, such as a window, it condenses, this is known as surface condensation. It is quite common for single glazed windows to fog up during the cold evenings and water will also run down in droplets. Another form of condensation is known as interstitial condensation and this occurs within the structure of the walls and roof of a building, usually when warm, moist air reaches the colder areas.

Condensation, whether it is surface condensation or interstitial condensation, can cause the following:

  • Damp
  • Mildew
  • Mould
  • Staining
  • Migration of salts
  • Damage to insulation
  • Electrical damage
  • Rotting timber

Water ingress and condensation can cause damp, mildew and mould, all of which can have a potentially serious effect upon the health of you and your family.

The staining and migration of salts is unsightly and can often be difficult to deal with as it may keep returning.

Should condensation reach your garden room insulation layer it can reduce its performance or even render it completely ineffective and requiring replacement.

Any kind of electrical damage will require immediate attention as this is not only a risk to the health of you and your family but also the building itself.

Structural damage may result from rotten timbers caused by condensation or water ingress.

The British Standard 5250 Code of Practice is a useful source of information and guidance on the control of condensation in roofing structures.

Installing a Breather Membrane

One manufacturer may differ from another in how a roof membrane should be fitted, just as not all kinds of membranes are fitted in the same way, this is why it is crucial that the manufacturer’s instructions are followed closely, step by step.

The membrane should be draped to provide channels rather than pulled taut, this will allow water to run off easily. Each length of breather membrane should be fitted with a generous overlap according to the manufacturer’s instructions, additional battens can be fitted where necessary, to correspond with the overlaps. The membrane should be cut and turned to fold up against chimneys, pipes, etc., tape should be used to hold the membrane in place.

Whether you are considering building a garden room, are already in the throes of construction, or you are retrofitting to an existing garden building, we hope you have found our series of three articles about the membranes used in garden rooms to be both enjoyable and informative. If you are about to place an order for a garden room, either from an “off the shelf” supplier or a company that designs and builds bespoke garden buildings, we recommend that you approach a representative of the company for information regarding the membranes they will be using in your garden room.

When purchasing any kind of membrane for a garden room it is important that you always check with the supplier that the product is suitable for your requirements. When installing a membrane, it is imperative that you follow the instructions fully, seeking advice if necessary. Some types of foundation waterproofing membranes can be hazardous to your health and should only be installed by professionals. Power tools should always be handled with care and only by a competent person. Working on a roof is a hazardous activity that should be carried out by professionals, the government has published useful guidance for advice and information regarding roof work, which can be found here.

For further information regarding the membranes used in the foundation, also known as the base or concrete base, of garden rooms, please click here. If you would like to know more about the membranes used in the external walls of garden rooms, please click here.

Special thanks Yersinia pestis for providing the image

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