Reducing condensation within a flat roof structure

Condensation can cause a number of problems within a building, particularly interstitial condensation, which can occur inside the roof’s structure. Over time, this excess moisture can lead to the unseen decay in roof timbers and fixings. It can also have an effect on thermal insulation materials, as the measured thermal performance reduces when increased moisture is present. Therefore, it’s important to ensure that your flat roof structure is free from condensation, otherwise you’re going to have serious problems later down the line.

How does condensation occur?

While tradesmen will no doubt be familiar with why condensation occurs, it’s important to understand exactly what causes excess moisture to form. Essentially, condensation takes place when the hot air meets cold air. In the case of buildings, hot air from within rises through the roof and meets the cold air outside, which will create damp within the roof’s structure – unless it has been properly constructed. Most people will see evidence of condensation on their windows during the winter, especially if they aren’t double glazed, as the heat generated by central heating clashes with the very cold air outside – creating moisture on the inside of the window.

Ethylene propylene diene monomer (M-class) rubber – also known as EPDM – roofing systems are 100 per cent waterproof and are ideal for reducing the risk of damp, as they let no moisture in through the roof. Of course this is dependent on the system being correctly installed. If it hasn’t, it won’t be able to do this job to the best of its ability.

Is condensation more of a problem than it used to be?

In many ways, condensation can occur more often now than perhaps it did a few years ago. Climate change is partly responsible, because now the air is much more humid than it used to be. Increased humidity means increased moisture, which can be particularly problematic in the winter months. Higher winter temperatures combined with increased vapour pressure can create severe problems, especially in roof spaces, so it’s vital to get roof structure right.

Thanks to thermal regulations, there’s more pressure for builders to include as much thermal insulation within roof structures as possible. However, doing so increases the risk of condensation forming on the elements not surrounded by insulation, such as cold roof decks. This problem may become more common as existing buildings are altered to become eco-friendly.

The cold deck solution

There are two forms of flat roof construction which can reduce the risk of condensation forming. Essentially, thermal insulation can either be placed above or below the structural roof deck or waterproofing layer. When it is placed below the structural deck, ventilation is needed at the underside of the deck, this is known as a cold deck construction. It is the most common form of the two, but it is also the least thermatically efficient.

A cold deck construction isn’t the preferred solution, as the humid, temperate climate in the UK can cause some issues. It’s more difficult, and sometimes impracticable, to effectively ventilate restricted roof voids, particularly in the sheltered and built-up parts of Britain. If possible, this type of construction should be avoided and a warm deck solution should be selected instead.

The warm deck solution

In warm deck construction, the thermal insulation is placed above the structural deck and either above or below the waterproofing layer. There is no need for any additional ventilation. Not all buildings can accommodate this design, but it’s recommended where possible, as it’s the most modern method of construction. Warm deck construction also usually includes a vapour controlled layer (VCL), which is placed on top of the structural deck. This stops moist air from entering the roof space and condensing.

As long as your flat roof has been properly installed and you choose the right deck construction for the building, you shouldn’t have any more problems caused by condensation. Of course warm deck construction is always best, but remember this might not be possible on certain buildings. Condensation is a problem that should be taken seriously and it’s even more important to consider the damage that might be occurring within the roof’s structure, as it is much harder to detect than surface condensation.