Flat roofs are a common feature to many types of building in the UK. They are quick and straightforward to build, easy to access, and come with a range of finish options. Unfortunately, issues can appear with a poorly built flat roof, or a roof which hasn’t been maintained correctly. In this week’s blog, we’ll discuss several different flat roof types and some common issues that a building owner can encounter.
What is a flat roof?
A roof is generally considered ‘flat’ if it has a pitch of less than 10 degrees. This makes a roof unsuitable for waterproofing using more traditional materials such as tiles or slates; instead, sheet materials are needed to create a roof covering that’s able to support standing (or ‘ponding’) water without leaking.
Flat roofs are generally built in several ways:
Cold flat roofs
A flat roof is ‘cold’ if the insulation is located below the roof structure, or in between joists at the base of a roof. This keeps the insulation close to the habitable area below the flat roof and prevents the heating of unnecessary space between the structural elements. These can be formed with timber, concrete or steel.
Cold roofs require ventilation to the structure. If the ‘cold’ space above the insulation isn’t ventilated, condensation can occur; this can cause damage to internal finishes, as well as corrosion or timber decay to the structure. It’s also essential that a ‘vapour control layer’ is installed under the insulation to prevent moisture from passing through the insulation or structure.
Warm flat roofs
Warm flat roofs are built in the opposite way to a flat roof, with insulation laid on the top. This eliminates the need for ventilation to the structure which is kept warm by the insulation above, reducing the risk of condensation.
Warm roofs often provide better thermal performance, as there’s less chance of the structural elements getting cold and creating a ‘thermal bridge’. However, as the insulation is on top there’s more chance of it being compressed or damaged by walking on or storing heavy items (such as air conditioning plant).
Inverted flat roofs
An inverted flat roof is similar to a warm roof, with the key difference being that the waterproofing layer is located under the insulation. This achieves two things: firstly, the waterproofing is protected from physical damage, making in inverted roof more suitable for roofs being walked on or heavily used. Secondly, the waterproofing layer is protected from the sun or from frost; this can significantly increase the lifespan of the roof before any major refurbishment work is needed.
Because the insulation is laid over the top of the roof, it needs to be secured down to prevent movement. This is usually achieved with paving slabs or gravel.
Common problems
Whilst flat roofs are a tried and tested construction method, they are also a common source of failure. In fact, if you have over 25-30% of flat roofs to your home, you’ll likely need specialist insurance.
Here are some common reasons for flat roof failure:
UV damage
Many sheet materials used to cover flat roofs are vulnerable to ultraviolet (UV) damage. This can cause shrinkage or deterioration to the bonding materials used in many bitumen-based materials, causing the roof covering to fail.
Many polymer-based materials are more resistant to UV, decreasing the risk of this occurring. Other ways to prevent UV damage is to choose an inverted roof, shielding the waterproofing layer from weather and increasing its lifespan.
Blistering
A common sight to many flat roofs is the bubbling of the sheet covering. This is usually caused due to moisture being trapped beneath the covering, which expands and contracts through heating and cooling. Moisture can become trapped through damage or tearing to the covering, or if the roof covering was installed poorly or in poor weather conditions. Whilst blistering can be unslightly, it’s often a good idea to leave them alone if they aren’t likely to be disturbed by foot traffic.
Large blisters, or those that will likely be damaged when walked on, can be patch repaired to prevent the eventual failure of the covering. They can also be prevented through adequately maintaining the roof and choosing a competent contractor when building or refurbishing a flat roof.
Interstitial condensation
Condensation occurs when moisture vapour cools and returns to liquid. When this happens on a surface, the water left behind can cause mould, corrosion or timber decay.
Condensation can happen within a wall or material, as water vapour passes through; this is known as interstitial condensation. In a flat roof, this can occur in the roof structure; if left unnoticed, the amount of condensation can begin to drip back into the rooms below, or come into contact with electrical wiring.
Interstitial condensation usually happens in a roof buildup due to poor detailing, such as a missing vapour control layer or lack of ventilation. By carefully placing these elements during the construction of the roof, it is usually possible to maintain a flat roof for many years with little to no risk of condensation occurring.
Flat roofs are a common source of problems for property owners, due to the number of potential issues or mistakes that can be made in their construction. The good news is that most of these problems can be avoided by working with an experienced contractor, and by maintaining a flat roof properly.
If you’d like additional information or advice concerning a flat roof, please feel free to contact us.