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UKFR - ROOFING AND INSULATION SPECIALISTS
There are many types of flat roof build-ups, including warm deck, cold deck and inverted roofs. There are also many types of flat roof coverings, including EPDM, Felt and GRP. In this guide, we’ll break down the benefits and differences between these and why they might be right for you.
A flat roof is generally defined as a roof with a pitch of less than 10 degrees. While it may appear completely level, it is always designed with a slight incline to allow rainwater to run off effectively and prevent pooling.
In contrast to pitched roofs, which are more common on traditional residential houses, flat roofs are typically found on extensions, loft conversions, garages, and sheds. They are also widely used in commercial buildings, particularly where large roof spans are required.
There are many types of flat roof build-ups, including warm deck roofs, cold deck roofs and inverted roofs.
A warm roof is where the insulation is positioned above the roof deck and beneath the waterproofing layer. This creates a continuous thermal barrier that minimises thermal bridging and makes it easier to achieve a lower U-value, as there are no restrictions with the timber joist size/celling height.
Warm roofs have no ventilation, as the insulation sits above the joists. This allows the waterproofing layer to be applied directly onto the insulation. The insulation used in a warm roof set-up must be compatible with warm roof systems, as these have a higher compression ratio compared to standard PIR boards.
Visit our warm deck insulation page – we stock flat roof specified boards from some of the biggest manufacturers, including Kingspan, Ecotherm and Rockwool.
If you are not permitted to add extra height to the roof, a warm deck roof will not be suitable, as this would add extra height to the roof. In these cases, an alternative Optim-R system can be used for a thinner finish.
A mechanically fixed system
A mechanically fixed system is where the insulation and roof are secured using physical fixings (screws and plates). We recommend using warm deck fixings for this, which consist of a plastic tube and a metal screw, helping reduce the risk of cold bridging.
This system can be installed using a foil-faced insulation board, such as Kingspan TR26 or Rockwool Hardrock, and/or an upper timber deck if required. You need an 18mm lower deck and a vapour barrier installed below the foil-faced insulation. It’s advisable to use a bitumen-backed VCL, such as ALUTRIX, to ensure that punctures from the screws self-seal.
A fully adhered system
A fully adhered system is one where all layers are bonded together using adhesives.
This method requires a fully adhered vapour control layer, such as ALUTRIX, installed using a compatible primer, like FG35 Primer, to improve adhesion.
This system is installed using a tissue-faced insulation board, such as Kingspan TR27 or Ecotherm Innobond, adhered to the board with an insulation glue. We recommend Insta-Stik roof adhesive.
As shown in the graphic above, the waterproofing layer can then be installed in two ways:
A single-ply membrane is adhered directly to the insulation using appropriate adhesives.
An additional timber deck is installed above the insulation, followed by a waterproofing layer such as EPDM or GRP.
Cold deck roofs are most commonly used on older flat roofs and loft spaces where the structural deck is exposed. They require a ventilated air gap above the insulation to prevent condensation issues.
In a cold roof, the insulation is positioned below the roof deck, typically between the joists or rafters. This means the structural elements sit within the insulated zone rather than above it, which helps make use of the available joist space and ensures no space is wasted.
As seen in the graphic above, there is a gap between the insulation and the timber deck. This is intentional. A cold deck roof always requires a minimum ventilation gap of 50mm with cross ventilation to allow the structure to breathe. This can either be at the front and back, or left and right. If the cold roof spans over 5m, it will require extra vents for further ventilation.
This ventilation prevents any moisture buildup and allows the timber joists to breathe. Without this, you will end up with rotting timber and mould issues.
Where a vent isn’t possible, i.e. an extension against a house wall, you can use flat roof breather vents that span directly over a joist and into 2 air gaps. This allows trapped moisture to escape within the roof structure.
As the system requires a minimum breather gap of 50mm, the size of the joist will impact the thickness of insulation, which will subsequently lead to heat loss if the size of the joist is limited.
Cold roofs also require a vapour control layer (VCL) on the warm side of the insulation to prevent excess moisture build-up in the roof space. Without a VCL, it will lead to dampness. A VCL can be in the form of an additional layer or all the foil on the PIR tapered and sealed.
The most common way to install a cold roof is with a 50mm air gap at the top, PIR filled to the rest of the space, and then an extra board placed internally, which spans over the full area. This means you can use more insulation and achieve the required U-value, whilst still maintaining a gap for airflow.
For example, if you have 150mm joists, you can use 100mm PIR between the joists with a vented 50mm gap above and then 50mm PIR underneath. This is just an example; to know exactly how much insulation you need, it’s always best to get a U-value report done, as it does depend on joist spacing.
A vapour barrier is also required in the installation, placed on the warm side of the build-up (below the insulation and above the ceiling).
An inverted roof (also known as a protected membrane roof) is where the insulation is installed above the waterproofing layer and structural deck. This is different to traditional warm roofs, where the insulation sits below the waterproof membrane and structural deck.
This setup is commonly used on buried structures, such as green roofs or houses sunken into the ground, and large-scale commercial projects where waterproofing is the highest priority. This would be applications such as podium decks or roof terraces built over other occupied spaces; by placing the waterproof layer beneath the insulation, the membrane is protected from physical damage and heavy footfall, while the layers above take the wear instead.
The insulation used in an inverted roof system must be EPS or XPS insulation. This has a closed-cell structure and is designed specifically to keep out moisture. To ensure the insulation doesn’t move around, it must be ballasted in place, which is often achieved with gravel, paving slabs or a green roof system.
Inverted roof systems are less thermally efficient than warm roof systems, as the materials have lower thermal values. This, however, should not be a deciding factor for an inverted roof, as they are used for risk management; to achieve target U-values, you can simply use thicker insulation.
The most common way to install an inverted roof is a ballasted system, where the insulation is held in place by weight (typically from gravel, paving slabs or a green roof system).
In this system, the waterproofing membrane is installed directly onto the structural deck, where it acts as both the waterproofing layer and the vapour control layer. This layer must be fully sealed and continuous, as it sits beneath all other components of the roof.
Insulation is installed over the waterproof layer, and a breathable waterproof membrane is then installed on top to allow water to drain. You would then ballast over the top using gravel, paving slabs or a green roofing system.
There are several ways to waterproof a flat roof. We’ve broken down all the options for you below.
Best suited for cold deck roofs, warm deck roofs & inverted roofs
EPDM rubber is a highly versatile synthetic rubber that’s suitable for flat and low-slope roof applications. It’s one of the most popular choices for flat roofs, and for a good reason, too.
It has a lifespan lasting up to 70 years, which is at least twice that of other flat roofing materials. It’s also incredibly durable, remaining weather-resistant all year round and withstanding temperature changes from -45°C to +120°C without cracking or degrading.
EPDM is also very lightweight and DIY-friendly to install, typically supplied in large rolls that can be cut down to size, and cold-applied using a water-based adhesive. This makes it perfect for simple flat roofs that do not have complex detailing, such as garage roofs, shed roofs, or extensions.
If there is complex detailing, such as pipes or skylights, EPDM might not be the right material. This is because you would need to cut the EPDM roll to fit around them, which can be harder to detail if you’re not experienced in detailing EPDM.
Best suited for cold deck roofs, warm deck roofs & inverted roofs
Torch-on felt roofing, also known as bitumen felt or modified bitumen roofing, is a traditional flat roofing system made up of multiple layers of bitumen-based felt. The name “torch-on felt” comes from its installation process, as it requires the use of a flame torch to bond the felt directly to the roof surface to create a continuous waterproof barrier.
Once installed, the system forms a durable and weather-resistant layer capable of protecting against rain and general exposure. However, because the material is heat-welded, we recommend hiring a professional to install it for you.
Older felt systems deteriorate over time and can become brittle, making them susceptible to cracking and splitting, which can lead to water ingress and damp. Repairing this damage is a very complex and costly process compared to modern alternatives, which is why its popularity has decreased over the past few years.
Best suited for cold deck roofs & warm deck roofs
GRP roofing, also known as fibreglass roofing, is a popular flat roofing system that combines the strength of glass fibre matting with the hard, non-permeable layer of resin. This creates a strong, fully bonded waterproof membrane that, once cured, forms a rigid, seamless and watertight surface.
These systems are highly durable and support heavy foot traffic, while remaining resistant to harsh weather conditions and UV exposure all-year-round. GRP roofs typically have a lifespan of between 15 and 25 years, depending on the system:
During installation, GRP is very malleable and can be formed to almost any shape. This is ideal for complex roof layouts, such as those with intricate detailing, pipes or skylights.
However, the installation process is not DIY-friendly. GRP must remain dry throughout the installation and curing process. If it’s exposed to moisture before fully curing, impurities can form on the surface, which can lead to water ingress over time. This is why we always recommend hiring a professional who knows what they’re doing.
Best suited for cold deck roofs & warm deck roofs
Liquid roofs, more commonly known as liquid rubber roofs, are made from liquid polyurethane and cure to create a rubber-like waterproof membrane.
They are very similar to GRP roofs as they form a completely seamless, watertight membrane without the use of joists or seams. They also form a near-impenetrable, highly rigid surface which handles weather conditions like snow, ice and rain with ease all-year-round.
Liquid roofs have a lifespan up to 25 years and remain flexible after curing, which means they accommodate building movement and thermal expansion without cracking.
Similar to GRP, the installation process for liquid roofs is not DIY-friendly and requires a perfectly dry surface throughout the installation and curing process. It’s also vital to get the liquid-to-matting ratio correct. This is why we recommend hiring a professional to install the system for you.
For an accurate calculation on how much your flat roof will cost, try out our roof kit calculators. We’ve built this ourselves, and it covers everything we need from you to provide you with an accurate quote.
