Commonly associated with the sleek lines of mid-century modern architecture, the flat roof is one of the oldest styles of roof. From the residential buildings of India’s ancient cities to the ultra-modern commercial buildings of the United States, the flat roof is used the world over.
In this guide to flat roofing, we cover everything you need to know about flat roof design, from the pros and cons of flat roofs to the different materials used to construct them. But first, what exactly is a flat roof?
What is a flat roof and when are they used?
Technically speaking a flat roof isn’t always perfectly flat. Rather, a flat roof is defined as a roof with a pitch of between one and ten degrees. Like skillion roofs, flat roofs are typically comprised of one large flat surface. However, unlike skillion roofs or traditional gabled roofs which are set at a steep pitch, flat roofs are pitched just high enough for water to drain away.
The flat roof has been used in residential buildings all over the world for centuries. While it is especially common in countries with warmer, drier climates, such as Egypt, India and Israel, flat roof houses are growing in popularity in Australia, particularly amongst homeowners seeking an elegant, contemporary aesthetic. They are also well suited to the Australian climate, given it experiences relatively modest rainfall and doesn’t see the heavy snowfall of other countries which makes flat roofs a less appropriate design.
There are three main types of flat roofing available to homeowners
The materials used to construct a flat roof have come a long way since the old-school felt used to construct traditional flat roofs. As a result, a properly installed and maintained flat roof can now rival a gable or skillion roof in terms of longevity. Below we outline the three main types of flat roofing available to homeowners.
Built-up roofing (BUR)
Built-up roof (BUR) is the oldest technique used to construct a flat roof, and is also the most common. As the name suggests, BUR is comprised of multiple alternating roof layers that work together to create a continuous seal over the flat roof surface. To construct a BUR flat roof, layers of asphalt are applied between ply sheets—which are also known as roofing felt—to build up the roof, before a surfacing material is placed on top. The surface material is typically comprised of stone or gravel and is an important component of the roof as it helps to protect the lower layers from both UV damage and the impact of any falling debris.
A flat roof constructed from BUR has the advantage of being cost effective—BUR is often the cheapest form of flat roof available—and relatively fire retardant. However, BUR roofs take longer to install than other types of flat roof design, require a professional to install and can be very heavy given the multiple layers used to construct the roof.
Single-layer membrane roofing
Single-layer membrane roofing is a relatively new type of flat roof material that consists of a single membrane that is applied over the building’s underlying roof structure. In modern builds, Ethylene Propylene Diene Monomer (EDPM)—a durable synthetic rubber—is the most commonly used material for this style of flat roof, however other options are available too, including neoprene, PVC and polymer-modified bitumen.
For this style of flat roof, a thin membrane (30 – 60mm thick) is applied in a single layer and anchored to the underlying roof structure using either adhesives or mechanical fasteners. As they are comprised of either rubber or polymer, single-layer membrane flat roofs can withstand temperature fluctuations well and are extremely durable. Furthermore, this type of flat roof is lightweight and relatively quick to install, making it a suitable choice for DIY installation. However single-layer membrane roofs are typically more expensive than other flat roof options and can be more vulnerable to wear and tear.
Modified bitumen systems (MBS)
Modified bitumen systems (MBS) were developed in the 1960s as an alternative to the BUR flat roof, and have grown in popularity ever since. For this type of flat roof, a single layer of asphalt modified with polymers is applied to the roof structure and then reinforced with fibreglass or polyester. A base coat and top coat are then applied above these two layers to increase resistance to the elements.
In comparison to BUR flat roofs, MBS flat roofs are faster to install because they come in easy to roll sheets. They are also thicker than single-layer membrane roofs, which helps them to stand up better to heavier foot traffic if the roof is used as an outdoor patio. However, they will be more vulnerable to water pooling than single-layer membrane roofs, and the modified bitumen seams don’t handle standing water as effectively as flat roofs with a thermoplastic membrane, such as PVC.
Pros and cons of a flat roof
Like all roof styles, flat roofing has its pros and cons, and will suit some building applications better than others. Below we outline the pros and cons of a flat roof design.
Flat roof pros
Flat roofs have numerous advantages that make them a versatile option for residential buildings. These include:
- Flat roofing is space efficient and can be used to create additional outdoor living or storage space. A flat roof can be designed to enable homeowners to use it as a roof deck, providing a great space for entertaining guests or even establishing a rooftop garden. Similarly, flat roofs can be designed to provide excellent storage space for household equipment like water tanks or heating systems. In areas with small block sizes, where space comes at a premium, a flat roof can help make the most of the available space.
- A flat roof is easily accessible, enabling homeowners to inspect them more easily than traditional gabled roofs. Because of the flat pitch, homeowners can safely inspect a flat roof on a regular basis for any leaks or damage that may have occurred over time. This also helps to make regular roof maintenance tasks such as gutter cleaning, a more straightforward process.
- A flat roof can be just as durable as a gabled roof. Provided your flat roof is correctly installed by a flat roofing specialist and is regularly maintained, it can last up to 30 years. While older style flat roofs had a lower lifespan than gabled roofs (about 10-15 years as opposed to 30+ years), modern building materials have significantly increased the longevity of flat roofs and they are now a durable, sustainable roofing solution. For instance, modern EPDM rubber can last up to 50 years if properly installed and maintained, in comparison to the felt that was traditionally used for flat roofs, which has a lifespan of 10-15 years.
- A flat roof is less expensive to install than a gabled roof. This is because flat roofs require fewer materials than a gabled roof and also has a simpler structure. Having said that, it’s important to ensure you pay to have your flat roof installed by a trained roofing specialist as proper flat roof installation is vital for minimising maintenance costs in the longer term.
Flat roof cons
As with any roofing option, there are some downsides to the flat roof design that you should bear in mind when deciding whether a flat roof is the best option for your house. These include:
- Flat roofs aren’t designed to withstand heavy rainfall, making them unsuitable for climates that regularly experience heavy rain and snow falls. This is because the low slope of a flat roof is significantly less effective at draining water or shedding snow than a traditional gabled roof. In areas that experience consistently high volumes of rainfall, this can cause water to pool in the centre of the roof, which increases the chance of leakage or water damage. For this reason, it’s vital to ensure your flat roof is installed by an experienced roofing professional.
- Less optimal performance in high winds. Because they are comprised of a single flat surface, flat roofs don’t perform as well in high winds as gabled roofs. Traditional gabled roofs are designed to intercept the wind and channel it down to the ground. While the low profile of a flat roof might seem ideal for coping with high winds, flat roofs are actually more susceptible to wind damage than other roof designs due to upward forces.
- Flat roofs can be costly to maintain in the long run. While flat roofs are cheaper to install than gabled roofs, they require more regular maintenance than gabled roofs, which can increase their cost in the long run. It is advisable to inspect the roof at least once per year, and regularly remove debris that may interfere with the roof’s ability to drain water—tasks that can be costly if you are unable to perform these yourself.
- A flat roof will absorb more heat than a gabled roof. Because of their low pitch, flat roofs are exposed to the sun all day, unlike gabled roofs which only receive direct sunlight for part of the day. This absorbed heat can make your house warmer in summer. It is possible to reduce the amount of heat flat roofs absorb by applying particular coatings—or even installing a rooftop garden—so we recommend talking to your roofing professional if you live in a particularly warm area.
Best uses of a flat roof
There are some types of buildings that will suit a flat roof more than others. These include:
- Houses in warm climates. Because they are less efficient at shedding water and snowfall than steeply pitched roofs, flat roofs are best suited to warmer, drier climates, such as those found in most of Australia. Although flat roofs tend to absorb a lot of heat, they are still suitable in warm areas as they can be designed to reduce the amount of heat they retain.
- Modernist houses. Along with skillion roofs, flat roofs are a staple of modernist architecture thanks to their clean lines. They help to create a minimalist feel, and are a more distinctive architectural choice than the traditional gabled roof.
- Houses on small blocks. Installing a flat roof can be a great way to maximise the amount of living space you have on your block, as it will allow for useable rooftop space. This is particularly good for homeowners who may otherwise struggle to fit an outdoor entertaining space on their block, or for homeowners who want to store large household equipment, like heating systems or water tanks, out of sight.
- On houses with solar panels. Solar panels can be installed just as effectively on a flat roof as on a traditional gabled roof. On a steeply pitched roof, the direction the roof faces determines whether it will be suitable for solar panels. However, a flat roof gives homeowners the flexibility to orient the solar panels in the optimal direction to maximise the amount of sunlight they are exposed to.
Flat roof design – examples
Below we provide several examples of flat roof design to illustrate the different ways flat roofs can be used.
In the example above, a flat roof is used across both storeys of the house, as well as the garage, to create a consistent look. This is an example of a flat roof design that has been chosen to maximise the internal height of the building, rather than provide additional outdoor space on top of the roof. The addition of the flat roof on an otherwise traditional looking home helps to create a more contemporary aesthetic—and is a great example of how a flat roof can be used to modernise your home if you’re thinking of renovating.
The examples below illustrate how a flat roof can be paired with other elements of contemporary architectural design to create a sleek, modernist home. In each of these homes, the use of a flat roof creates a clean horizontal line that runs either parallel or perpendicular to other lines used across the buildings. Indeed, the flat roof makes it appear as though there is no roof at all. These houses are great examples of how a flat roof can be used to create a private entertaining space that is hidden from view by the front wall of the house.
Image from Houzz
Image from Rawson Homes
Image from Custom Homes Magazine
Image from BAAHouse