What is a flat roof?

Flat roofs are designed to provide waterproofing coatings for a variety of properties and buildings. 

A flat roof is a roof which is almost level in contrast to the many types of pitched or sloped roofs. A flat roof should be designed with a fall no less than 1:40 to ensure a finished fall of 1:80 can be achieved, allowing for any inaccuracies in the construction based on BS8217:2005 and BS6229-2018.

Benefits to a flat roof 

When a flat roof has been installed correctly using high-quality ingredients, there is a range of benefits:

  • Full, durable waterproofing
  • Create surface to walk on
  • Quick to install
  • Low cost
  • Sleek and aesthetic
  • Increase the value of your property

Customising your flat roof

Flat roofs can be built with many considerations in mind. These centre on aesthetics, speed of installation, waterproofing capabilities and cost. In addition, a range of benefits can be incorporated into the flat roof:

  • Non-slip coatings: making your roof safe to walk on

Property owners expecting foot traffic from maintenance workers or window cleaners should consider installing a non-slip, anti-skid coating. This is a grit-based layer incorporated into the outer layer of the flat roof, creating additional grip. Likewise, those planning to create a balcony, walkway or terrace on their flat roof should also consider this.

  • Accessories for flat roofs: from gutters to downpipes and lead flashing

Flat roofing materials and systems come with a range of accessories, from gutters to downpipes and insulation. The insulation itself can come in different types depending on the design as well as the standard. For the most part, insulation for a flat roof is made from PIR, short for polyisocyanurate, is the standard material for Warm flat roofs, yet vacuum insulation panels (VIP), designed with a gas-tight enclosure surrounding a rigid core is more of a high-end system with superior performance yet a higher upfront cost.

  • Reinforcement: when to use a thicker, stronger material

Flat roofs expecting foot traffic or equipment placement should be designed to incorporate additional weight. As well as sufficient structural support, the waterproofing membrane should be resistant to damage from foot traffic, ladder placement and general impact. Some membranes are more resistant than others: GRP fibreglass, for instance, is impact resistant and able to withstand foot traffic by incorporating added layers. Single-ply, on the other hand, is more susceptible to tearing and impact-related damage.

  • Sustainability: clean and environmentally friendly additions

Gone are the days when a flat roof was just a waterproof barrier to keep your property dry. Many progressive property owners are choosing to install green roofs on their flat roof. These are sedum-based layers that provide aesthetic as well as environmental benefits, from carbon capture and pollutant removal to improved insulation.

  • Renewable Energy: solar panels and wind turbines

With increasing energy prices and increasing levels of air pollution in the atmosphere, the potential for solar power to be used as a viable energy source is promising. By converting the Sun’s rays into electricity, solar panels are able to generate clean, zero-emission energy.

Whilst pitched roofs are associated with solar power, flat roofs can also be designed to generate solar power.

Standard flat roofing applications

  • Garages
  • Extensions
  • Balconies
  • Walkways
  • Terraces 
  • Retail centres
  • Factories 

Flat Roofing Materials

Various flat roofing systems exist that demonstrate a range of different performances. 

Traditional: felt, bitumen and asphalt

  • Felt: this is a sheeted form of flat roofing material. It is constructed from bitumen,  which is produced from fossil fuel distillation and usually contains some form of grit like sand or limestone.
  • Asphalt

New: fibreglass GRP, single ply, polyurethane (liquid applied)

Fibreglass (GRP): these flat roofing systems are created by combining glass fibre mat with polyester resin. Once cured, the covering is solid and waterproof.

Single-ply: this is a synthetic rubber-like polymer that is installed on the flat roof as a sheet. Examples of single-ply include PVC and EPDM. Expected lifetimes are between 20 and 40 years depending on thickness.

Polyurethane Resin

Polyurethane (liquid-applied) resin, like glass fibre, is a polymer-based waterproof coating that is created by combining a resin (polyurethane) with a textile fabric-like backing.

Polyurethane resins have attainted popularity with the construction industry, owing to durable and flexible properties, as well as excellent fatigue resistance. Polyurethane polymers are excellent wood adhesives, protective waterproof coatings, rigid and flexible foams, and fibers. These qualities make them perfect as a flat roofing material.

Guarantees and Warranties 

A range of guarantees and warranties exist to provide protection.

These can be largely categorised into 3 types:

  • System: where the manufacturer or specifier offers a set system and design covering insulation through to waterproofing. This is usually BBA certified and requires NVQ level training
  • Installer: this could be a company backed guarantee that covers workmanship.
  • Insurance: third-party guarantee regulated by the Financial Conduct Authority (FCA) and offered by an independent warranty association. 

It is also worth noting that customers or property owners who incur their own damage to the flat roofing membrane are not covered. Examples include foot traffic-induced tears and punctures, holes from ladders, etc. As such, instances where a more demanding work environment is expected to warrant a more durable flat roofing material like polyurethane or fibreglass, whereas those expecting no foot traffic or equipment placement could benefit from a milder flat roofing system like single ply. 

Types of flat roofing designs

Warm flat roof

Warm flat roofing designs are considered the gold standard in flat roofing. 

They offer notable benefits in thermal efficiency when compared to Cold roofs. This is because the insulation is placed on top of the wooden rafters, rather than between it. This complete coverage means that thermal bridging is virtually eliminated, translating to reduced heating bills and better temperature control within the underlying property. 

Whilst it does improve insulation, the Warm roof design does add an extra layer onto the roof. If a flat roof is being built upon to create an extension or walkway and an access door is required, then a Cold roof is preferable. 

Cold flat roof

The term ‘cold’ is somewhat misleading, but relevant. Unlike a Warm flat roof, the Cold flat roof design is installed such that the rafters are not insulated because the insulation (typically rock wool) is laid between them but not over them. When the temperature drops, cold air will conduct through them (wooden rafters are not effective insulators). This leads to unnecessary heat loss, especially when the design is compared to a Warm flat roof. The installation of rockwool between the rafters is more time-consuming and by nature leads to higher heating bills. Therefore we do not recommend a Cold flat roof design.

How to build your own flat roof

One of the most important considerations when building a flat roof is the underlying structure, as much as the outer waterproofing membrane.

Guidelines and detailed information about how to perform fibreglass flat roofing are available. 

Problems with Flat Roofs

The National Federation of Roofing Contractors outlines the following problems that occur with flat roofs. These are summarised below and are not exhaustive:

Roof sagging with puddles – an indication of condensation or excessive load.

Membrane split in the line of deck boards – an indication of movement in the decking, possibly due to incorrect fitting of boards or structural movement.

Waterproofing membrane separation around the perimeter – structural movement, perhaps between the walls and the roof. This could be due to leaking lead flashing, leaking gutters (due to improper maintenance or damage) or incorrect specification where roofing trims have been incorrectly installed.

Roof blisters – a sign of excess condensation and water ingress, or in the case of a liquid-applied membrane, incorrect resin application that would accelerate such blistering.

Widespread upstand movement – roofing materials may not be able to deal with structural movement, usually an indication of poor design or incorrect materials

Flat roofing maintenance 

Professional installers – what to look for when choosing your flat roofer:

  • Guarantees
  • Accreditations
  • Vat reg
  • Company reg
  • Insurance 
  • Testimonials and references
  • Track record of work 

Additions to flat roofing systems 

In addition to the flat roofing construction and system, a variety of additional structures can be placed on the flat roof. These range from green sedum roofs to solar panels, wind turbines and more.

  • Green sedum roofs 
  • Blue roofing 
  • Solar 
  • Wind
  • Roof gardens 
  • Solar reflective paint 

The future of flat roofing 

In spite of recent advances, flat roofs have largely remained the same across for the past few decades. 

Many of the problems encountered 50 years ago still exist today, from membranes susceptible to tearing and poor installation procedures. 

New types of flat roof

In addition to new polymers on the market, flat roofs are also likely to benefit from several advances in other industries. Some of these centre on the inclusion of sensors into and around the roof to detect moisture and leaks well in advance.

Integrated systems

The flat roof also presents unused space, which in inner cities is at a premium, particularly when renewable and sustainable technologies are considered. In our opinion, the flat roof provides an ideal platform to support a variety of accessories and systems beyond simple waterproofing. 

The traditional purpose of a flat roof has been to provide structural waterproofing – that is, protection from the weather and in some cases to support foot traffic and equipment placement. Yet with increasing levels of urbanisation and it’s known side effects (increased pollution and lower air quality), this purpose is looking decidedly basic. 

Instead, the flat roof is becoming a multi-functional structure, that is able to address both waterproofing and renewable energy/sustainability at the same time. Indeed, this is why we expect future flat roofing systems to focus on two key themes:

Monitoring & leak detection 

Without flat roofing inspections to identify leaks and membrane breakdown before they occur, the first sign of a leaking flat roof is water ingress (ceiling stains, water pooling, dampness and a leak). 

Sensors: the average cost of moisture and temperature sensors are falling rapidly. One of the reasons property bills are so high is because of damaged roofing structures and systems. This presents the opportunity for water and moisture detection to be incorporated into the flat roofing system that would alert the property owner of any leak of insulation defect before it gets worse.

Sustainability & renewables

Increasing energy requirements and inefficient energy consumption are all contributors to global warming, climate change and a variety of health problems.

The introduction of electric vehicles is an excellent answer to this, but with energy generated from solar, wind or any other renewable technology, their ability to offset carbon and other atmospheric pollutants is limited.

In heavily urbanised areas, traditional energy generation (wood burners, etc.), are known causes of air pollution.

A shift to solar and wind-generated power is an excellent way to combat these problems. They generate clean energy with virtually no carbon or toxic pollutant emissions. In addition to solar farms, solar and wind turbines can be placed on both pitched and flat roofs. This is particularly useful in urbanised areas like Bristol city centre that have limited space.

The second part of this theme is sustainability. Flat roofs are also an excellent support structure for sedum, a vegetative plant. As well as providing improved aesthetics, it also presents environmental benefits that centre on carbon dioxide clearance and pollutant removal. Green roofs have also been proven to translate to reduced cooling and heating bills since the green sedum roof adds an extra layer of insulation.

Choosing a Reliable Flat Roofing Specialist

Choosing a flat roofing system can be daunting. However, there is a range of systems available on the market that have excellent characteristics. These range from fibreglass to polyurethane and in some cases single ply.

Finding a reliable flat roofer

Yet when it comes to choosing the right system, it’s also necessary to choose the right installer too. Since there is no industry standard that is enforced, anyone can install (or claim to install) your flat roof.

Leaking flat roofs cause a lot of damage. Water ingress can penetrate into the underlying roofing structure and if left unnoticed can cause rot and mould. Ask any major insurance company and they’ll tell you that one of the main causes of insurance claims on leaking flat roofs centres on poor installation.

So what should you look for when choosing a flat roofer?

  • Minimum 15-year guarantee, ideally 20+ year guarantee
  • Insurance-backed guarantee regulated by the FCA
  • An excellent track record of workmanship spanning decades
  • Accredited in industry-recognised construction schemes
  • Reliable testimonials and references from past customers

Traditionally, flat roofs have been built from lead, zinc, bitumen, asphalt and felt. Yet these membranes have drawbacks, from limited lifespans (under 8 – 15 years) to the use of fossil fuel-derived ingredients.

Which is the best flat roofing material?

Like anything, there is no one size fits all approach to flat roofing. Combining the demands placed on flat roofs (to be completely leak-free and durable for decades) with variable and uneven surfaces, yet cost-effective, the recommended system can vary between projects.

In most instances, fibreglass GRP, single ply or polyurethane provides effective protection.

Regulations on flat roofing installations

Understanding the existing regulations for your flat roof is also important. The replacement or even just the repair of your flat roof can trigger building regulations surrounding the Conservation of Fuel and Power – seen in Building Regulations Part L. More specifically if more than 25% of your flat roofing surface is undergoing repair, you are required to install new insulation compliant with existing standards.