Flat Roofing
(8 Products)Flat roofing is a specialised roofing system characterised by a low or minimal slope, designed to provide effective water drainage while maximising the usable area of a roof. Flat roofs are particularly valued for their ability to accommodate additional functions, such as the installation of mechanical systems, solar panels, or green roofs. In some cases, they can even be used as rooftop terraces, creating valuable outdoor spaces in urban environments where ground space is limited.
Flat roofing systems are typically made from a variety of materials, including built-up roofing (BUR), single-ply membranes, and liquid-applied coatings, each providing specific benefits in terms of durability, installation methods, and long-term performance.
What is Flat Roofing?
Flat roofing refers to roof structures that are essentially horizontal or very low-pitched, typically with a slope under about 10°. Unlike steep roofs, flat roofs appear level but in practice include a slight fall to allow rainwater drainage. This roof style has been used since ancient times (especially in arid climates where roofs double as living spaces) and remains popular today, notably on many modern commercial buildings.
Flat roofs (also called low-slope roofs) maximise usable space - the roof surface itself can sometimes be used for plant equipment, roof terraces, or even green roof installations - and offer a streamlined, utilitarian profile suited to contemporary architecture.
Flat Roof Construction and Design
Structurally, a flat roof comprises a load-bearing deck (often plywood/OSB, concrete, or metal) topped with layers that ensure weather protection and insulation. Any material covering a flat or low-pitch roof is typically called a membrane, whose primary function is to waterproof the building.
Because the roof is nearly level, designing effective drainage is critical - even “flat” roofs are built with a slight incline (for example, a fall on the order of 1:40) so water naturally runs toward gutters, internal drains or scuppers (openings or outlets built into the roof's parapet or edge that allow water to flow off a flat roof).
In this context, the term "1:40 fall" refers to the slope or incline of the roof. This is a ratio that indicates how much the roof drops vertically for every horizontal distance. Specifically, "1:40" means that for every 40 units of horizontal distance across the roof, the roof will drop by 1 unit vertically.
Rainwater is usually directed into a guttering system to keep walls and foundations dry, or into outlets leading to the storm drainage network on larger buildings. Without an adequate fall, water can pond on the surface and increase the risk of leaks or structural load issues.
Warm Roofs vs Cold Roofs
Flat roofs are generally built in one of two configurations: warm roof or cold roof. In a warm roof design, the insulation layer is installed above the structural deck (often a rigid insulation board) with a vapor control layer beneath it, meaning the deck and the interior below are kept warm. This is the most common modern design as it avoids condensation issues by keeping the deck on the “warm” side of insulation.
In contrast, a cold roof places insulation below the deck (leaving a cold void above it); this void must be ventilated to expel moisture. Cold flat roofs were traditionally used but are less common now due to the condensation risk if not perfectly ventilated.
A third approach is the inverted roof (a type of protected membrane roof), where the waterproofing membrane lies beneath the insulation. In such systems, water-resistant insulation boards (e.g. extruded polystyrene) are laid above the membrane, often held in place with ballast like gravel or paving slabs.
The inverted design protects the membrane from thermal shock, UV exposure and mechanical damage, but it adds weight and requires a structurally sound deck to support the load of ballast or a green roof layer.
In all cases, building regulations mandate that flat roof assemblies meet thermal performance standards and resist moisture ingress, so the build-up typically includes a vapour barrier, ample insulation to meet target U-values, and adequate weatherproofing detailing at edges and penetrations.
Types of Flat Roofing Systems and Materials
Flat roofing systems are designed to provide durable, watertight protection and come in various material categories, each with unique benefits.
Built-Up Roofing (BUR) and Modified Bitumen
Built-Up Roofing (BUR) uses multiple layers of bitumen-coated felt or similar sheets to create a robust, waterproof covering. Modern BUR often uses modified bitumen membranes, SBS (Styrene-Butadiene-Styrene) and APP (Atactic Polypropylene) ) that are heat-applied or self-adhesive. These roofs are known for their durability but require regular maintenance to prevent issues like water infiltration at seams.
Single-Ply Membrane Roofing
Single-ply membranes are large, flexible sheets that provide a seamless, waterproof layer. The most common materials are EPDM (Ethylene Propylene Diene Monomer), PVC (Polyvinyl Chloride), and TPO (Thermoplastic Polyolefin).
EPDM is widely used for its durability and flexibility, especially in commercial applications, while PVC and TPO offer excellent UV resistance and are ideal for energy-efficient roofs. Single-ply systems are quick to install and typically offer better wind uplift resistance compared to multi-layer systems.
Installation Methods
- Ballasted Systems: Membranes are laid loose and held down with stones or pavers.
- Mechanically Attached Systems: Membranes are fastened to the deck with screws and plates.
- Fully Adhered Systems: Membranes are bonded directly to the deck using adhesive for a strong, uniform seal.
Each method has its benefits, depending on the roof size, building type, and desired performance.
Frequently Asked Flat Roofing Questions
How Does Drainage Work On A Flat Roof?
Flat roofs are designed with a slight slope (typically 1:40) to direct rainwater toward drainage points such as gutters, internal drains, or scuppers (drainage outlets built into the roof's edge). Installing an adequate drainage system ensures the roof remains functional and protected from water-related issues such as water pooling, which can cause leaks or structural damage.
How Do I Maintain A Flat Roof?
Maintaining a flat roof involves clearing gutters and drains of debris, inspecting the roof surface for damage such as cracks or punctures, and checking the drainage system to ensure water flows freely off the roof. For flat roofs with membranes like EPDM or PVC, check seams and flashing to prevent leaks. Additionally, periodically applying protective coatings can extend the lifespan of the roof.
How Do I Fix A Leaking Flat Roof?
To fix a leaking flat roof, it's essential to first identify the source of the leak. Common causes include damaged seams, punctures in the membrane, or clogged drains. Small leaks can often be repaired using a patch of matching material, such as EPDM or TPO.
However, if the leak is more extensive or if damage is widespread, the affected area may need to be replaced entirely. It’s crucial to inspect the entire roof, as leaks on flat roofs are often caused by accumulated debris or poor drainage systems.
While minor repairs can be handled with some DIY knowledge, consulting a professional roofer is highly recommended, especially for larger leaks or extensive damage. Professionals have the expertise to properly assess the roof’s condition, identify hidden issues, and carry out repairs to the highest standard. Ensuring the correct materials and installation methods are used will help prevent further damage and extend the lifespan of the roof.
