As per the textbook definition from a leading flat roofing manufacturer, a flat roof is any roof set at an angle between 1-12.5 degrees. Anything from 12.5 up to 85 degrees falls into the pitched roofing category. As for the materials used in their construction, a flat roof can be made from a wide variety of products, which we will cover in this article.
Whilst called "flat", almost all modern flat roofs are constructed with a minor pitch to allow rain water to run off. Flat roofs have gained a bit of a bad reputation in the past for having a short lifespan, but the development of modern flat roofing materials and improved installation techniques has led to great improvements in flat roof reliability and are now considered a cost effective and safe alternative to a traditional pitched tile roof.
Where To Find a Flat Roof
Flat roofs can be found installed on many types of building, however flat roofs are typically seen on the following -
- Garages
- Extensions
- Dormer windows
- Commercial buildings/offices
- Tenement blocks
Flat roof design
Warm Flat Roof
A warm roof is built with the insulation above the deck, providing a barrier from the cold. This type of flat roof is most recommended as it avoids the needs for extensive ventilation of the roof structure.
Cold Flat Roof
A cold roof is built with the insulation below the deck, separating it from the structural joists and ceiling. This format requires considerable ventilation to stop condensation issues arising.
How Are Flat Roofs Made?
Structural Deck
Plywood - Minimum of 18mm exterior grade (BS) 5268-2 WBP.
Timber Boarding - Tongue & grooved with preservative treatment.
Orientated Strand Board OSB -Minimum of 18mm only OSB/3 or /4.
Chipboard - Not recommended for living areas, ensure good ventilation.
Concrete - Often used for flats, ensure 100% dry before waterproofing.
Vapour Control
Warms roofs should be constructed with a Vapour Control Layer (VCL) between the insulation and the structural deck. Usually consisting of one or two layers of a bitumen membrane, polyethylene or foil cored felts. The vapour control layer should be selected based on the expected humidity levels of the room below. The wrong sort of VCL or not using one at all will result in early failure of the roof due to excessive condensation build up.
Insulation
Poly Isocyanurate - Very popular, it's lightweight and readily available in various sizes and thicknesses. An appropriate grade should be selected that is compatible with the waterproof layer.
Mineral Wool - Good heat and fire resistance with a wide range of thicknesses.
Expanded Polystyrene - An effective insulator, but must be protected from heat by a protective overlay like cork or wool fibre if bitumen is to be applied over it. Only high duty or extra high duty grades to be used.
Composite Boards - There are multiple products that combine materials, such as cork and polyurethane. The pros & cons should be researched before use to ensure suitability.
Cork - Very stable and a good insulator, it is well suited for use with a reinforced bitumen membrane layer for waterproofing.
Waterproof Covering
For residential flat roofing, its most common to find some form of Reinforced Bitumen Membrane (RBM) for the waterproof layer. It's normally applied in two to three layers and sealed together with hot bitumen. On a "warm roof", the membrane is applied on top of the insulation. This setup is very effective and creates a reliable flat roof.
Ways to apply it -
- Pour & Roll - The bitumen is heated until it is molten, then poured out as the RBM is unrolled over it, acting as an adhesive and sealing the laps.
- Gas Torch - Using RBM which is already coated in bitumen, a torch is used to heat the RBM directly as it's applied. This is best suited to repair work or smaller jobs.
- Cold Applied - Working the same way as heated bitumen, a special adhesive is used, which doesn't require heating for application.
- Self Adhesive - The RBM uses a high tack layer for waterproofing that is applied during manufacturing. This type is not suitable for cold temperatures of uneven surfaces.
Surface Protection
Most types of RBM need some form of protection applied to protect against early wear. Depending on the expected traffic there are several common options.
Low Traffic
Stone Chippings - 12mm+ chips bonded with a gritting compound.
Mineral - Small flakes of slate are stuck into the felt during manufacture.
Painted - Various paints are available for use, although they need regular reapplication to maintain their protectiveness.
High Traffic
Concrete Tiles - Applied with a suitable adhesive.
Bitumen/Rubber/Plastic Tiles - Applied with a suitable adhesive.
Concrete Slabs - Roof must be designed to take the weight.
Types Of Flat Roof Waterproofing
Single ply
Single ply flat roofing has been used in the UK for many years and can be found over a wide range of buildings, such as supermarkets, schools and domestic properties. It works best in a "warm roof" construction, standing up well to the British weather and repeat exposure to the sun. With good maintenance and proper installation, a single ply flat roof can last up to 40 years.
Liquid applied solutions
Typically reinforced with some sort of fiberglass or fleece matting, a liquid flat roof is commonly referred to as a Glass Reinforced Plastic (GRP) roof. To adhere to UK building regulations, they must be fitted to a suitable quality plywood or OSB deck only.
There are also products that utilise polyurethane, polyester or acrylic to create a more flexible system over GRP and can be applied to a larger range of substrates to create a seamless membrane. Like the single ply, a properly maintained roof that uses high quality products can last up to 40 years.
EPDM roofing
Ethylene Propylene Diene Monomer (EPDM), is a single ply waterproof layer that is installed without heat. A strong bonding adhesive is used to glue it to the deck, with trims installed around the edges to create a clean looking and fully waterproof layer.
EPDM is lightweight and very durable, with good thermal expansion properties, making it a wise choice for larger flat roofs which may succumb to cracking over time. A properly installed flat roof made from high quality EPDM with appropriate maintenance can be expected to last 40+ years.
If you have any questions about flat roofing or have concerns about your own flat roof and would like to seek expert advice, please feel free to visit our flat roof repair service page or give the Trusted Roofers a call on 01382 671028