There are many specification considerations for flat roof design for both performance and aesthetic requirements of the roof. This list represents most commonly asked enquiries from flat roof designers and specifiers; topics include roofing interfaces, compatibility with other roofing products and how to make savings in the rainwater drainage systems.

Here are our top 7 Fundamental design considerations to make sure your flat roof provides a high performance watertight solution regardless of design, chosen waterproofing, and installation.

  1. Ensure your ‘flat roof’ is tilted

    It’s important to ensure that your flat roof is designed with adequate provision for drainage. The ‘tilt’ can be formed by using timber fillets or concrete laid falls, or tapered roof insulation as shown in the below diagram. Falls are created to ensure any excess water is able to drain away and not pond on the surface of the membrane. Without this design consideration, standing or ‘ponding’ water can result in fungus/algae build up, or put a strain of the roof structure that hadn’t been accounted for. It can also cause some membranes to degrade, impacting the integrity of the waterproof roof covering and life expectancy. Guidance on the correct drainage design can be found in BS6229:2003. It is recommended that roofs should have a design fall of 1:40 to achieve a minimum fall finish of 1:80.

    Cut to fall

  2. Make sure your roof can withstand the wind.

    Wind uplift calculations take into account geographic location and height of the building to determine the correct method of attachment. This is commonly understood as the number of fixings and fixing pattern required to restrain a mechanically fixed roof assembly to the structural deck. Successful wind uplift calculations are a combination of knowing the wind uplift design pressures for the perimeter, corners, and main areas of the roof, before selecting a roofing system with performance of the whole build up.

    If you’ve specified a fully bonded system then the wind uplift calculation will determine if this is correct. It is common for northern and coastal areas to require a fully mechanically fastened roofing system, and inland southern areas typically use an adhered system.
    It is usually the waterproofing manufacturer that will provide the required calculations to ensure the performance of the whole roof system.

  3. Optimise thermal performance.

    While this is really a blog post in its own right ultimately you want to deliver the best U-value calculations with insulation to meet (or even exceed) the requirements of part L. The thermal performance of a building is about the total envelope not just the roof, but correct specification of insulation can compensate for large glazed areas of a façade, all contributing to BREEAM and the buildings insulation properties. Design out any cold bridging – Heat loss can be prevented by making sure the complete roof build up delivers the best thermal performance. The whole roof system contributes to the thermal performance and a relatively simple inclusion such as thermally broken fasteners can help to prevent cold bridging, without compromising the thermal integrity of the roof.

  4. Ensure the compatibility of the whole roof system

    The differing components of a flat roof assembly must be compatible with each other to ensure the roofing system is durable and reduces the risk of future defects and system failure. For example, Insulation designed for use in an adhered system is required to undergo external testing with individual membrane manufacturers. The robust testing ensures that it is not only a suitable adhesive bond, but also that none of the adhesives will affect the performance of the insulation.

  5. Consider durability and guarantee

    Longevity of materials has a direct impact on the environment and how often materials or roof assemblies need replacing. In financial terms it is the period over which the depreciated initial capital cost and annual maintenance cost do not exceed the annual cost of a replacement roof. The BBA assess the durability of waterproof membranes and typically are given a lift expectancy of anywhere between 25 – 40 years. Individual BBA certificates will confirm. Guarantee requirements should also take into consideration the whole roof systems as any issues with any failing element of the roof system will impact the whole roof build up.

  6. celotex-crown-bondMeet the required fire performance

    Approved document B sets guidance for fire safety in all building types. When it comes to flat roofing the whole of the roof assembly is required to resist fire penetration from external sources. The roof assembly consists of the deck, VCL, insulation and roof covering. The test measure both fire penetration and surface spread of flame.The results range from the highest EXT FAA (which stands for External surface flat, penetration grade A and surface spread of flame A) to EXT FCD (the lowest).

  7. And last but definitely not least, choose a trusted installer.

    Even the most comprehensive specification needs to be installed correctly for the roof to function as intended. Many manufacturers have ‘approved’ installer schemes to make sure that operatives have not only been trained in the installation of a specific product, but also been assessed to ensure they can deal with difficult details and any product specific installation considerations.


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