Roofconsult Website An Interview with SIG Design & Technology’s Steve Cleminson on Designing Flat Roofs
Contact Us
Industry News
Check out our web directory of the UK roofing and cladding industry

Sign up for our monthly news letter.

Flat roofs have long held a place in architects’ hearts for reasons of aesthetics, cost and functionality. While their conceptual and visual simplicity is undoubtedly part of the appeal, specifiers should be under no illusion as to the detailing rigour that is required to achieve a solution that is fit for purpose and provides long-term protection from the elements. An Interview with SIG Design & Technology’s Steve Cleminson on Designing Flat Roofs
Adding to the complexity of the design process is the number of different flat roofing materials and systems available – each with its own individual characteristics and technical requirements. So how should architects approach this critical aspect of building design? We ask SIG Design & Technology’s technical director Stephen Cleminson.
What are the most important things to consider when designing flat roofs?
SC: “Undoubtedly it’s the failure to always take a holistic view. Architects should pay particular attention to project sequencing and preceding trades. For example, the dressing of roofs around openings and reveals needs to be completed before doorways, rooflights and clerestorey windows are installed. Achieving adequate long-lasting weather protection around these critical junctions can be nigh-on impossible if these elements are already in place.
“Procedures such as fitting balustrades and mansafe systems also need to be properly co-ordinated to avoid damaging the roof covering. Many manufacturers have conducted mansafe testing, so are well placed to advise on correct detailing and site co-ordination for different systems.
“Another important consideration is whether the roof will be used for material storage or as a loading deck during construction. For a warm roof on a concrete deck, it may be worth upgrading the vapour control layer to a robust bitumen-based product, and delaying installation of the insulation and waterproof membrane. When the roof deck has served its purpose as a loading or storage platform, any necessary local repairs can be made to the VCL before the rest of the roof is installed.
“This approach not only avoids the cost of temporary roof protection, but also ensures that the finished roof does not look secondhand or like a patchwork quilt! Obviously, it’s important to carefully control access once the roof is finished.”
What should architects pay special attention to during the installation phase?
SC: “Specifiers should insist on full QA and QC procedures to ensure the roof is properly constructed. Hold points are also important, as they give architects the opportunity to check key elements of the roof design as they are completed, such as VCL laps. This can include photographic evidence of correct procedures.
“SIG Design & Technology does not sell its products to installers who have not completed one of our product-specific, SPRA-approved (Single Ply Roofing Association) training courses. SIG field technicians then support these newly accredited installers by visiting site regularly. Ultimately, we want installers to use our products with a high degree of expertise and confidence.”
How should flat roofs be maintained to maximise their life expectancy?
SC: “Maintenance should always be in accordance with the manufacturer’s recommendations and the British Standards. Flat roofs should be inspected at least twice a year – more if they are located in areas with lots of trees. Among the biggest threats to the long-term performance of flat roofs can be leaves, saplings and other vegetation on poorly maintained installations.
“SIG D&T roofing warranties are issued with a maintenance plan, which must be adhered to as a means of validation. Regular scheduled inspections resulting in clearance of detritus and early detection of damage is essential to ensure flat roofs meet and surpass their guarantee period.”
What are the advantages and disadvantages of cold and warm roofs?
SC: “Cold roofs are currently less popular in the UK due to the need for increased depths to meet current U-values and the requirement for through-ventilation. For architects it’s more a question of whether to specify a warm or inverted roof. The latter places the waterproof layer at the base of the roof build-up where it is less susceptible to damage and UV-degradation. On the downside, the WPL can be difficult to access in the case of water ingress.
“More often than not, our recommendation for a concrete deck would be to use a hot-melt system. It’s fully adhered to the substrate so water can’t track through the roof layers in the case of damage or poorly designed or unplanned penetrations. This makes finding faults much easier. A single-ply membrane is often recommended for flat roofs that do not require multiple penetrations or a mass of plant and equipment, as it is simple, economical and quick to install.
“One of the advantages of SIG Design & Technology is that it supplies a wide range of different flat roofing systems and can therefore recommend the best product for the application. When it comes to the advantages and disadvantages of different flat roofing systems, such as hot melt, cold-applied or membrane, we tend to take a scenario rather than product-based view. Specification will ultimately depend on answers to specific questions, such as how complex is the roof? Are there additional NHBC or FM requirements? What if any penetrations are required?”
What are the latest flat roofing products from SIG and what are their benefits?
SC: “Our most notable recent product is Hydrostop AH-25, a wet-on-wet, cold-applied liquid waterproofing system that is particularly suited to complex roofs and balconies, or where there are lots of plant and equipment. It is also suitable for refurbishment and repair applications, and can be installed on inverted or green roofs. A further benefit is that the system is virtually odourless and does not require heat, flames or hazardous chemicals on site, making it installer-friendly and non-disruptive to building occupants on refurbishment projects.
“Overall, the main recommendations I would make to architects are to seek early engagement with flat roofing experts, take a holistic approach, and go beyond the minimum standards where possible. In other words, design from the roof down!”
About SIG Design & Technology
SIG Design & Technology is part of SIG Roofing, a leading division of SIG plc, a FTSE 250 listed company and the UK’s market leading specialist supplier to professionals in the building and construction industry. We design and supply flat roofing solutions including green roofing, zinc, copper and stainless steel roofing and cladding and pitched roof coverings including natural slate and clay tiles.
We have put together an 8-step guide to identify the challenges and ensure that a roof’s design meets a building’s requirements.
We have put together an 8-step guide to identify the challenges and ensure that a roof’s design meets a building’s requirements.

Our know-how is just part of the service that is provided absolutely free to customers.


For further information on SIG Design & Technology see
Home > Industry News