Daniel Bosworth, Design & Technical Manager at SIG Design & Technology, discusses the challenges that specifiers face when designing BS 6229:2018 compliant tapered insulation schemes with Architecture Today’s Technical Editor John Ramshaw

Tapered insulation provides a cost-effective and reliable means of combining both insulation and falls on flat roofing projects. Central to the success of tapered schemes are their ability to eliminate ponding – by employing adequate gradient – while also ensuring sufficient thermal performance at their lowest/thinnest points, for example around sumps and rainwater outlets. However, revisions to BS 6229 introduced in 2018 have made achieving these two prerequisites significantly more challenging and onerous. So, what does BS 6229:2018 recommend, and what can be done to ensure compliance? Furthermore, how should architects approach the design of tapered insulation schemes, and how can roofing specialists, such as SIG, help them to make the right choices? Daniel Bosworth, Design & Technical Manager at SIG Design & Technology, discusses these questions with Architecture Today’s Technical Editor John Ramshaw.

What is tapered insulation and what are the main benefits of using it?
Tapered insulation or ‘cut to falls’ insulation eschews the need for screed, firings or a sloping deck by providing the appropriate fall for the roof’s waterproofing layer. In other words, the fall is designed into the insulation rather than the substrate. Tapered insulation can be applied to flat and/or negative fall substrates; in order to improve drainage.

The main benefit of tapered insulation is that it can be designed ­by experts, such as SIG Design & Technology and their tapered insulation partners, thereby relieving architects of this time consuming and potentially tricky task. Tapered insulation can also result in time and material savings as the roof fall does not need to be fabricated onsite in timber or concrete. Added to this, it can provide a more sophisticated and flexible solution than a simple deck-to-falls design, which can be beneficial on larger and more complex flat roof projects.

This scheme has been designed with falls at 1:60 into a flat perimeter gutter, and as such, could be deemed to not satisfy current guidance

What are the limitations or challenges to using a tapered insulation scheme?
Restricted numbers of rainwater outlets can be challenging on both refurbishment and new-build projects. Existing upstand heights, thresholds and outlet positions can also be problematic on retrofit projects in terms of achieving roof gradients and thermal performance. Schemes with multiple interlinked roofs can be particularly challenging in terms of creating falls that facilitate obstacle-free drainage from all areas.

How should architects approach the design of tapered insulation schemes?
When working with a specialist tapered insulation supplier, such as SIG Design & Technology and their tapered insulation partners, it is important that architects communicate a number of key project criteria. One of the most important of these is building type. A brewery or swimming pool, for example, may pose a significant threat of interstitial condensation due to high levels of heat and humidity. Other key data include geographic location, predicted occupancy levels, project status (new-build or refurbishment), type of waterproof finish specified for the roof and U-value requirements.

With regards to thermal performance, it is important to differentiate between achieved and minimum U-values for the entire tapered scheme. For example, a project with an achieved U-value of 0.18 W/m²K – calculated to BS EN ISO 6946:2017 Annex E – may only be achieving the minimum recommended U-value of 0.35 W/m²K around sumps and gutters. To counteract this, the insulation would need be thicker at the deepest point to achieve an overall U-value of 0.18 W/m²K for the whole roof. By contrast, the insulation on a tapered scheme designed to achieve a minimum U-value of 0.18 W/m²K would be significantly thicker (usually not less than 120mm with PIR or 210mm with mineral wool, at the thinnest point).

The type of waterproof finish chosen for the roof should not directly affect the design of tapered insulation schemes either. Single ply membrane roofs, for example, were installed historically with slightly shallower falls/gutters, as lap build up was minimal; by contrast, the laps on a built-up felt roof system are more susceptible to standing water, so the system would always be required to incorporate minimum 1:80 tapered gutters; however, guidance now stipulates the required given fall in all areas of the roof, irrespective of the waterproofing selection.

This fully mitred tapered scheme (for the same project) is compliant at 1:40, and the design reduces the overall insulation depth

How does BS 6229 affect tapered schemes and what can be done to ensure compliance?
BS 6229:2018 recommends that all flat roof surfaces, including gutter beds, should be designed with a fall of 1:40 to ensure finished drainage falls of 1:80 are achieved. Some building insurance and warranty providers (i.e. NHBC) suggest that design falls should be no less than 1:60, however in roof valleys where two 1:60 falls meet the valley fall will be shallower, meaning that the recommended 1:80 fall across the whole roof may no longer be achievable.

BS 6229:2018 also stipulates that no part of the roof, including internal gutters, should exceed 0.35 W/m²K. This generally equates to minimum required thicknesses of insulation in these areas, for example 70mm of tissue-faced PIR insulation board or 105mm of mineral wool, and makes the detail design of the tapered insulation schemes even more critical. A fully mitred tapered scheme, for example, may provide a more efficient way of draining the roof than a standard mono- or duo-pitch design, while keeping the overall insulation thickness to a minimum.

SIG, in association with its tapered insulation designers, is able to provide fully compliant BS 6229:2018 schemes in a range of insulation materials and with fully annotated drawings for construction and Building Regulations purposes. Schemes will include 1:40 falls to the main roof areas (facilitating full compliance in valleys), 1:40 falls to tapered gutters, and assurance that no part of the roof, including gutters and sumps, will exceed 0.35 W/m²K. Adjustments can be made to the scheme if a 1:40 falls is not achievable, for example on refurbishment projects or due to value engineering requirements – subject to compliance with the necessary warranties.

What should specifiers ask their roofing specialist to provide on a tapered scheme?
Architects should ensure that the U-value for the tapered scheme is stated on the specialist supplier’s drawings. They should also ask for a U-value condensation risk analysis calculation in order to guard against condensation. Added to this, the tapered scheme should be backed by an NBS roof specification as part of the proposal. One of the benefits of using SIG is that it can produce a tapered insulation scheme as part of a wider flat roof waterproofing design. This system-based approach ensures that the insulation scheme is suitable and acceptable for both the building and its flat roofing system(s).

Contact Details
For more information please visit the SIG website.