The AT Technical Seminar, in partnership with AccuRoof, saw experts in the field present and explore best practices for roofing design and specification.
On Wednesday, May 15th, at The Building Centre in London, AccuRoof, a part of SIG Roofing, hosted a technical seminar focusing on best practices in roofing design and specification. Experts Ross Finnie, Steven Vincent, Simon Walker, Daniel Bosworth, Chris Collins, and Terry Collins provided valuable insights into various roofing systems, materials, and techniques. The day included detailed presentations followed by interactive sessions where attendees could engage with the specialists.
Bio-Solar Roofing: Best Practice Design and Specification
Steven Vincent, head of design and estimating, Eco Green Roofs, started the seminar with a presentation on Bio-Solar roofing. He emphasised the integration of non-penetrative PV mounting into green roofs, using the growing medium as ballast. This design allows light and water to reach the plants, supporting their growth. The system is designed for larger, next-generation panel sizes and aims to achieve both Carbon Net Zero and Biodiversity Net Gain without compromise.
Vincent also explained the what was needed for biodiverse roofs to thrive, with a substrate thickness of 80mm to 150mm considered best practice for a variety of different species to thrive. Vincent then went on to outline how a water drainage board can be employed to spread the load of the PV array, however, if the building is to be above 35 metres, then wind factors come further into play, with more ballast being required.
Factors for establishing the client’s brief.
Hard Metal Roofing Systems: From Specification to Installation
Simon Walker from SIG Hard Metals discussed the specification and installation of hard metal roofing systems. SIG Hard Metals specialises in traditional metal roofing and cladding, working with materials such as KME TECU Copper, VMZinc, Prefa Aluminium, and Rheinzink. Walker highlighted the importance of material selection, construction methods, and accurate detailing to deliver on architectural intent. He also addressed the pros and cons of warm versus cold roof construction, warranty options, and the impact of market volatility on material prices.
Walker pointed out how clients need to factor in the wider context of material selection, with this pertaining to two key points: site environment – such as being aware of how to properly specify steel if being used in a coastal environment; and being aware of minimum order numbers for certain materials and how that can impact your project, particularly with material prices being subject to a volatile market.
View of concealed box gutters in zinc above high-level dormers, as well as bespoke snow guard at the Lucent W1 project in London.
Counting Costs: Where Not to Cut Corners in Roofing Projects
Walkers conclusion on material orders and pricing led into AccuRoof commercial director Ross Finnie’s presentation on managing costs in roofing projects, which covered the latest inflation data and regulatory changes. With construction material inflation at 4.8 per cent as of June 2023, down from 10.4 per cent the previous year, and new requirements from the Building Safety Act, Finnie described how it is crucial to understand where to avoid cutting corners as well as how different insulation boards compare and what systems are compatible and tested. Options such as foil-faced, tissue-faced, and bituminous-faced PIR boards vary in performance, and each has implications on design due to their differing lambda values and thickness requirements.
Closed cell insulation boards, like XPS, XPS+, and EPS, are popular choices, and specifying the correct type is crucial. In some cases, vacuum insulation panels (VIP) may be necessary due to their superior insulating properties despite higher costs. The compressive strengths and loadings of these materials also play a significant role in their selection.
Insulation is one of the main cost areas in flat roofing projects. Ensuring adequate fire safety through Broof(T4) testing is a key stipulation. While materials like Foamglas or mineral wool offer excellent fire resistance and acoustic performance, they can be expensive. Early understanding of the fire strategy is crucial, as is designing to BS 6229:2018 standards for effective water drainage.
Detailing is another major cost factor. Early engagement in the design process can prevent costly complications, and on-site support helps identify and address issues during construction. (Architects don’t carry the liability for waterproofing, pointed out Finnie, but it’s important the drainage details are correct so the architectural intent isn’t impacted if and when these have to be amended). Using thermally broken fixings and specifying the correct, fully sealed air and vapour control layer (AVCL) optimised for the building’s use are critical for achieving the best performance.
Flat Roof Drainage and Design
Dan Bosworth, technical director was on hand to detail how effective rainwater management is vital for flat roofs, including gutters, to prevent ponding and ensure longevity. Insurers emphasise that well-designed flat roofs should have proper drainage to avoid water accumulation, which can lead to greater water penetration and damage. Ponding not only exacerbates leaks but also encourages debris build-up, potentially blocking drainage outlets and fostering insect infestations.
Example image of no falls design leading to ponding water.
Design requirements for flat roof drainage are specified in BS6229:2018, recommending a 1:40 design fall to achieve a 1:80 fall. Tapered insulation is often accepted with a 1:60 fall design by insurers, though BS6229 does not explicitly state this, necessitating consultation with the design team. For systems with third-party accreditation for “zero falls,” a 1:80 design fall is acceptable if no backfalls occur, requiring deflection analysis to mitigate potential issues.
Best practices for flat roof falls and drainage include:
- Adherence to BS6229:2018: Ensure 1:80 achieved falls to all roof areas, including gutters.
- Two drainage outlets minimum recommend: Positioned away from door thresholds, calculated in accordance with BS EN 12056-3.
- Use of tapered insulation: Achieving falls of 1:40 or 1:60 to direct water to outlets efficiently.
- Proper upstand installation: Waterproofing upstands should be at least 150mm above the finished roof surface, crucial for abutments and to avoid water ingress.
- Examples of poor design include flat gutter zones and inadequate falls leading to ponding, which are often rejected by insurers. Good practice involves early consultation with insurers and thorough design planning to avoid costly corrections. For complex designs, such as zero falls to a valley line, ensure the use of accredited systems and verify that ponding is managed appropriately.
Ultimately, maintaining effective drainage and adhering to best practices in flat roof design not only meets insurers’ requirements but also ensures the roof’s durability and performance.
Natural Slate Roofing
The final segment, presented by Chris and Terry Collins, covered in detail natural slate roofing design. Together, they explained how the degree of a building’s exposure to driving rain significantly influences the required lap for roof slates and tiles. Proper roof design, governed by BS 5534:2014+A2:2018, is essential for ensuring durability and effective weather resistance.
The pitch of the roof plays a key role in lap requirements. Generally, a lower pitch necessitates a greater lap to resist wind uplift and rain penetration. For 500mm by 250mm slates, a moderate exposure requires a pitch of 25 degrees with a 90mm head-lap, while severe exposure needs 27.5 degrees. For 500mm by 300mm slates, moderate exposure requires a pitch of 20 degrees, and severe exposure needs 22.5 degrees. Maximum roof pitch can be up to 90 degrees (vertical). Natural slate is recommended for pitches as low as 12 degrees due to its unique breathability and condensation control.
Slate roof installation and product guarantees
Permavent Easy Slate allows for pitches as low as 12 degrees, keeping battens dry and extending the roof’s lifespan. It has been tested by the BRE and certified for use with Gallegas 39, approved by NHBC, and comes with a 40-year product guarantee.
For maximum rafter lengths, moderate exposure can be up to nine meters, and severe exposure up to six meters. Incorporating a suitable natural slate into the H62 specification ensures optimal performance.
SIG Roofing offers a comprehensive warranty program, ensuring preferred materials achieve the desired aesthetic and performance standards. By following these guidelines and utilising high-quality materials from the SIG Natural Slate Range, roof designs can achieve optimal performance and longevity, even under severe weather conditions.
Map showing weather exposure across the UK. SIG has an introductory guide to designing slate roofs on its website, available here.
UK legislation and standards for roofing slate
Under the Construction Products Directive, UKCA marking is required for products meeting Designated Standards such as BS EN 12326-1:2014, which covers slate and stone products for roofing and cladding. While CE marking is accepted until June 2025, all SIG Roofing natural slates are already UKCA marked. British Standards, including BS 5534 and BS 8000:6, also regulate roof design, installation, and material control.
Regarding compliance and testing, manufacturers must provide a “Declaration of Performance” (DoP) for slates, ensuring traceability and compliance. These slates undergo annual testing, or for every 25,000 tonnes of material, specific to one quarry or vein, with results printed on crate labels. Tests assess water absorption (frost resistance), thermal cycle (rust test), and sulphur dioxide resistance (acid rain test).
An example of “Bird mouthing” which produces gaps between courses.
Water Absorption (W):
- W1: Less than 0.6% by weight, indicating good frost resistance.
- W2: Greater than 0.6% by weight, requiring further tests.
Thermal Cycle (T):
- T1: No running rust or discolouration.
- T2: Discolouration without perforation.
- T3: Rusting through, unsuitable for roofing.
Sulphur Dioxide (S):
- S1: No damage or softening.
- S2: Increased thickness due to softening risk.
- S3: Use slates at least 8mm thick, potentially unsuitable for roofing.
The NHBC, the Collins’ pointed out, mandates W1, T1, and S1 ratings for slate roofs under their insurance.
Compliance labels are another factor for ensuring quality. Crate labels must display the CE or UKCA mark, product performance details, DoP reference, production date/year, EN Standard, and manufacturer contact information. Common issues with low-quality slate include oxidization, poor roof finish (“bird mouthing” gaps), and improper sorting or grading. Using second or third selection products can exacerbate these problems, leading to costly re-roofing. Proper grading and high-quality materials are essential.
More to come
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Source: Architecture Today