In recent weeks, a number of retrofit installers have contacted us to ask for clarification on the official guidance regarding the use of verge trims, and details of proposed revisions to the design principles associated with them. In this post, we aim to explain the current situation and the progress that has been made to date.
Background
As many in the industry will know, PAS 2035: 2019 Incorporating Corrigendum No. 1 – issued in 2020 – included in Table B.4 a line stating:
‘ii. interfaces with roofs at eaves and verges (where metal or plastic cappings and trims shall not be used)’.
This effectively banned the use of traditional verge trims, top trims and all other metal or plastic cappings to protect the top of EWI systems where there is insufficient existing roof overhang at eaves and verges. This change in the PAS had been brought about as a result of a series of failures associated with traditional verge trims (and other eaves and verge profiles). Problems included water penetration and thermal bridging, often as a result of poor installation and / or design.
As a result of this new line in Table B.4, Retrofit Designers were compelled to incorporate roofline extensions into their Retrofit Designs, a process which introduced significant additional cost and complexity. In some cases – e.g. mid-terrace properties – extending a roof in isolation is impossible or, at the very least, greatly impractical. It can create problems with aesthetics, planning consents, rainwater drainage and other issues. These challenges have led some clients to remove such properties from their programmes, leading to delays in the introduction of badly needed retrofit insulation measures.
Consequently, the solid wall insulation industry has sought to work with government, BSI, The Retrofit Standards Task Group and Trustmark to develop new, robust, practical solutions that will enable a reintroduction of the use of metal profiles and trims at roof level, albeit in a strictly controlled fashion.
Proposal
PermaRock’s Technical Director, Dr. Jeremy Richings is an active member of the Working Group formed jointly by the National Insulation Association (NIA), the Solid Wall Insulation Guarantee Agency (SWIGA), and the Insulated Render and Cladding Association (INCA). The group has been working closely and openly with RSTG, Trustmark, BSI and government. Describing the sector’s response, he said:
“Historically, the use of metal verge trims has not been inherently problematic. However, there have been several high-profile projects where water ingress into EWI systems has led to dampness inside of houses. This has been found to be associated with poorly designed or improperly installed metal cappings. As a result, the ban on the use of such metal (and plastic) profiles was incorporated into PAS2035.
“The external wall insulation industry has expressed the view that metal profiles can be employed successfully if the way that they’re used is changed. The key is working to effective and practical design principles that call for multiple layers of weathering protection, and which address potential thermal bridging issues. In addition, these principles require effective quality control during installation by trained operatives, and the use of good quality materials that meet minimum specification standards.
“A Working Group of EWI system designers, product manufacturers, guarantee providers and specialist EWI contractors came together to develop a set of design principles; principles that would ensure the safe and robust use of metal profiles to protect the top of EWI systems at roofline interfaces. This group represents the breadth of the UK solid wall insulation industry, and is coordinated by the Solid Wall Insulation Guarantee Agency (SWIGA), which effectively chaired the group.
“The design principles are intended to provide flexibility in design and detailing. They call for measures to mitigate against thermal bridging, and for multiple levels of water ingress prevention, and they set out minimum specification requirements for materials and their use. In addition, they require that installation is undertaken by trained installers.
“The Working Group has adopted the term “roofline closure systems” to replace the former term, “verge trims”, to represent this new approach to weathering and thermal bridge mitigation at eaves and verges, etc.”
Current Status
“As of the beginning of January 2024, we are still waiting for these proposals to be signed off by government and Trustmark. It’s fair to say that everyone who needs to be involved in making the final decision has already reviewed the proposals, but that final step has not yet been taken.
“This means that any talk of an official approval decision is, as yet, premature. It’s still possible that Trustmark or the British Standards Institute could come back to the Working Group with queries or calls for clarifications, so we aren’t there yet. However, a great deal of work has gone into this and progress has been good.
“The Working Group established an initial set of Design Principles which formed the basis for a series of in-situ trials at the end of 2022, running through to autumn of 2023. The data from these initial case studies were used to refine the Design Principles, which have been augmented by a series of detail drawings. These show possible approaches to the use of verge trims, in combination with thermal bridge mitigation measures and secondary membranes to enhance weathering resistance capabilities.
“The entire process will require an initial survey of the existing roofline to determine if one of the ‘standard’ details in the Weathering Guide is suitable. If not, a bespoke solution (that meets the requirements of the Design Principles) can be developed with the system provider and Retrofit Designer prior to the works being undertaken.
“The Design Principles also include a requirement that photographic evidence of key stages of the installation of Roofline Closure Systems be prepared and retained for the period of the guarantee. This evidence will include close-up photographs of representative examples of all moisture and thermally sensitive details – thus being compliant with PAS 2035 requirements.
“The Design Principles and drawings have been incorporated into a new version (not yet released) of the document ‘External wall External Wall Insulation Specification for Weathering and Thermal Bridge Control’ which is referenced in PAS2035.
“The reaction from government, BSI and RSTG has been good – generally supportive – and the three industry bodies, NIA, SWIGA and INCA have all put their weight behind it. SWIGA’s Executive Director, Andrew Champ, has been a key figure in this campaign – not only instigating the setting up of the Working Group – but also being very active in maintaining a productive dialogue with RSTG, Trustmark, BSI and governmental departments.”
SWIGA’s View
Speaking on behalf of SWIGA, Andrew Champ said:
“The project has been challenging and taken longer than expected. It is clear, though, that the site trials and feedback from these proved critical in determining the conclusions drawn, which guided us to the robust practical detailing and guiding principles. The Working Group should be commended for the commitment and support with expertise during the length of the project.
“I believe we have met the original objectives of developing robust and practical solutions for roofline closures that reduce the need for maintenance, and await the final approval, allowing the group to undertake the industry communication.”
Summary
There are, as yet, no official PAS 2035 guidelines regarding the use of roofline closure systems, and nor is there yet any approved set of design principles. The updated version of ‘External wall External Wall Insulation Specification for Weathering and Thermal Bridge Control’ has not been formally released.
These are still being worked out, though recent feedback to the industry Working Group suggests that an official response can be expected shortly.
We will, of course, alert our clients, supply chain partners and online followers as soon as any formal change is announced.
Pictured: Roofline closure profiles, used as part of in-situ trials. Note that these trials began in 2022 and are currently being evaluated, so the product shown does not necessarily depict a final or approved design.