Architect Robert Griffiths of AWW, and Terry Fearfield, Specification Manager at SIGA Natural Slate discuss best practice design for slate roofing at Brandon Yard in Bristol, with Architecture Today’s Technical Editor John Ramshaw
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Located on Bristol harbourside, Brandon Yard is an award-winning regeneration project comprising 58 one, two and three-bedroom apartments. Designed by AWW, the £14m scheme occupies a site that had been derelict for 40 years and contained two ruined grade-two listed buildings: West Purifier House and Engine House. Forming a key part of the successful restoration of these structures has been the design and installation of new high-quality slate roofs supplied by SIGA Natural Slate. Architect Robert Griffiths, and Terry Fearfield of SIGA Natural Slate, in discussion with Architecture Today’s Technical Editor John Ramshaw, explain what type of slate was chosen and why, the importance of slate selection, and how the detail design and installation stages were developed and implemented.
The new SIGA 39 slate roof from SIGA Natural Slate incorporates a large zinc-clad dormer
Why was slate roofing chosen as part of the refurbishment of West Purifer House and Engine House?
RG: Slate was chosen for conservation reasons and because it fulfils the original design concept of sympathetically restoring the listed masonry structures in line with East Purifier House (also designed by AWW and located adjacent to Brandon Yard). Added to this, slate’s robustness, durability and longevity are ideally suited to tough marine environments, such as Bristol Harbour.
Why did you choose SIGA slates over other brands?
RG: SIGA slates were chosen for their consistent high-quality finish and aesthetic appeal. It was important for our client and us to satisfy the project’s conservation requirements, while also having confidence in the material’s long-term performance.
The SIGA 39 slates installed on Engine House come with a 75-year warranty
How did the slate selection process work? And what aspects were important in terms of performance and aesthetics?
RG: The selection process began by examining the local area and matching as closely as possible the slate roof of East Purifier House. It was paramount that the slate should complement the development’s material palette, as well as the wider context. In addition to the two historical buildings, a new-build structure completes the development, and the slate needed to be a blue-grey colour to tie-in with the other finishes and fenestration.
Which SIGA slate product was specified and why?
RG: SIGA 39 from SIGA Natural Slate’s Excellence range was chosen for its flatness, uniformity and consistency of thickness. The high-performance S1 (exposure), T1 (thermal cycling) and W1 (water absorption)-rated slates also require minimal sorting – facilitating quicker installation – and come with a 75-year warranty. The slate’s blue-grey appearance and physical attributes also allowed it to harmonise with both the new-build and historical elements of the scheme.
How did you approach the detail design of the slate roofs?
RG: The project was procured using a Design & Build contract. AWW worked closely with SIGA Natural Slate to produce a comprehensive tender package, which was key to ensuring that the correct slates and fixings were selected and carried forward to the construction phase. The site’s harsh marine conditions made this process even more critical than usual. Post-tender, AWW collaborated with the main contactor to ensure that the various slate detail interfaces were designed to a high standard. We provided typical design intent details for the roof and interface conditions, while the main contractor managed the detail development with the roofing subcontractor.
West Purifier existing gable wall to zinc dormer slate roof design intent detail (AWW)
What were the main technical challenges and how were these successfully overcome?
RG: The main technical challenges were detail-based and related to the provision of ventilation, weatherproofing and integration with the historic walls. For example, Purifier House incorporates two types of gable wall – one being original and the other partly reconstructed. The constructional build-up of the slate had to integrate fully with the lead detailing of each gable wall, as well as a new zinc-clad dormer. In light of the site’s exposed nature, the specification of the slate headlap and fixings were key to ensuring minimal reliance on the membrane beneath the tiles. An increased headlap distance, extra large clout head nails and pre-holed slates were all selected to ensure robust detailing.
What is the material build-up of the slate roofs?
RG: The roofs employ a traditional build-up comprising rigid insulation between the timber rafters, a layer of rigid insulation above the trusses, then a breathable sarking membrane followed by counter-battens and nail-fixed slates.
Is the slate warranted, and if so what is the nature of this?
TF: SIGA 39 is a first selection product, which means that it is the flattest, smoothest, and highest-quality slate available. As such, it comes with a 75-year material warranty.
Slate roof under construction (ph: SIGA Natural Slate)
How were the slates installed on site, how was quality maintained, and how long did it take?
TF: SIGA Natural Slate’s Design & Technical Team provided an NBS full-slate specification, which was closely adhered to by Camilleri Roofing during the installation process. The slate installation is very traditional – in keeping with the buildings’ historic status – with the only exception being a maintenance-free, dry-fix ridge. Camilleri took extra steps to ensure consistency, sorting the slates for thickness and smoothness, and graduating thickness up the roof. This has resulted in a uniform, flat appearance, without any large gaps or slates kicking up. In total, the slates took around 4-5 weeks to install.
For more information please visit the SIGA Natural Slate website.