Nigel Fields, Midlands Area Field Technician at SIG Design and Technology, discusses best practice for a technically challenging single ply roof refurbishment at Chester Zoo with Architecture Today’s Technical Editor John Ramshaw.

Prior to the roof refurbishment scheme, June’s Food Court had been closed due to water ingress (ph: courtesy of Chester Zoo)

Chester Zoo at Upton-by-Chester in Cheshire is one of the UK’s largest zoos housing more than 35,000 animals over a 51 hectare site. The attraction is visited by around two million people per annum, with many choosing to eat at June’s Food Court – a pavilion-like structure whose unusual roof comprises nine hexagonal peaks linked by seven flat areas. A major roof refurbishment scheme undertaken by SIG Design and Technology, in association with flat roofing specialist Tapered Plus, and roofing contractor Lester Cladding, has allowed the restaurant to successfully reopen after a period of closure caused by water ingress.

Central to the project has been a robust yet technically complex cut-to-falls design utilising an Armourplan P roofing membrane from IKO and polyisocyanurate TR/MG insulation from Xtratherm. Nigel Fields, Midlands Area Field Technician at SIG Design and Technology, in conversation with Architecture Today’s Technical Editor John Ramshaw, explores the main design challenges, and how the roof specification process was developed and implemented.

What was main problem with the existing roof?
It’s generally accepted that areas where roofing membranes join are more vulnerable to deterioration and eventually water ingress, if they are not properly maintained. While beautiful and visually interesting, the design of the June’s Food Court roof meant that there were many vulnerable areas, some of which had been leaking for years. This made it necessary to renew the entire build-up. The roof, with its trapezoidal features is hugely complex, and this is without doubt the most complicated cut-to-falls scheme I have come across in my 35-year career!

The pavilion roof is made up of nine hexagonal peaks linked by seven flat areas (ph: Property Photographix)

How did SIG approach the design process?
The renewal of the roof involved an extremely intricate design process, requiring the expertise and involvement of SIG D&T as technical product experts, Tapered Plus as experts in designing cut-to-falls schemes, and the contractor Lester Cladding. Collaborative thinking, joint site visits and constant communication throughout the project were essential.

The design process began with an initial site visit, where SIG D&T technical advisors discussed the project with the client and the contractor, assessed the condition of the roof, and decided what should be done. Meanwhile, Tapered Plus measured the roof area and ascertained the exact position of the existing outlets. The company then produced a series of detailed technical drawings illustrating this; ensuring clear communication from the start.

An additional complication was that the first site surveys were carried out during Covid-19 lockdown, and all the companies involved had to ensure compliance with social distancing and the wearing of appropriate PPE. While the lockdown provided an opportunity to work on the building during its forced closure to the public, there was nevertheless a tight timescale to contend with as the zoo needed to be ready to open at short notice when the government guidelines allowed.

What is the new roof covering and why was this specified?
The two main products chosen for the new roof were IKO Polymeric’s Armourplan P roofing membrane and Xtratherm TR/MG insulation. The latter is a high- performance polyisocyanurate with mineral coated glass facers. It’s suitable for use below single ply waterproofing systems and is compatible with partially bonded built-up felt and many liquid waterproofing systems. Furthermore, the insulation boards are easy to handle, cut, and secure with insulation adhesive or mechanical fixings.

Armourplan P single ply PVC roofing was chosen for its flexible and lightweight properties, high UV-resistance and secure seam welding quality – essential for this type of project, exceptional mechanical performance, long service life, and sleek finish.

The new roof build-up combines an Armourplan P roofing membrane from IKO with polyisocyanurate TR/MG insulation from Xtratherm (ph: Property Photographix)

What fixing method did you use and why?
The roof was mechanically fixed with steel screws from fixing specialist Eurofast, employed as part of the IKO roofing system. The build-up comprises a vapour control layer, followed by insulation, and then fixed through with a thermal break tube and screws.

There were a number of reasons for specifying the Armourplan P membrane with mechanical fixings as opposed to the fully adhered system or a cold-applied liquid approach. First, the project was carried out during the winter and we didn’t want to run the risk of weather-related project delays – a potential factor when using adhesive or liquid systems. This was a particular issue on the perimeter upstands where the choice to use mechanical fixings was a priority.

Second, the Armourplan P membrane is fully hot air weldable (there’s no fleece layer) allowing the product to be applied to the laminated metal flashings along all perimeter edges, and at changes of angles within the roof area. The use of this material also means that cut sheets of membrane can be re-used, significantly reducing site waste.

Lester Cladding used its in-house fabrication workshop to manufacture the bespoke detailed flashings required to complete and fit the project to an impressive standard.

The roof is mechanically fixed with hot air welded joints (ph: Property Photographix)

Were there any other challenges to overcome?
Yes. One of the key challenges was achieving the correct U-Value for the new roof. This was important in terms of both energy efficiency and customer comfort in the restaurant. The previous roof design, it transpired, included very little insulation. Coupled with the deterioration of the roofing membrane, this meant that it did not meet current U-value requirements.

SIG D&T advised on the correct insulation and the amount needed to meet a U-Value of 0.18W/m²K, which is required by the updated Building Regulations Part L2 (published in 2021). The new roof has significantly improved the energy efficiency of the food court in line with the zoo’s overall energy reduction programme.

Contact Details
For more information, please visit the SIG Design and Technology website.