Andy Wells, National Supply Chain Manager at SIG Design and Technology, in conversation with Architecture Today’s Technical Editor John Ramshaw, explores best practice design for a complex, large-scale membrane roof overlay project at the International Electric Maintenance Depot in Crewe.

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The building’s original built-up felt covering has been overlaid with an Armourplan PSG Fleece Backed PVC membrane from IKO (ph: Michael Cameron)

DB Cargo UK’s International Electric Maintenance Depot in Crewe, Cheshire, provides essential maintenance and repair facilities for electric train engines. The 2,880-square-metre building has been in continuous use since 1960, and unsurprisingly, the roof had started to leak badly as the original built-up felt covering degraded over time. The solution was to overlay the felt with an Armourplan PSG Fleece Backed PVC membrane from IKO.

But with more than 40 per cent of the roof comprising glass rooflights, together with the close proximity of live high-voltage cables, and the need to keep the building operational throughout the works, the scheme was far from simple. Andy Wells, National Supply Chain Manager at SIG Design and Technology discusses the roof design and specification, as well as how the main technical challenges were overcome, with Architecture Today’s Technical Editor John Ramshaw.


The depot has been in continuous use since 1960 (ph: Michael Cameron)

How did the project come about and why was an overlay solution the preferred choice?
The client had approached several contractors asking “is it fixable?”, and the only one to return an answer that was both practical and within a reasonable budget was Elgin-based GL Contracts. The roof structure is a trapezoidal steel deck, which is still in very good condition, and the contractor concluded, that given the constraints of the project, an overlay of the existing roof was the best solution. Working with the support of SIG Design and Technology, GL Contracts designed a bespoke specification using IKO’s Armourplan PSG Fleece Backed PVC membrane.

The need to keep the building functional was a big factor in electing to use an overlay system, as it eliminated debris internally and meant there was no risk to people working in the area below. Time was another constraint and design driver. The contractor originally worked on an 18-month programme allowing for a step-by-step phased approach, and for time to trial different solutions. The client, however, needed the project to be completed within just eight months, so the team proposed a single overlay solution and stuck with it to complete the project on time.


A combination of under flashings and standard drip flashings were successfully used around the existing rooflights (ph: Michael Cameron)

What is the roof build-up?
The existing decayed roofing has been overlaid with heavy-duty 18mm OSB boards, which are screw-fixed through the existing roof and into the steel subframe (the fixing details were provided by the SIG technical team). Bonded to the OSB boards using Armourplan Spectrabond Low Foaming PU adhesive is the new fleece-backed membrane covering. The membrane itself is gas-permeable, so any moisture remaining on the original roof build-up will evaporate over time (surface temperatures on the roof can reach more than 60 degrees centigrade in the Summer).

There were limited options for this installation. For example, the finished build-up could not be much more than 20mm thick to integrate effectively with the rooflights, and the detailing around these aging areas of glass needed to be designed specifically for the project.

So how did the team approach the detailing around the rooflights?
Cautiously! Like the felt roof, the glazing is also original and causing its own water ingress problems. Sadly there was no budget to replace all the glazing, so the roofing team had to figure out a way to provide a watertight seal around the rooflights without damaging the glass.

The solution was to use an under flashing for the top details, i.e. sliding the flashing up under the glazing. For the bottom details running onto the next run of glazing, a standard drip flashing was used. This is reinforced with a galvanised bracket that is mechanically-fixed over the Armourplan membrane. A capping strip is then welded over the top of the joint so that the whole system is sealed.

It was hugely fortunate that the space between the rooflights was the same width as the OSB sheets, minimising the cutting needed on site. The Armourplan, provided in 2.1-metre widths, also fitted neatly into the grid pattern of the roof, minimising the number of hot air welds required for seams.


SIG provided full materials warranties for the project (ph: Michael Cameron)

How was the roof installed and were the main issues on site?
Installing the roofing system involved repeating the same complex detailing again and again along the 90-metre-long runs. Cranes were not an option because of the electrical cables around the perimeter, so all the equipment had to hoisted onto the roof using a scissor lift. Without perimeter catch nets or crash mats under the roof, a mansafe system, consisting of horizontal safety cables to which all contractors fixed their own safety lines, was used to ensure the safety of the operatives.

A team of four contractors worked continually for eight months to complete the project. It was a small team, but that focus ensured that everyone involved was completely familiar with the solutions designed for the project. This was a pragmatic but very effective solution to a complex problem.

Overall, the installation was completed quickly on a site that posed huge challenges in terms of time frame and access restrictions. With full materials warranties provided by SIG, the contractor is highly confident that this robust renovation will provide effective waterproofing for decades to come.

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