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Roof and wall cladding case study – Marks & Spencer – ProLogis Bradford Project Lima

10 May 2010 No Comment

Colorcoat® was specified for Marks and Spencer retail distribution shed, the largest in the UK with 124,000 m2 of building envelope.

Colorcoat HPS200 Ultra® in Goosewing Grey was used for the roof, supplied with supply chain partner CA Group’s RiverTherm® system. Colorcoat Prisma® in Silver Metallic, Grey Aluminium and Slate Grey was specified for walls.

Aiming for an excellent BREEAM rating on the project Colorcoat® pre-finished steel is 100% recyclable and made in the UK for a lower carbon footprint.

Over 3 million kg of unavoidable CO2 emissions from the project have been offset in audited climate friendly projects overseas with Confidex Sustain®,  the worlds first CarbonNeutral envelope solution.

This is no ordinary roof; this is a CA River-Therm® roof!

For almost two years, CA’s technical department has been working alongside ProLogis, a leading global provider of distribution facilities and its architectural team to deliver a cost effective solution for Marks & Spencer’s new distribution warehouse in Bradford, Yorkshire.

The project involves a roof size of 514 metres long, consisting of 1.5 m radius eaves-to-eaves, with individual lengths of CA’s River-Therm® secret fix roofing sheets of 178 metres. Previously, the longest ever roofing sheets to be roll-formed on site were 118 metres long, so this job is being heralded as truly ground-breaking.

Stuart Brown, CA Building Products technical services, explains the challenges faced when designing buildings of this scale:

“Often, huge buildings of this type need to be of a certain height at eaves level to accommodate the tall racking systems inside, but cannot be significantly higher at the apex of the roof, owing to stringent planning restrictions. This means the architects design a vast expanse of roof that is as close to flat as possible.”

“In the case of this new build, owing to the flatness of the roof only a standing seam or secret fix roof design was a viable option.”

“Due to the sheer scale of this building, the radius of the curve is 1.5km, which means there is a 20-30m wide area at the apex which is almost flat because the curve is so shallow. The risk the specifier faces is that rainwater run-off will take approximately 20 minutes to drain from this area into the gutter, so the roofing material must ensure that the exposure to wind and rain over such a large surface area would not risk water ingress into the building.”

In facing this challenge, the client, architects and specifier had to focus particularly careful consideration on three areas of the roof before they could choose which standing seam roof product to use. These areas were 25 metres either side of the apex, the central section of roof on either side of the apex, and 30 to 35 metres from the eaves.

In order to prevent rainwater ponding occurring at the apex, coordination between the steelwork contractor, architect and roof supplier is critical. It was found that the main cause for concern would be the area 30 to 35 metres from the eaves, where investigation showed that during storm conditions the wind would have the potential to dam the water flowing from the roof into the gutter, allowing ponding to the extent that water levels could rise above the level of the roof profile.

This was a cause for concern for a major reason: one of the key benefits of all standing seam or secret fix roofing products is that the seam is breathable. This, combined with the risk of ponding rainwater rising above the profile, presented a potential risk of water ingress into the building. If the profile were to be submerged it would form a path through which water could flow into the roof assembly.

River-Therm®

The River-Therm® eliminates this risk with its engineered side lap incorporating a built-in drainage channel, so that even in extreme cases where heavy, wind-driven rain causes water levels that overflow the top of the profile, any water ingress is channelled away to the eaves edge and into guttering.

Stuart Brown continues, “History has proved that it is always advisable to have a positive fall on any roof to ensure that water flows quickly from the roof into the rainwater management system and does not enter the building.  The use of secret fix solutions, such as the River-Therm®, will provide security ensuring that this doesn’t occur. However, on a roof of this scale, it is critical that the entire construction team is aware of what has to be achieved and delivered.

“Most standing seam roofing systems would offer insufficient rainwater protection on a roof of this scale. Even when they are roll-formed on site as River-Therm® is – to offer unbroken sheet coverage over the width of the building – there is still the issue of unsealed or vented seams between sheets along the building’s length. A traditional standing seam could, if inundated by ponding rainwater, potentially leak straight inside of the building, unless it had River-Therm®’s patented drainage channel designed into the joint.

“We believe, therefore, that River-Therm® is the only metal roof system available on the market today that could meet these architects’ requirements.”

To demonstrate the unique design of River-Therm® and to prove how it would operate in storm conditions, CA invited the ProLogis architectural team to Verona, Italy, where an 80 metre long purpose built test rig is permanently set up to illustrate this issue and to provide visual reassurance of how the self-draining system works, under extreme conditions.  A separate demonstration unit resembles a swimming pool with the bottom made of a life-sized River-Therm® installation. When the pool is full, not a single drop appears on the underside of the River-Therm® sheets.

As well as solving the rainwater issue, River-Therm® adds several other benefits to projects of this nature. When installed, unlike virtually any other standing seam roof design, a River-Therm® roof is strong and safe enough to be fully walkable, without any need for specific walkways or high density insulation.

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