As specifiers and developers strive to achieve faster build times, with less site waste, lower emissions and simpler site logistics, the term 'offsite' has quickly become synonymous with 'Modern Methods of Construction'. Yet as David Lowe, Technical Director of SpeedDeck Building Systems explains, there is still a place in today's fast-moving world for ingenious onsite manufacturing solutions and such methods should not always be overlooked.
Offsite solutions to architectural challenges are nearly always delivered by product manufacturers who have responded to the end user demand for reducing the amount of time spent on site, easing the construction process, improving site safety and profit margins. However, not all roofing challenges have a simple offsite solution that can deliver all these elements.
The design and construction of Tesco's inaugural 'dot com' warehouse at Priory Park in Aylesford, near Maidstone in Kent, is a prime example of an instance where offsite methods would not suffice. Roofing and cladding contractor, Arrico Roofing Ltd, was faced with a very unique challenge when presented with specialist distribution and logistics architects, Ashton Smith Associates' design for the roof of the new building.
Arrico Director, Gavin Harriman explains, "Due to the complexity of the site and because the SpeedDeck secret fix roof sheets had to be 90m long, delivery of the finished product from off site was impossible and on site manufacture was the only option to consider."
This conclusion was borne out of a unique set of complications on the Priory Park site where Tesco's new distribution centre is located. Covering an area of almost 11,000sqm, the building is 120m long and 90m wide. Its roof is 11m above the ground, but due to the location and design of the building, Arrico had to engineer a solution to get the 90m long sheets to roof level. "The building is located on a corner plot, so only one of the long sides was accessible. This was complicated still further by a canopy that projects 9m and runs the whole length of the building," said Gavin.
Although there were three possible options it became clear, by the nature of the site, that only one was going to be viable. "There was no room to ramp roll the coils, due to the presence of the canopy and the lack of ground space available on the site," said Gavin, "Another alternative considered was to use a lifting beam to get the sheets up to roof height, but no beam would have been long enough to accommodate the 90m sheets. The 90m single length roof sheets were the only option due to the barrel vault roof design.
The design of the roof was something both SpeedDeck and Arrico had to work with. Due to the nature of the location, Ashton Smith Associates had to design the building with a gradually curving barrel-vaulted roof in order to ensure it remained in keeping with the surrounding buildings. With a pitch of just 4¢ª at the eaves and a smoothly curving vault with a 640m radius, there could be no risk of end laps or visible fixings to compromise the weather-tightness of the roof.
Arrico's solution came in the form of onsite manufacture from SpeedDeck Building Systems. "We knew SpeedDeck had the ability to roll its products at eaves height," explains Gavin, "So we placed the order with them before the technical team even saw the site!"
Following a comprehensive onsite risk assessment, the SpeedDeck solution involved a 160 tonne crane located 10m away from the building to suspend the onsite rolling machine, thus avoiding the incumbent canopy. This enabled the 250 sheets of SpeedDeck in 0.6mm Corus Colorcoat HPS200® Moorland Green steel to be rolled directly onto the roof at eaves level. Working with at least eight to ten men on the roof at one time, it took just four days for the roof to be rolled from the manufacturing unit with the crane working from only three positions along the length of the building. The entire installation including 180mm Glass Quilt Insulation and 0.7mm Steel Walkliner was fully complete in just two weeks.
The resulting roof achieves a 'U¡' value = 0.25W/m²K, gives a sound reduction of Rw = 45dB and has Corus Confidex guarantee cover for 30 years.While the complexity of the Priory Park site rendered all other options virtually impossible, it was the ingenuity and ability to adapt the SpeedDeck onsite rolling at eaves height to the Tesco Priory Park situation and to keep the architectural specification that led Arrico to turn to an onsite manufacturing solution. Whilst many onsite efficiencies are achieved today by building components being manufactured offsite, onsite rolling can provide the answer to this sort of challenge.
