The Met Bar at The Metropolitan Hotel, is one of London's premier celebrity hang outs. They have very high standards for entry...'members' only!

The ten storey Metropolitan Hotel stands on its prominent site with stunning views across Hyde Park. The refurbishment of the old Londonderry Hotel has given birth to an ultra modern world class hotel that pays great attention to detail.

There were many critical decisions made when the flat roof area was redesigned. As a world leading brand, the STYROFOAM™ Solution, ROOFMATE™ LG-X rigid insulation board with integral ballast from Dow Construction Products was specified.

The first design decision was to retain the existing waterproof membrane (as is often the case with a refurbishment). This was performing well and the programme did not allow for its removal and relaying.

An inverted or 'upside down' roof (USD) was the only feasible solution. Dow developed this design over 20 years ago. There were key benefits with this choice. There were to be high levels of plant on the roof for the new air conditioning requirements of such a high quality hotel. In turn, these areas would require frequent access by maintenance personnel. A USD roof would protect the waterproof membrane from damage and give a defined area as walkway. In addition, the membrane would not be exposed to UV degradation and high temperature differences that can be experienced within each day and throughout the year.

ROOFMATE LG-X was chosen because the 900m² roof could accommodate an additional load of 25kg/m2. In this case, paviors were added as additional ballast around the perimeter to counter the effect of wind uplift. Wind uplift calculations are critical for a USD roof design. At Dow, this service is an integral part of the consultancy offered to designers and specifiers. The geographical location of the site, the building aspect and height are all key factors in the design that will dictate the ballast, the bonding and the pattern for the interlocking ROOFMATE LG-X boards. The contractors, London & Home Counties Roofing, were very familiar with the benefits of USD roof applications. The main contractor Komojo, also had experience of this STYROFOAM Solution.

The short programme and a congested site - with all trades working with cross over times - meant product delivery times were critical as goods hoist times were strictly rationed. The distributor Solent Insulation Supplies London, worked closely with the installer to ensure timely delivery. The comment from Alan Richardson, London & Home Counties Roofing, summarises the views of his company, "Using ROOFMATE LG-X was more expensive than an inverted system ballasted with paving slabs or gravel. But on a ten storey building the cost of lifting materials up to the roof would have been very high anyway, so it was economical. The ease of site handling was a real benefit to my team who had to work around the M&E contractor and every other trade under the sun that seemed to be up there with us!"

There were other site specific issues that needed to be addressed. The vast expanses of glazing to the suites on the top floors needed to be balanced with high levels of insulation. The thermal conductivity for ROOFMATE LG-X is 0.028W/mk. The specification of 100mm of insulation was close to the limit for the height of the parapet walls to the roof perimeter (see site photographs). If traditional ballast was employed the parapet would have needed to be raised. By using ROOFMATE LG-X, no alterations to the existing roof were necessary. The ease with which the material was handled on site was a bonus, normal hand tools were all that were required for trimming and cutting boards around plant penetrations, skylights and upstands, plus all handrails and walkway balustrades. The boards were even carried by hand to the roof in the hotel goods lift!

1. Ponding water was a problem with the original flat roof
2. The mortar topping provides gives a suitable walkway for maintenance foot traffic
3. The easy to cut feature of the board enables accurate detailing around plant projections and walkway upstands
4. The low parapet wall dictated the maximum height for the new inverted roof. Note the new level and detailing around the balustrade.

Insulating inverted roofs
ROOFMATE LG-X