Owned and operated by the British Film Institute, the BFI Southbank is a very popular venue and the UK’s leading destination for film lovers from across London and beyond. The building sits directly underneath Waterloo Bridge; a Grade II* listed heritage structure, but extends to both sides, where roof areas, totalling 350m2, needed refurbishment. 

The roof areas are composed of 38 raised GRP decks with a rooflight in each section and an expansion joint between sections, with vented vertical surfaces and metal decked gulleys that also needed refurbishment. Only a wall approximately 1m high separates the gulleys and the roof areas from the bridge pavements.

The challenge was to specify a roof covering that could provide a high level of waterproofing performance, along with a neat finish, while addressing the idiosyncrasies of the unique roof design and overcoming the operational challenges of refurbishing the roof so close to vehicular and foot traffic. The ARBOFLEX® PU cold-applied liquid waterproofing membrane from CARLISLE® was chosen as an ideal system to address these challenges, installed by Martins Roofing Contractors Limited. 

GRP sections

The BFI’s in-house facilities team stipulated that a cold- applied liquid roofing system should be used for the project, and, having used the ARBOFLEX®️ PU waterproofing system on multiple previous projects, Martins Roofing recommended the CARLISLE liquid system.

All the roof sections had to be thoroughly cleaned using CARLISLE’s G500 cleaner, and the prepared roof sections were then regularly cleaned with an acetone wash as the project progressed, due to constant fouling in the busy central London location. 

Each of the GRP roof sections have raised channels a few millimetres high running from side to side across the thin strip of roof, creating a continuous pattern of horizontal lines from one end of the roof to the other, parallel to each side of the bridge. Because of the undulations in the roof sections, it was not possible to include the matting usually installed as part of the ARBOFLEX PU system in the specification, because this would not have delivered a smooth and seamless result. Instead, two additional coats of liquid waterproofing were applied, with each coat allowed to cure prior to application of the next, forming a robust waterproofing layer. The U/V topcoat was then applied to the cured surface to provide a fully-warrantied 20 year system.

Installation of the waterproofing system had to be co-ordinated with replacement of the rooflight units, and a cure accelerator was added to the ARBOFLEX PU liquid system for the sections around the rooflight apertures so that each new unit could be fitted within an hour of the waterproofing being completed.

For the vented vertical sections, the ARBOFLEX PU system was applied with the correct ratio of thickener, providing suitable viscosity and adhesion to enable the cold-applied liquid membrane to be used to waterproof these roof areas and provide consistency across all aspects of the GRP roof.

Expansion joints

The expansion joints between the GRP roof sections needed to be stripped and replaced and Arbomeric MP20 sealant from ARBO, part of CARLISLE, was specified for these joints. A single-part, high modulus modified polymer sealant, MP20 cures to form a tough but elastic rubber, providing excellent adhesion and durability in demanding construction applications such as this.

The sealant was installed at each expansion joint and colour matched to the grey liquid waterproofing, with a conventional ARBOFLEX PU installation of cold-applied liquid waterproofing, matting and U/V topcoat applied onto the cured sealant.

Gulleys

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The profiled metal deck gulleys had to be stripped back to the original substrate and cleaned and primed, with a vapour control layer laid onto the prepared deck, followed by a fully-adhered, non-combustible mineral wool insulation. A further application of primer and a carrier membrane were then installed, followed by the liquid waterproofing system.

One of the challenges of refurbishing the gulley areas was the stipulation that no modifications could be made to the fixings, due to structural implications and the listed status of the bridge. To overcome this, the Martins Roofing team terminated each side of the gulley with a termination bar and metal flashings, which had to be adapted to fit the existing bridge structure.

Operational and logistical challenges

Around 45,000 vehicles are driven across Waterloo Bridge every day, and the bridge is also in constant use from pedestrians, with commuters walking across it from around 6am every morning, and tourist footfall all day long. To work around this, the Martins Roofing team arrived on site at 5.30am each morning offloading materials before the bridge got busy, and temporary fencing was erected to provide additional separation between the site and the adjacent bridge pavement.

The BFI building remained fully operational throughout the project and space restrictions in front of the building meant that it was not possible to erect a scaffold, so instead, the Martins Roofing team relied on MEWPs and a man safe system for accessing some areas of the roof and managing health and safety.

Duncan McKeich, head of facilities at the BFI commented: “BFI Southbank is a challenging building, being mostly directly underneath Waterloo Bridge with no separate roof, except for this GRP roof built in the mid-1980s.  

“The solution allowed us to carry on with our normal operations below with very limited disruption and gives us confidence in the roof again, which is especially important as we have public space and exhibitions below.”