Commercial buildings have a history, even if it is only 50 years. The government’s Non-Domestic Building Stock project (NDBS), which is creating the first record of non-domestic building stock in the UK, has identified that nearly half of UK office buildings were built before 1985[i].
The energy performance of commercial buildings is closely linked to their age, and as the UK works to meet its net zero commitments, many of these buildings will need to be upgraded.
As a building owner or construction professional looking to refurbish your commercial building, you may be expecting the “as built” drawings to show you what you are working with, but this could be a risky endeavour. In this article we use the example of an iconic 1970s office building refurbishment to show how a roofing specialist like AccuRoof can help you understand the building you have now, and help you develop a workable approach to upgrading the building for the coming decades.
About the building
Lloyds Banking Group’s Trinity Square Head Office was designed by BDP and completed in 1974 as the headquarters of the Halifax building society. It is generally considered a remarkable building for its period and described by BDP as ‘one of the most architecturally influential buildings of the 20th century’. The building was Grade II listed in 2013.
Designed along Burolandschaft principles, the main office space is raised above the surrounding buildings on four huge York stone clad legs which protrude through the diamond-shaped mass of the building and form part of a complex roofscape. Alongside the main roof and a central service core, there are two plant areas and four raised octagonal roofs over director’s suites looking onto sunken courtyards.
AccuRoof was asked to carry out a survey of this roof in May 2023 as part of an overall refurbishment of the building by Wates Living Space. The brief for the roof refurbishment included addressing the reinforced autoclaved aerated concrete (RAAC) roof deck which had been identified on the architects’ drawings, to install new areas of roof glazing and to raise the insulation values of the roof to bring it up to current regulations.
What the flat roof survey found
Our initial flat roof survey identified:
The original asphalt roof had been repaired with a liquid roofing product which had blistered and failed.
Large amounts of ponding water suggesting inadequate roof falls in several areas. In some places the drainage system was inadequate, leading to water cascading over copings and cladding.
Damage to the existing roofing system, including the liquid roofing overlay:
Areas of saturated PIR insulation and water below the surface confirmed the failure of the waterproofing.
Whilst most of the roof deck appeared to be RAAC, the plant areas had been roofed with a metal deck. This deck was in good condition but had been laid to no or minimal falls. The area had single ply roofing (as opposed to asphalt), which had also been overlaid with liquid which had also failed.
The survey also identified a range of features that would need to be adjusted to allow for increased insulation including interfaces with the York stone cladding and copings, door openings and rooflights, drainage features, plant and safety systems. Some doors had been painted shut during the liquid roof repairs.
Core samples suggested that on much of the roof area the deck was RAAC, but underneath there was an unexpected void which was not shown on the as built drawings. Meanwhile, inspection of the roof below suggested that the deck was conventional reinforced concrete and not RAAC.
Following the initial roofing survey a telescopic camera inspection was carried out to further investigate the void below the main roof areas. This identified that the RAAC was laid over sleeper walls onto a concrete slab below. These sleeper walls were not on the as built drawings.
The void was 600mm deep, giving the AccuRoof design team the opportunity to address both the inadequate pitch in the roof and the need for additional depth for insulation.
Roof refurbishment design
AccuRoof’s comprehensive survey had identified an opportunity to remove the RAAC deck and create a new roof on the concrete slab below. However, with a tight programme, there would be insufficient time to use conventional wet screed to create falls on the concrete slab.
An inverted roof system using Hot Melt was ruled out due to the existing roof structure not being able to withstand the structural load of both IKO Permascreed and the inverted roof ballast. The AccuRoof design team therefore worked with Gradient to design a cut-to-falls PIR insulation scheme to provide the falls and insulation, topped with a two-layer SIGnature 25 Torch On Reinforced Bitumen Warm Roof System. With a great deal of plant and maintenance required to the roof over the coming years, a robust reinforced bitumen system will be capable of handling the traffic. Small areas of hot melt inverted roofing in places will also provide amenity space for the staff once the building is complete.
To ensure the installation would be possible within the tight programme, we took advantage of the ability of Permascreed to both address any imperfections in the concrete slab and provide a temporary watertight finish over the slab before the PIR insulation was installed. This enabled Wates LivingSpace to make a start on the interior fitout before the bitumen waterproofing was installed, which was a huge advantage for the critical path of the project.
Find out what’s there
As built drawings may not provide reliable evidence and should not be relied upon. Instead it is wise to have a specialist survey carried out to confirm the structure and finishes of the roof.
A specialist roof survey will identify all the features of the roof which have been added over the years and may need replacing, repair or reconfiguration in a refurbishment project.
A flat roof survey can confirm the presence of RAAC and other fragile decks and recommend the best remediation.
Whilst the specialist roof consultant is on your roof, they may also be able to develop a design that makes the most of the opportunities the roof provides.
[i] National Buildings Database phase 1: non-domestic building stock in England and Wales (Department for Energy Security and Net Zero, Feburary 2024), Part 1: Stock description, page 48 https://www.gov.uk/government/publications/national-buildings-database-phase-1-non-domestic-building-stock-in-england-and-wales
