When it comes to the conservation of heritage buildings, roof refurbishment can play a crucial role. Authentic materials are able to retain the character of the building or structure in question, whilst also providing long-lasting, waterproof protection.
For many years mastic asphalt has been relied upon for roof refurbishment works on heritage projects. Mastic asphalt is frequently specified by the National Trust and English Heritage, as well as other public and private owners of prestigious buildings in this sector.
Buckingham Palace is testament to the longevity of mastic asphalt. On top of the Royal household is a rooftop of mastic asphalt that has kept the building dry for well over 80 years. Other heritage applications include the Tower of London, Tower Bridge, Westminster Cathedral and Edinburgh Castle.
Mastic asphalt is one of the few construction activities still regarded as a ‘craft trade’ and there are many time-served mastic asphalt roofing applicators working across the country. The skilled work involves ensuring that asphalt is at the correct temperature, and then spreading it using traditional techniques to coat the surface. A thermoplastic material that changes shape when heated, mastic asphalt cures to form a hard, durable, finished product to suit a wide range of roofing and waterproofing applications.
As mastic asphalt is laid in molten form, it is often confused with other types of waterproofing membrane that require naked flame or torch on application. In reality, there is no naked flame at the point of installation.
The Mastic Asphalt Council (MAC) recognises the heritage sector as a crucial part of the work carried out by its contractor members and as such, heritage projects are recognised at the MAC Annual Awards.
At the recent MAC Awards 2024 hosted in London, Keble College – one of the largest colleges at the University of Oxford with over 800 students – was crowned winner of the Heritage Project of the Year award. The college dates back to 1870 and mastic asphalt had been widely used for protection of the roof areas over the years due to its suitability for heritage buildings.
The award-winning project related to a high quality 350m2 installation of IKO’s Permaphalt system on the college’s student bar roof. As mastic asphalt had previously been used, the college wanted to maintain the aesthetic consistency, however the asphalt finish needed to be painted with white solar paint, which would reflect onto the higher-level windows surrounding the single-storey building. To address this issue and enhance the environmental credibility of the project, a green roof was proposed and ultimately chosen by the college.
The Oxford Asphalt Company and IKO collaborated on this project, utilising the IKO Permaphalt system to create a durable, watertight, aesthetically pleasing roof. A 20mm thick layer of IKO Permaphalt was applied in two 10mm coats on IKO black sheathing felt separating membrane, ensuring a seamless and impermeable waterproofing barrier. The final coat was rubbed with clean coarse sand for a refined finish, enhancing the roof’s appearance.
On insulated sloping timber surfaces over 5 degrees, a 20mm thick layer of IKO Permaphalt was laid in three coats on EML over IKO black sheathing felt fixed to the slopes. This application not only provided robust waterproofing, but also improved the building’s thermal efficiency by reducing heat loss through the insulated roof.
To ensure complete protection, even at vulnerable points, a two coat solid IKO Permaphalt angle fillet was formed at junctions to flat and upstand areas. This attention to detail guarantees the long-term performance and integrity of the waterproofing system, safeguarding the building from water damage.
This project presented several challenges that required expertise and innovative solutions. Firstly, some roof areas had slopes that necessitated input from IKO regarding mechanical fixings. Pull-out tests were conducted on the fixings to ensure their stability and effectiveness. Due to the building’s listed status, the low parapet could not be altered. This limitation meant that the detail had to remain the same and only 50mm of insulation was able to be installed.
The Oxford Asphalt Company’s commitment to health and safety was evident throughout. A segregated route to the roof was established to keep students clear of the work area. A shallow ramp was created for safe access to the roof for asphalt delivery. Despite the challenges faced, the Oxford Asphalt Company, in collaboration with IKO, delivered an outstanding project that met Keble College’s requirements and exceeded their expectations.
Clients in the heritage sector, such as roofing contractors, architects, surveyors and other specifiers, can benefit from MAC’s comprehensive technical information service, which provides advice on the design of all types of mastic asphalt installation. As part of this service, MAC has produced a series of Technical Guides which can be readily downloaded from the MAC website by visiting this link.
Back in 2008, the Mastic Asphalt Council made history as the first industry in the world to achieve the CarbonZero standard. Since then mastic asphalt has offset thousands of tonnes of carbon dioxide – underlining its status as a green waterproofing material.
Although mastic asphalt is typically known for its durability, it is increasingly being specified for heritage roofing projects where green credentials are taken into consideration. When mastic asphalt has reached the end of its useful life, it can be recycled or used as roof screed. The Building Research Establishment (BRE) has officially stated that mastic asphalt roofs are capable of lasting 50-60 years, but MAC has many examples well in excess of this. For instance, mastic asphalt was first laid at London’s St Paul’s Cathedral in 1906 and it provided well over 100 years’ of effective waterproofing before it needed replacement.
This article featured within the September / October 2024 edition of Roofing Today magazine – click here to view the article.