Inverted roofs - the upside-down revolution
For decades, the roofing industry has relied on a tried-and-trusted method of waterproofing buildings – where it has placed the insulation layer on top of the waterproofing membrane. This concept has grown significantly in the last 80 years or so, a revolutionary approach that has literally turned the traditional roofing installation on its head. It is development now universally accepted and known as the inverted or upside-down roof.
It is believed that the first modern inverted roof was installed on the Florida State University Student Union Building in Tallahassee, Florida in 1961. It was designed by the architectural firm of Welton Becket & Associates, and the waterproofing membrane was made of chlorinated rubber. The inverted roof was chosen for its durability and energy efficiency.
That said, the concept of an inverted roof is not new. In fact, there is evidence of inverted roofs being used as early as the 9th century in Scandinavia. These roofs were typically made of turf or sod laid over a layer of wood or stone. The turf or sod provided insulation and protection from the elements, and the weight of the material helped to keep the roof in place.
We now know why inverted roofs were so popular in those early days. Firstly, the region's harsh climate made it necessary to have roofs that were durable and could withstand the elements. Inverted roofs, with their layer of turf or sod, were well-suited to this challenge. Secondly, inverted roofs were relatively inexpensive to construct. The materials used were readily available, and the construction process was fairly simple.
Thirdly, inverted roofs had a number of environmental benefits. The turf or sod helped to insulate the buildings, which reduced the need for heating and cooling. This, in turn, helped to reduce the environmental impact of the buildings.
Archaeological excavations have uncovered several examples of inverted roofs in Scandinavia dating back to the 9th century. One of the most well-known examples is the Viking longhouse in Hofstaðir, Iceland. This longhouse, which was built in the 9th century, had an inverted roof made of turf and sod.
Another example of an inverted roof from the 9th century is the Viking burial site in Hedeby, Denmark. This burial site, which was excavated in the 1950s, contained the remains of several Viking longhouses with inverted roofs made of turf and sod.
However, it was not, as we have already heard, until the mid-20th century that inverted roofs began to be used in a more widespread way. In the years following the installation of an inverted roof on the Florida State University Student Union Building, a number of other high profile buildings were constructed using the same concept. These included the Ford Foundation Building in New York City (1963), the Sears Tower in Chicago (1974), and the John Hancock Centre in Chicago (1969), Denver International Airport (1995), Bank of America Tower (2009) and The Shard (2012)’
In the 1980s and 1990s, inverted roofs began to gain in popularity as the benefits of this type of roofing system became more widely recognised. Today, inverted roofs are now used on a wide variety of buildings, including commercial, industrial, and residential properties.
As we have already stated, inverted roofs, also known as upside-down roofs, flip the conventional design by positioning the insulation layer above the waterproofing membrane. This ingenious shift unlocks numerous advantages, making inverted roofs the preferred choice for discerning building owners and architects seeking superior performance, sustainability and long-term value.
It is claimed that an inverted roof can last up to three times longer than a more conventional installation. This remarkable durability translates into lower maintenance costs and reduced disruptions throughout the building's lifetime.
In addition to their exceptional durability and energy efficiency, inverted roofs offer a range of other benefits, including improved sound insulation, enhanced fire protection, and aesthetic appeal. These versatile systems can also seamlessly integrate with green building initiatives, providing a platform for thriving rooftop gardens that enhance air quality, reduce stormwater runoff and promote biodiversity.
As the demand for sustainable and energy-efficient building solutions continues to grow, inverted installations are increasingly poised to become the roof of choice, representing a significant step towards a greener, more resilient future for the built environment – and that’s excellent news - even if it is upside down.