Choosing the Right Single-Ply Roofing System: Ballasted, Fully Adhered, or Mechanically Attached?
Single-ply roofing membranes are a popular choice for waterproofing flat roofs due to their durability, limited weight, cost-effectiveness and ease of installation. These membranes are secured in place over the support using one of the following attachment methods: ballasted, adhered or mechanically attached. Each method offers distinct advantages and poses unique challenges.
Whether it’s creating a brand new flat roof or refurbishing or renovating an existing one, building owners, architects and contractors need to evaluate which system is best suited for the specific needs of the project. This may include addressing factors such as whether the roof is accessible or not, or if there are plans to add a green roof or a solar installation down the line.
Ballasted roofs
One of the oldest single-ply roofing systems, ballasted roofs date back to the 1960s and are still widely used today for their performance and cost-efficiency. On a ballasted roof, the insulation layer and waterproofing membrane are loosely laid over the roof deck and are held in place using ballast material (e.g. river washed gravel, concrete pavers, etc.) placed over the membrane, usually with a protection layer in between. This method is most suitable on roofs with a minimal amount of openings and penetrations.
A ballasted roofing system is relatively easy to install. Labor time is reduced, which leads to lower installation costs compared to other systems. The larger the sheet size of the roofing membrane, the faster the installation can go. In addition, ballasted systems can also be installed during almost any weather. When designed properly, ballasted roofs provide excellent resistance to wind uplift, as the weight of the ballast keeps the membrane firmly in place.
In addition to their straightforward and fast installation, ballasted roofs are also sustainable and energy-efficient. Because no fasteners are needed, the incidence of thermal bridging is virtually zero. Single ply roofing membranes which have longer service life, such as EPDM, can be reused when installed in a ballasted application: they can be easily removed and used on another roof.
The main challenges posed by a ballasted roof are related to its weight. The roof's slope, height and structural deck can be limiting factors. The roof deck must be able to support the weight of the ballast, which may mean carrying out structural reinforcements that can increase the overall cost of the roofing project. Old buildings may not be suitable for ballasted systems without expensive modifications.
Ballast, particularly gravel, can also make it difficult to inspect and maintain a roof. Eventual damage to the roofing membrane may be harder to detect and debris such as leaves can accumulate unseen under the ballast.
Fully adhered roofs
On a fully adhered roofing system, the waterproofing membrane is adhered to the insulation layer, or directly onto the roof substrate using adhesive. This attachment method offers a smooth and aesthetically pleasing appearance and can be installed on roofs with varying slopes and complex shapes, even ones where the structure is not flat at all. Almost every roof is a good candidate for an adhered system thanks to its flexibility and limited weight.
A very important advantage of a fully adhered roofing system is that the strong adhesive connection provides excellent wind uplift resistance when adhered to a firm fixed substrate, making it ideal for regions prone to high winds.
On the downside, fully adhered roofs can be more labor-intensive than ballasted or mechanically attached systems. The application of adhesives is sensitive to weather conditions, so if the temperature is too cold or the humidity too high, the adhesive may not bond properly and this may impact installation times.
The recent emergence of self-adhered single-ply roofing membranes, where the underside is factory-coated with a strong adhesive, eliminates the need to apply adhesives on site and wait for them to dry, providing a time-saving alternative.
Mechanically attached roofs
In mechanically attached roofing systems, the insulation layer is mostly attached to the roof deck using plates and fasteners. The roofing membrane is then laid on top of the insulation, and is mechanically attached to the roof deck using different fastening systems, mostly fastened in the seam area of adjoining sheets. The roof upstands can be mechanically attached or fully adhered, while penetrations and openings are flashed.
Mechanically attached roofs can be installed in a wide range of weather conditions. Temperature and humidity do not affect the installation process, which means fewer delays and more flexibility in scheduling. It is also a lightweight system that offers an aesthetic look.
This attachment method is relatively affordable but has some drawbacks. Many of the fastening systems used in mechanically attached roofs can act as thermal bridges, allowing heat to escape through.
Non-penetrating variations of the in-seam mechanically attached method are the induction welding system for thermoplastic membranes, and the linear attachment system using special strips. In the induction welding attachment method, the thermoplastic roofing membrane is heat-welded to special coated metal plates by means of an induction tool. This method, where the induction plates are fixed into the substrate as per a symmetrical design pattern, provides high wind uplift ratings and reduces labor and material costs.
