Common Defects in Flat Roofing: Blistering in Felt Systems
Posted on 21st October 2024 at 16:49
Blistering is one of the most frequently encountered defects in flat roofing systems, particularly in felt roofs. Though it may not always cause immediate issues, it can be a sign of underlying problems that, if left unaddressed, could lead to more serious and expensive repairs. In this post, we'll dive into the causes of blistering in flat roofs, why it occurs, and how modern materials and techniques can help prevent it. Whether you're a building manager, contractor, or property owner, understanding this common issue can help you maintain the integrity of your roofing system and avoid unnecessary costs.
Causes of Blistering
Blistering occurs when air and moisture become trapped in voids within the roofing system during installation. Even under ideal conditions, some voids are inevitable, often caused by skips in bitumen application, debris, or irregular bonding between felts and bitumen. These voids expand when heated by the sun, creating pressure that lifts the membrane and causes blistering.
Blisters typically form in two key areas:
Between the roof membrane and the substrate.
Between the layers of the membrane.
In the past, older pour-and-roll systems that fully adhered the membrane to the insulation were especially prone to blistering. As temperatures rise, the trapped air and moisture expand, causing pressure to build within these voids and lifting the membrane.
Bitumen, which is widely used in built-up roofing, softens at temperatures above 60°C. This weakens the bond between layers, making blistering more likely. As the membrane stretches to accommodate the pressure, the blister can grow over time, especially if cooling at night doesn’t allow the membrane to return to its original state.
Cork’s Role in the Past
Historically, cork was a common insulation material in flat roofing systems. Known for its durability and flexibility, cork was popular due to its resistance to environmental and chemical degradation. Being an inert material, cork did not react to temperature changes or moisture exposure, which helped reduce blistering risk.
However, cork had some limitations, particularly in terms of its thermal performance. As building regulations increasingly emphasised energy efficiency, cork’s insulation properties were insufficient compared to newer materials like polyisocyanurate (PIR) or polyurethane (PUR). Cork also became more expensive due to its natural sourcing and processing costs, leading to its replacement by synthetic insulation materials. PIR and PUR offered better thermal performance at a lower cost, making them more suitable for modern roofing systems.
PIR and Modern Insulation
Today, PIR insulation has largely replaced cork in flat roofing systems due to its superior thermal properties. However, PIR is not without its challenges, particularly when used beneath bitumen-based membranes. The closed-cell structure of PIR makes it impermeable, which can trap air and moisture between the insulation and the membrane, increasing the likelihood of blistering.
In older systems, materials like wood fibre or cork were used as overlay boards to provide a separation layer, allowing vapour to escape. In some cases, a 3G perforated layer was installed to prevent full bonding and manage vapour dispersion, helping to reduce blister formation.
Modern roofing practices now favour partially bonded venting underlays in torch-on systems. These venting underlays are designed to create a controlled path for vapour to escape, reducing the pressure build-up that can lead to blistering over PIR insulation. They are now a critical component in modern flat roof installations, significantly improving the system’s durability and resistance to defects like blistering.
Blisters Between Membrane Layers
Blisters can also develop between the membrane layers if air or moisture is trapped during installation. These blisters expand during hot weather and contract when temperatures drop. However, built-up membranes are not fully elastic, so they may not return to their original shape, leading to permanent deformation and further blister growth.
Modern venting underlays help minimise this issue by allowing trapped vapour to escape, thereby reducing stress on the membrane layers and preventing the bond from failing at the blister’s edges.
Repairing Blisters
Larger blisters, particularly those in high-traffic areas, should be repaired promptly. The typical method involves cutting the blister open, peeling back the affected membrane, and patching the area with layers of bitumen and felt. This ensures the membrane is properly resealed, preventing further moisture ingress.
Prevention and Best Practices
To minimise blistering, it is essential to use modern venting underlays that allow vapour to escape from beneath the membrane. Ensuring that all materials are dry before installation and working in suitable weather conditions are critical steps in preventing defects. For modern flat roofs, particularly those using PIR insulation, partially bonded venting underlays provide the most effective solution for vapour management and help prevent blister formation.
Conclusion
Blistering is a common issue in felt roofing systems, but with modern materials and techniques, it is increasingly preventable. Cork, once a popular insulation choice, has been replaced by more efficient materials like PIR, paired with venting underlays that manage vapour and reduce the risk of blistering. Regular inspections, proper installation practices, and timely repairs are key to maintaining a roof’s long-term performance and avoiding costly repairs.
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