A roofing membrane is a specialized layer of material installed as part of a roof system, serving as a barrier against the elements. This material can be a sheet laid beneath the primary roof covering, known as an underlayment, or it can be the exposed, finished surface itself. The membrane provides a continuous, protective layer that shields the underlying structure from moisture intrusion. Its composition is designed to handle the unique stresses of a roof environment, including temperature fluctuations and exposure to water.
The Essential Role in Roofing
The primary function of a roofing membrane is to act as a secondary water barrier, offering protection to the roof deck if the main roof covering fails. This safeguard is particularly important during severe weather events, such as wind-driven rain or when water backs up due to ice dams. By preventing water from reaching the structural components, the membrane helps maintain the integrity of the entire roof assembly.
Some membranes are also designed to manage moisture transmission, functioning as a vapor barrier or a vapor-permeable layer. A vapor barrier restricts the movement of water vapor from the warm interior of a building into the roof assembly, which prevents condensation from forming within the insulation or on the roof deck. Conversely, vapor-permeable membranes allow trapped moisture to escape outward while still blocking liquid water from entering from above. This moisture management is important because even slight wetting of insulation can significantly reduce its thermal performance and lead to structural damage like rot or mold growth.
The membrane also plays a protective role during the construction process itself. It shields the exposed roof deck from rain and sun damage before the final roofing material, such as shingles or tiles, can be installed. This temporary protection ensures the structural deck remains dry and clean, providing a proper surface for the subsequent layers of the roof system.
Key Types of Roofing Membranes
One of the oldest types of underlayment is asphalt-saturated felt, which is made from a base of organic materials or fiberglass saturated with asphalt. This traditional material is available in different thicknesses, such as 15-pound and 30-pound weights, and provides a basic level of water resistance. However, asphalt felt can wrinkle when wet, become brittle or crack in cold temperatures, and its asphalt compounds can dissipate over time when exposed to heat.
A more modern alternative is synthetic underlayment, which is typically spun or woven from polymers like polypropylene or polyethylene. Synthetic membranes are significantly lighter than felt, yet they offer superior tear resistance and durability, making them easier and safer to install. These materials repel water and are highly resistant to fungal growth, maintaining their integrity even when exposed to ultraviolet light during construction.
For areas highly susceptible to water pooling, such as eaves, valleys, and around roof penetrations, self-adhered membranes offer a higher level of waterproofing. These products, often called ice and water shields, are composed of a rubberized asphalt adhesive layer backed by a film of high-density polyethylene. The adhesive bonds tightly to the roof deck and seals around fasteners like roofing nails, preventing leaks caused by water back-up or wind-driven rain.
Where Membranes are Used
Roofing membranes are utilized in two distinct contexts depending on the roof’s pitch: as an underlayment on steep-slope roofs and as the primary waterproofing layer on low-slope roofs. On pitched roofs, the membrane is installed beneath the shingles, tiles, or metal panels, functioning as the secondary defense against water intrusion.
For low-slope or flat roofs, where water drains slowly, the membrane itself is the exposed, finished surface that provides the main waterproofing. These single-ply systems are made from materials like Thermoplastic Polyolefin (TPO), Ethylene Propylene Diene Monomer (EPDM), or Polyvinyl Chloride (PVC). TPO and PVC are thermoplastic materials that are heat-welded at the seams to create a continuous, watertight surface, while EPDM is a synthetic rubber that is often adhered or ballasted.
The choice of membrane for a low-slope roof depends on the desired performance characteristics, such as energy efficiency. White TPO and PVC membranes are popular because they reflect solar heat. Conversely, EPDM, which is black, absorbs heat, making it a durable option for non-living spaces or in colder regions.