Underlayment is a water-resistant material that is installed beneath the top layer of roofing. As an important part of the roof system, underlayment protects the structure during and after the installation process by:
- Preventing moisture buildup and repelling water.
- Adding an extra layer of defense against the elements.
- Improving the roof’s resistance to rain, wind, snow, ice, and fire.
- Helps to protect the roof deck during the installation process.
Having the right underlayment on your roof will help stop moisture from infiltrating and collecting inside your attic. While underlayment is laid underneath most roofing materials, it is especially important for metal roofing. When it comes to underlayments for metal roofs, there are three many types:
- Felt paper underlayment
- Synthetic underlayment
- Peel-and-stick underlayment
Felt Paper Underlayment for Metal Roofing
Felt paper underlayment is generally made from a natural fibre or cellulose base and then treated with a waterproofing agent such as asphalt. Because of its low cost, felt paper was once the most commonly used roof underlayment. However, roofing contractors are quickly shifting to synthetic underlayment for metal roofing because it lasts longer and is more resistant to the elements.
Important tip: you want your underlayment to outlast your roof. When combined with a metal roof, the short lifespan of felt paper presents a challenge. Metal roofs can easily last for decades—or longer—which is far longer than the lifespan on felt paper.
Pros of Felt Paper:
- Cheapest type of roof underlayment
- Very easy to install and can use common tools
- Readily available and almost always in stock
Cons of Felt Paper:
- Short lifespan (won’t last as long as a metal roof)
- Breaks down in direct sunlight and high temperatures
- Is known to buckle, bubble and wrinkle in as little as 24 hours with UV exposure
- Contains asphalt (made from petroleum) which is highly flammable
Synthetic Underlayment for Metal Roofing
Synthetic underlayments are constructed from woven polyethylene or polypropylene fibres that have been treated with a resilient polymer to form a protective barrier between the roofing material and the underlying deck. Because they last longer and perform better in high temperatures, winds, and exposure to the elements, synthetic underlayments are growing in popularity. The majority of synthetic underlayments are mechanically attached to the roof with roofing caps and staples.
Interesting fact: synthetic underlayments can substantially increase the energy efficiency of a metal roof. Metal roofs are one of the most energy-efficient roofing materials on the market when used alone, but when combined with advanced synthetic underlayments, the energy efficiency of metal roofing is amplified by 4! For example, a metal roof installed over felt paper improves thermal performance by 48%. Thermal performance increases to 218% when the same metal roof is installed over an advanced synthetic underlayment.
Pros of Synthetic Underlayment:
- Way more durable than felt and is the longest lasting type of underlayment
- Quick and easy to install
- Can withstand up to a year of direct exposure to UV without drying out.
- Tough thermoplastic polymers that resist tearing, movement, and impact damage
Cons of Synthetic Underlayment:
- More expensive
- Needs specialty tools (more than a standard hammer-stapler)
Peel-and-Stick Underlayment for Metal Roofing
Also known as ice and water shield, self-adhering membrane or rubberized asphalt.
“Peel-and-stick” self-adhering synthetic underlayment products that are applied directly to the deck typically have a rubberized asphalt adhesive on one side that is protected by a peel-off membrane.
Because of the rubber-like properties of self-adhering synthetic underlayment, it is self-healing, which means it seals extremely well around fasteners and penetrations. Because it is so effective at preventing water penetration, peel-and-stick synthetic underlayment is also known as an ice and water shield. Building codes frequently require ice and water shield underlayment in areas prone to heavy rain and snow.
Pros of Peel-and-Stick Underlayment:
- Very effective for ice dam protection
- Creates a water-tight seal
- Lasts a long time and performs better than felt
- Contributes to a more resilient air barrier for better thermal performance.
Cons of Peel-and-Stick Underlayment:
- Self-adhering underlayments must be installed correctly. Careless installation can cause the membrane to peel back over time, exposing the roof deck to damage
- More expensive than felt and synthetic materials