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With so many choices to make when building or remodeling a home, the choices that you can’t actually see are the easiest to omit. Roofing underlayment is one of those unseen choices that really can make a difference in the overall performance of the roof system and longevity of the home.

Originally roofing felt was used only for temporary protection against the elements during construction or renovation. It’s still meant to do that, and much more. Modern underlayment is an essential part of a roofing system, functioning as a secondary protection to help preserve the life of the roof. Nowadays underlayment is designed to:

    • protect the sheathing
    • help keep moisture out of the building, and
    • act as a weather barrier if the primary roof covering is compromised.

A good underlayment leads to stronger roofs that should hold up better against weathering.

Traditional felt underlayment usually consists of felt (made from cellulose or fiberglass) coated with asphalt, marketed in #15 and #30 rolls. Synthetic underlayments, on the other hand, are generally made from engineered materials like polypropylene or polyester. Although traditional felt is sometimes referred to as “organic” when it contains some organic materials, it’s also saturated with asphalt. When it comes to roofing underlayment, “organic” doesn’t have the same green connotation that the organic foods aisle in the grocery store has, and some manufacturers market synthetic underlayments as a more sustainable choice.

The best type of underlayment depends on variables like the roof slope, climate, and roofing material (such as wood, metal, or asphalt shingle). The main advantages of asphalt felt are that it’s often a low-cost, readily available option, and most people are familiar with the standards. On the other hand, synthetic underlayment’s advantages are that it is designed to be resistant to rotting, cracking, fungal growth, and wrinkles. Synthetics are generally lighter and more durable and less prone to puncture and tear. And when weather or schedules delay the completion of a roofing project, synthetics are designed to protect the building materials for up to 6 months.

Felt Underlayment

Synthetic Underlayment