The roof underlayment or most commonly known as “felt paper” is the first layer of waterproofing materials that goes before the shingles or roofing materials. It is mainly a roll of paper or fiberglass saturated with asphalt and other waterproofing materials.
There are many new types of underlayment used beneath roofing materials but basically there are three types of undenlayment and it can be difficult to know which underlayment to choose.
- asphalt-saturated felt
- rubberized asphalt
- non-bitumen synthetic
Asphalt-saturated felt
Asphalt-Saturated Organic Felt Definitions: ASTM D226 is the “Standard Specification for Asphalt-Saturated Organic. Felt Used in Roofing and Waterproofing” ASTM D4869 is the “Standard Specification for Asphalt-SaturatedOrganic. Felt Underlayment Used in Steep Slope Roofing”
Rubberized asphalt
Rubberized asphalt is more expensive than other types pf roofing underlayment. Why? It contains higher percentages of asphalt and rubber polymers, making it waterproof.
- Rubberized asphalt roofing underlayment often has a sticky back with a protective membrane that is removed prior to installation. This special backing creates the waterproof seal between the underlayment and a clean roof deck.
- In severe winter regions, waterproof roofing underlayment at the eaves of a roof is helpful. The underlayment protects the edge of the roof deck from water damage.
- Rubberized asphalt roofing underlayment should be added at valleys and around roof protrusions. These spots often get leaky first.
Non-bitumen synthetic
Synthetic underlayment (technically non-bitumen synthetic underlayment) is the preferred underlayment of most roofing professionals today.
- Synthetic underlayment basemat is saturated in asphalt. The addition of fiberglass gives synthetic products superior tear resistance and stability.
- High-quality synthetic underlayment like CertainTeed’s DiamondDeck™ has the added benefit of scrim reinforcement for added slip resistance, even when wet.
- Synthetic underlayment is designed for application across the entire roof deck. It is sometimes used with waterproof products.
Underlayment Size
Felt underlayment It’s available in two thicknesses: 15-pound and 30-pound.
Fifteen-pound felt has a perm rating of about 5, although this number can rise in high-humidity conditions.
Thirty-pound felt is more resistant to damage during installation of the roof-covering material, and will protect the roof longer if it should somehow become exposed to weather. The difference is obvious, once you see them together. Thirty-pound felt is much thicker and stiffer.
When choosing a type of underlayment always try to go with the manufactures felt. It is always smart to go with the same brand of your roofing materials because in case of a warranty claim you would have a whole roofing system installed and not a mix of different brands.
Securing Methods
Staples are commonly used to secure felt to the roof
Plastic Caps offer better wind resistance than staples, and help prevent leakage through the holes made by the fasteners.
