Newton Roofing: Article About Three Basic Underlayment Types

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The underlayment on any roof is a critical moisture barrier that provides a foundation for the uppermost roofing materials. Newton roofing experts are familiar with the three most common types: asphalt saturated felt, rubberized asphalt, and non-bitumen synthetic, and should be able to explain the difference in a way that any home owner can understand.

Asphalt saturated felt is also referred to as "organic" mat because it is cellulose based. It is water resistant but not waterproof. Saturated felt is available in two weights 15 pounds and 30 pounds. Although 15 pound felt has a perm rating of about 5 when dry, the 30 pound felt is much thicker and stiffer which makes it less likely to tear during instillation and more durable should it become exposed. To prevent tearing and leaking in high wind regions, saturated felt should be attached with plastic or metal caps instead of staples. One disadvantage to asphalt saturated felt is that it can absorb water and wrinkle. Wet felt can transfer moisture to the deck below and irregularities to the shingles above.

Rubberized asphalt is an underlayment made from various rubber like materials and is usually made with a self adhesive backing.

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This backing is protected by a peel away membrane that should only be removed just before placement. Because of the rubberized material, this underlayment creates a better seal around fasteners. Rubberized asphalt may also have a polyethylene or polyester bond on the surface to increase its weather resistance. To create a stronger underlayment, many are reinforced with fiberglass. Naturally, rubberized asphalt is more expensive than saturated felt.

Non-bitumen synthetics are roofing membranes made of synthetic polymers like polypropylene and polyethylene. This underlayment is lightweight, high strength, and won't wrinkle. It is also resistant to fungal growth and UV damage. However, currently, there are no ASTM standards for non-bitumen synthetics and many do not meet the standards for local building codes. In addition, the use of this underlayment may void the manufacturer's warranty on certain roof covering like asphalt. Although synthetic underlayment can be moisture permeable, they are generally moisture barriers. Moisture problems that may occur due to the nonporous surface can damage the deck and roofing materials.

Since underlayment is the first layer of waterproofing for any roof system, homeowners need to discuss their options with their roofing contractor. A number of factors need to be taken into consideration when selecting the underlayment, including budget, building codes, regional weather conditions, and manufacturer's warranties.

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