Synthetic Roofing Underlayments
These materials are tougher than felt, but do they perform as well?
Synopsis: The traditional choice for roofing underlayment is asphalt felt, but as technologies have developed, manufacturers are offering more synthetic underlayments. Senior editor Martin Holladay takes a look at the various types of synthetic roofing underlayments and considers their pluses and minuses compared to felt. The biggest issue with many synthetics is permeance, which makes them effective vapor barriers. However, a strong vapor barrier is an asset only for certain roof styles. Care must be taken when attaching synthetic underlayments; cap fasteners are the best choice. Traction is also an issue as some synthetics can be slippery.
NOTE: All material prices fluctuate over time, so the prices listed in this article are more for comparison than for providing accurate cost estimates.
A milestone in any construction project is drying in, usually defined as the day the roof sheathing is covered with underlayment. Building codes require the installation of asphalt felt for several reasons: Underlayment keeps the sheathing dry until the roofing is installed, it provides some protection against leaks in case wind-driven rain gets past the roofing, and it provides a slight improvement in a roof’s fire resistance.
For years, roofers chose between basic #15 or heavier #30 asphalt felt, which are commodity products sold under many brand names. Both types of felt are made from recycled corrugated paper mixed with sawdust; to provide water resistance, the paper is impregnated with asphalt. These days, however, roofers also can choose from a variety of synthetic roofing underlayments: sheet products made of laminated polypropylene or polyethylene plastic.
Synthetic roofing underlayments look and feel similar to housewrap. Unlike housewrap, though, most synthetic roofing underlayments are vapor barriers, so they shouldn’t be used on unventilated roofs.
These plastic underlayments also offer higher resistance to UV radiation, better traction for roofers, and more square footage of coverage at a lower weight. They are not, however, intended or approved to replace peel-and-stick membranes in areas prone to ice dams.
Traditional felt still competes with newer synthetics
Although synthetic roofing underlayments have several advantages over asphalt felt, asphalt felt remains popular as a roofing underlayment for several good reasons.
While the price of asphalt felt fluctuates somewhat, it’s still the least expensive option. Builders typically pay about 5¢ per sq. ft. for #15 felt and 10¢ per sq. ft. for #30 felt. Synthetic underlayments cost more than twice as much as #15 felt — usually between 11¢ and 15¢ per sq. ft. Vapor-permeable synthetic underlayments are even more expensive — from 20¢ to 90¢ per sq. ft.
According to Dyami Plotke, a manager at Roof Services in Islip, N.Y., “For a standard roof assembly, where the felt and asphalt shingles are installed on the same day, it doesn’t make any difference what underlayment you use, so the lower cost of the standard felt is a big advantage. Where the synthetics outperform felt by a mile is in their tear resistance. Synthetic underlayment allows us to bring a building to a watertight condition just by papering it, without installing the roofing immediately — and it will stay watertight for months. That’s why we always use synthetic underlayment under specialty steep-slope products like slate and tile, which are slow to install.”
Cap fasteners aren’t optional
Although asphalt felt doesn’t seal around fasteners as effectively as peel-and-stick membranes, it is less likely to leak at nail and staple penetrations than a synthetic underlayment. Synthetic underlayment punctured by staples or common roofing nails can, with the help of capillary action, lead to leaks. That’s why plastic-cap nails or staples, which help to seal penetrations, are a must when installing synthetic underlayments. Cap fasteners can be installed with a compatible pneumatic tool or, in the case of cap nails, manually.
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