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Synthetic vs. Felt Underlaymen
What Homeowners Should Know

Published: 12/09/2021

new roof for historic home

When establishing the fundamentals for your new rooftop, the importance of selecting the right roofing underlayment can’t be ignored. Your choice can heavily impact the performance of your roof, its endurance, and whether or not your roof meets building code requirements.

To help you ensure that you achieve a successful installation, we’ll go over what underlayment is, and how the two main types (synthetic and felt) differ. By the end of this quick rundown, you should have a better idea of what your roof needs and how to proceed.

What is Underlayment?

Roofing underlayment is the protective layer that is laid over the sheathing/decking and beneath the shingles. It is meant to function as a weatherproof barrier to keep moisture from penetrating into the roof sheathing and the interior structure beneath it.

This can prevent serious problems from developing, such as mold, rot, electrical hazards, property damage, and other types of water damage. Underlayment can also contribute to a roof’s fire resistance.

Synthetic

Synthetic underlayments are composed of laminated polypropylene or polyethylene plastic. They are comparable in consistency to housewrap. One significant difference, however, is that basic synthetic underlayments also serve as vapor barriers. This makes them unsuitable for rooftops that have no ventilation. Vapor-permeable synthetics are available, but they are notably more expensive than standard synthetics (i.e. 20-90¢ per sq. ft. vs 11-15¢ per sq. ft.).

Advantages of synthetic underlayments include superior UV resistance and a lighter weight. They do not offer sufficient protection for spots that are vulnerable to ice dams, but here in South Florida, that isn’t a big worry.

Additionally, synthetic underlayments offer extra durability in the form of traction and tear resistance. This makes them ideal for installation on roofing projects that may not be completed for an extended period of time. The synthetic covering will adequately protect the roof structure until the shingles are finally installed. Such strength and weather resistance is especially a perk in our region, where we regularly have to worry about roofing damage due to hurricane-force winds.

A critical point with synthetic underlayment is that Florida’s building code imposes specific stipulations for what types suffice and how they are supposed to be installed. The latest code update calls for two layers of reinforced synthetic underlayment certified to comply with ASTM D226 Type II standards.

Pros

  • More lightweight than felt
  • Easy and quick insulation
  • Resistant to weather and foot-traction

Cons

  • More expensive than felt
  • Bound to specific code requirements
  • No vapor permeability except in more expensive products

Felt

Felt underlayments are a composite of recycled corrugated paper and sawdust, with asphalt infused into the felt for waterproofing purposes. Asphalt felt is the traditional choice for underlayments. The building code requires a #30 grade asphalt felt underlayment, with the exception of a few alternatives that are specified for their ability to provide equivalent protection.

Asphalt felt is the most economical option available, costing at least 50% less than synthetic underlayments. It isn’t as hardy as synthetic underlayment, meaning one can’t count on it offering adequate protection if left exposed. This is why it is advised that felt underlayments only be used on roofing projects that can be completed the same day, or at least shielded from the elements by a temporary covering system.

The vapor-permeability of asphalt felt makes it a better choice for attics that have little to no ventilation, or may have pockets that are vulnerable to moisture collection. Vapor-permeable synthetics are an option in these cases, but they are quite a bit more expensive.

Pros

  • More affordable than synthetic
  • Vapor-permeable
  • Readily accepted by building codes

Cons

  • Tears easily underfoot or in high winds
  • Wrinkles, cracks, and splits in exposure to cold and moisture
  • Heavier and less material per roll than synthetic

Get the Best to Build Your Roof

Whether you’re already clear on what underlayment is best for your roof, or you feel like you need some guidance, we’re ready to help you ensure your project is successful. 

AKVM has been helping homeowners throughout Bradenton and Sarasota install tough, durable rooftops. We’ll help you structure a roof that has the right composition at every level, deployed with expert technique and insight. 

To get your FREE contactless inspection and estimate, please get in touch. Call (941) 727-3996