Which Is Better: Synthetic v. Felt Underlayment

Roof Repair, Roofing

While most people think about the type of roof they want to install on their home, very few think critically about what underlayment they’re going to use. That’s because your roof’s underlayment acts as a critical but hidden layer that is vital to the durability of your roof. The classic debate for many is synthetic v. felt underlayment. There are several pros and cons to both. 

What is Underlayment?

Roofing underlayment is a material laid down between your roof’s deck and its roof covering (Concrete tile, asphalt or wood shingles, clay tile, metal panels, etc.) It works to protect your home and roof deck from natural disturbances like wind, rain, and snow and ice.Underlayment is installed directly onto roof sheathing before you install your actual roof. 

What is Synthetic Underlayment?

Synthetic roofing underlayment came on to the scene a few decades ago and is now often used for its water resistance and durability against weather events. Synthetic underlayment is typically made of polypropylene,  polyethylene or other thermoplastic polymers., . Synthetic felt is spun or woven in the manufacturing process and generally offers better tear and wind resistance.

Though they offer great benefits, you still have to be careful when using synthetic underlayments. Their materials are not yet regulated*, meaning different products can offer vastly different performance capabilities. Be sure to do your research and talk with a professional roofer who can help you choose the best underlayment for your roof.  *(In December of 2020, ASTM International came out with the first published product standard for synthetic underlayments.) 

Felt Underlayment

Felt underlayment, also called asphalt-saturated felt, is still the “go to” product in Arizona for underlayment on steep slope roofs (defined by OSHA as roofs with a slope of 4:12 or greater) Homes in Arizona with asphalt shingles, tile or metal are steep slope roofs.

Felt underlayment is made of blended plant fibers or inorganic fibers with asphalt. It also comes in sizes, which you can choose from based on thickness. Typically (but not always)  the thicker the felt underlayment, the stronger and more durable against weather conditions it will be. 

Which is Better?

Felt and synthetic underlayment both offer different pros and cons. The type of underlayment you choose depends on the climate where you live and the material of your main roof covering. In our experience to date,, most synthetic underlayments for tile roofs  are prone to leak. On the other hand, we have found synthetic underlayment under asphalt shingles to be effective in standing up to wind and storms during construction. It also stands up against the Arizona monsoon season.  

Felt Underlayment


  • Cost-effective
  • Comes in many different sizes and thicknesses
  • Long proven performance when quality products are used.


  •  More difficult to install in extreme temperatures of Arizona summer.
  • May tear in high winds before the main roof covering is installed.
  • Organic felt absorbs water if exposed, causing felt to wrinkle; Felt built around fiberglass mats do not absorb water.

Synthetic Underlayment


  •  Easy to install
  • Water-resistant


  • Quality of material differs vastly from manufacturer to manufacturer.
  • Not yet fully tested and regulated by UL. 

Get a Free Estimate On Your Roof 

Are you building a custom home or looking for a roof replacement in the Phoenix area? At Roofstar Arizona, we’re experts on Arizona and Phoenix roofs. We can help you choose the right roof for your home based on your roofing materials and local climate. Call our experts now at 480-426-1915 for more information or to schedule a free estimate on your next roofing project. We’ll tell you the truth about your roof!

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