Synthetic Underlayment VS Asphalt Felt


Generally speaking, roofing felt is a felt paper that is soaked in asphalt and other water resistant compounds to produce a membrane. Of course, it is a bit more technical than that, but this is the basic idea behind how felt works. Felt is commonly sold in either 15# or 30# rolls measuring 36 inches wide. Long ago, a 15# role actually weighed 15 pounds, but over the years, that has changed.


Synthetic underlayment was developed as a felt replacement. By weaving together polypropylene and polymer, a water resistant and vapor resistant underlayment was created. This new underlayment is lighter and stronger than its predecessor.

Reasons to Use Synthetic Underlayment

To decide which underlayment is the optimum choice for your project, you first have to understand the actual uses of underlayment and what they can and cannot do:

  • Water resistant (not waterproof) 
  • Protection from resins in the decking that can be damaging the asphalt shingles
  • Vapor protection
  • Temporary protection in the event of storm damage

Not every home needs the best roof underlayment available on the market and not every budget can afford it. A standard asphalt shingle installation does not require the same vapor protection that might be needed for a metal or slate roof, but a steep roof installation might be safer with the better walking surface provided by some synthetic underlayment.


One of the main barriers to using synthetic underlayment for roofing contractors is the cost is higher per square foot than traditional felt.  

Some of the major benefits to synthetic underlayment are that it is:
  • Incredibly lightweight
  • Safer for roofers to walk on while installing shingles
  • Stands up to high winds in the event of shingles being blown off
  • Both water-resistant and vapor-resistant
  • Great for metal roof applications
  • Likely to have fewer wrinkles (than what may occur with felt) due to laying flatter on the roof decking
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