Best underlayment for a metal roof

Roofing underlayment is designed to lay under metal roofs and provide thermal protection as well as waterproofing for your home. Without them, metal roofs tend to be loud when it rains, cold during the winter, and hot during the summer with high temperatures.

Improper selection of the roof underlayment may allow leakage or entrap moisture due to vapor diffusion or air exfiltration. Adding an underlayment will provide an extra layer of protection for your home, and it will improve insulation and waterproofing.

When it comes to metal roofing, there are three options for underlayment: felt, synthetic and High-temp underlayment. All three are great choices to use for metal roofing, but each have pros and cons which are outlined below.

Felt underlayment

30# felt underlayment (or two layers of 15# felt) are recommended for metal roof panels. Felt is an all-over roofing accessory made of a natural (wood cellulose, natural fibers, etc.) or synthetic (polyester or fiberglass) base that is then saturated in asphalt to make it water-resistant.

Metal Roofing Underlayment
Pros of felt underlayment:
  • It is the least expensive option.
  • It is water-resistant, which helps keep water from penetrating the surface if any leaks through the roofing structure.
  • It is available almost anywhere because it is more commonly used.
Cons of felt underlayment:
  • It does not last as long as synthetic
  • It may not respond well to high-temperature environments.
  • It cannot be left exposed for a significant amount of time. It can be vulnerable to rips from high winds.
  • Asphalt is made from petroleum, which is a highly flammable substance and brings a concern for fire resistance.
Synthetic sheet underlayment

Synthetic underlayment is a newer innovation that is taking over the metal roofing industry as a better, more advanced replacement for felt underlayment. Synthetic underlayment is a roofing accessory created by weaving/spinning together polypropylene or polyethylene and a polymer to form an all-overprotective barrier

Pros of synthetic sheet underlayment:
  • Synthetic is very light compared to felt.  Most roofers prefer this material because of its weight, it goes on faster and covers more area.
  • It typically has a longer lifespan than felt underlayment.
  • It is more durable because it is developed with thermoplastic polymers. It is more resistant to tearing, moving, and other types of damage.
  • It has a tolerance for high temperatures.
Cons of synthetic sheet underlayment:
  • It is more expensive than felt underlayment.
  • It has a low permeability rating (which is good for keeping out moisture), but that will also make it more difficult to have proper ventilation.
High Temp Underlayment
Pros of high temp underlayment:
  • Peel-and-stick allows a high temperature suitable for metal roofs.
  • Helps make low slope roof conditions weather-tight
  • Protects high risk areas on the roof.
  • Unlike traditional felt paper, a peel-and-stick underlayment’s rubberized features keep it from easily tearing thus providing consistent protection while being installed and against severe weather.
Cons of high temp underlayment:
  • It can be more expensive than the other options.

At the end of the day, each product has their own pros and cons. It will come down to what is most important to you. In every case, it is best to consult a professional to determine which underlayment would be best for you.

How long do you want to have the roof? Does your budget allow for a more expensive underlayment? Are you concerned about it being fire resistant? Those are typical questions you will have to think through when deciding what is best for you and your situation.

If you would like to discuss more about what underlayment would be best for your situation, please contact us at one of our 21 branch locations.

About Metal Sales

Metal Sales Manufacturing Corporation is the premier nationwide provider of metal panels for the construction industry. Metal Sales works with architectural specifiers and commercial construction professionals to create inspirational design solutions. With the industry’s largest and most knowledgeable sales and technical support team, Metal Sales has the expertise to address today’s challenges in high-performance, sustainable and Net-Zero building. Metal Sales has outreach around the world. Delivering outstanding roof, wall and fascia metal panels from its 21 facilities throughout the U.S.

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