Synthetic or Felt?

 In Uncategorized

This is a question that will be up for debate for many years to come, especially in the Roofing Industry.  As you may have heard, the choice of synthetic over traditional felt underlayment is a growing trend when it comes to roofs.  These days, most roofers are using synthetic over traditional felt. This could be a revolutionary positive change in the roofing industry or, the beginning to a costly end.  In this article, we at Aurum Roofing, will share our thoughts/opinions and compare the two as well as look at the pros and cons. So stick around because this is guaranteed to get you thinking like a seasoned roofer.

Although both are used for the same purpose, to keep water out, the difference between the two is what the products are made of.


Synthetic Underlayment

First things first, let’s start with the newer and younger underlayment known as Synthetic.  Synthetic underlayment is a part of your roof system made from polypropylene, polyethylene or polymer to form a protective barrier to put between the actual roofing material, regardless of your roof choice (I.e.: asphalt shingles, metal, tile, ect.), and the actual roof plywood decking.  But don’t get confused. Polypropylene, polyethylene and polymer is pretty much a blue plastic tarp that you buy at your local hardware store in the form of a roll. Here is the real definition for your reference:

A tarpaulin (US: /tɑːrˈpɑːlɪn/ tar-PAW-lin, UK: /tɑːrˈpɔːlɪn/ tar-PAW-lin), or tarp, is a large sheet of strong, flexible, water-resistant or waterproof material, often cloth such as canvas or polyester coated with polyurethane, or made of plastics such as polyethylene.

Synthetic Underlayment Top 3 Pros:

  1. Synthetic is very light compared to traditional felt.  Most roofers prefer this material because of its weight, it goes on faster and covers more area.  One roll of traditional felt covers only 4 roofing squares per roll as opposed to Synthetic, which covers 10 roofing squares per roll.  This equates to less up and down trips from the roof to the ground, to reload, which in turn, leads to saving time and being more productive.
  2. Synthetic has a much higher tear resistance because polyethylene is the key ingredient in this kind of felt.  Because of this, builders prefer to use this material as it helps with productivity since they are able to only hire 1 roofing crew instead of multiple crews.  Let me explain. Synthetic (again because its key component is polyethylene also known as plastic) can be left for days or even weeks at a time exposed to high winds.  It will not tear or blow off as easy as traditional felt so new home builders can hire 1 crew to only install underlayment (and worry about the actual roof at a later time) and not worry about water leaking into the new house, which means it won’t slow down the drywall, insulation, floor crews, etc.  The roof can then be installed at a different time, after all the other crews are completed with their projects. Hiring 1 roofing crew is usually cheaper than hiring multiple because they’re able to negotiate a lot more and use the bulk size job to their advantage. Have you ever driven by a new neighborhood and noticed only 1-2 guys roofing an entire house by themselves?  Do you wonder why? Well, now you know.
  3. Last but not least is advertisement.  Synthetic felt is a great way to advertise.  Unlike traditional felt, it’s mostly a lighter color and very easy to print high visibility lettering on it. The manufactures of the synthetic know this and take full advantage of this opportunity by marketing their name all over the product.  I like to think of it as a giant billboard but instead of paying for the billboard, the manufacturers are actually getting paid to advertise, while selling their product, for free. Some roofers are picking up on this trend and are now paying the manufacturers to place their roofing company name on the rolls and use this form of advertisement too.

Synthetic Underlayment Top 3 Cons:

  1. Our biggest problem with this material is heat and moisture.  Because synthetic is pretty much a blue plastic tarp, like what you buy at your local hardware store, wrapped around your plywood decking (roof), it’s bound to get hot.  Really hot! Heat is what raises our attic temperatures to an extreme amount in Texas summers which in turn drains our pockets with energy costs. There are probably multiple articles you can read that state that synthetic is really good but since this is such a new product, there is not enough evidence to prove otherwise.  We cannot say for certainty what can happen to your roof or plywood decking 15-25 years from now because of this. Let us be the first to go on record and say there is a huge possibility this plastic base material can ruin the plywood beneath your shingle and we feel it has a possibility of ruining your plywood along with asphalt shingles as well as it may cause it to become very brittle.   If you are an Austin native such as myself, you know the summer heat we get here in Central Texas. Ask yourself, is this worth the risk? After 20 years or so, our view on this may change, but for now I will say, stay away from synthetic. Traditional felt has been used for over 100 years and is still being used. Because it works!
  2. Synthetic has a much higher tear resistance because polyethylene is the key ingredient in this type of felt. Polyethylene, in case you didn’t know, is a chemical used to make plastic, accounting for over 34% of the world’s plastics such as plastic bags, trash cans, etc.  Because polyethylene is a key chemical ingredient in this material, there are a few things you should know. Not only is this chemical bad for the environment but it’s bad for humans because of its toxicity. This material is not biodegradable and decomposes like traditional felt does. Plastic can sometimes take centuries to decompose.  Much longer than what any house would last.
  3. Synthetic Underlayment, is 9 times out 10, installed wrong.  Like we mentioned before, synthetic underlayment has a very high tear resistance and roofers know this.  This material has specific prefabricated nail patterns on most rolls that indicate a very specific nail placement for its highest resistance possible and because roofing crews know it’s not going to tear, most simply tack a few staples to hold it in place and call it a day.  That’s right. Staples. Completely disregarding the instructions on how to properly install synthetic felt. Since they know it’s not going to tear beneath them anyway, how it’s installed or what they use to hold it in place, doesn’t even matter to them. Most rolls, and by most, we mean the most common used/popular rolls in the market, are labeled to follow a specific nail pattern and the instructions state to only install using nailed plastic caps (which by the way, most roofers hate, because installing plastic caps slows down production); not tincaps (those round gold or silver plates you see used to hold down felt), not nails and most definitely, not staples.  The roofing contractor you hire can be a certified roofing installer with whatever manufacturer they claim to be with, but guess what, the crew/subcontractor is more than likely not. And, if, for whatever reason you have to file a warranty claim with the manufacturer over any defect and your manufacturer’s regional rep comes out and inspects your roof, and notices your underlayment wasn’t installed correctly, your warranty is going to be 100% void and guess who’s going to be left on the hook? You! Next time you see a roof being installed and notice they’re using synthetic, I challenge you to take a close look and see for yourself if we’re right. It’s very rare to find a roofer installing this material correctly.


Traditional Felt Underlayment

Felt underlayment has been a part of most residential and commercial roofing systems for over 100 years and its formula has pretty much stayed the same (recycled paper with a thin coated layer of asphalt).  Felt underlayment is commonly found in two different grades, 15lb or 30lb. In case you’re wondering what 15lb and 30lb are, and why they’re categorized in this way, this is because this is it’s weight, in pounds, per roofing square.  A 10×10 area or 100 sq. ft, is considered 1 roofing square (15 or 30lbs).

Felt Underlayment Top 3 Pros:

  1. While felt is waterproof, it also allows your roof decking to breath.  This helps the longevity of your roofing materials, especially asphalt shingles.  Asphalt shingles are the most common roofing material today and if you live in our local market, Pflugerville/Austin,TX Metro area, you more than likely have asphalt shingles on your home.  Allowing your plywood to breath goes a very long way. It keeps plywood from becoming brittle and also keeps the roofing nails from backing up which in turn cause leaks.
  2. Felt is made of recycled paper/shredded compressed wood with a thin layer of sprayed tar, similar to what asphalt shingles are made from. It’s biodegradable unlike synthetic, and most importantly it isn’t toxic to humans.  Its formula has pretty much stayed the same since it was first introduced to the construction world in the 1920’s.
  3. Felt can be installed using either plastic caps or tin caps and as long as the material is properly lined and overlapping the way it should be, there really isn’t a wrong way of installing it.

Felt Underlayment Top 3 Cons:

  1. Felt Underlayment, unlike its competitor, synthetic, cannot be left exposed for a significant amount of time.  High winds make this material very vulnerable to blow offs or rips which in turn leads to exposed plywood decking.  It’s never a good idea to leave felt exposed to the sudden changes in our weather for any amount longer than a week.  Doing so would make your home susceptible to water leaks coming from your roof. We would highly recommend installing your roofing materials almost immediately (if at all possible) after installing the felt to prevent damage to the material.
  2. Felt, for some unknown reason, significantly contracts when left overnight.  If left overnight after being installed on a roof, the material wrinkles so much, making it almost impossible to install shingles.  Only in the daylight hours (late morning to early afternoon of direct sunlight) does felt naturally expand back to its normal position and lay back to be once again flat.  A very common, unsafe and un-craftsman like thing that roofers like to do, that will allow them to continue installing shingles in the morning hours as if nothing happened, is that they like to cut and slice through the air bubbles of the felt allowing it to relax and lay flat onto the plywood decking.  The problem with this is that the felt, because it’s sliced and cut in multiple areas, isn’t waterproof anymore. If water were to get past your roofing material, it cannot depend on the underlayment for waterproof back up anymore.
  3. For steeper roofs, 10/12 and above, 30lb felt is recommended but because of what it’s made of its tear and slip resistance is still very high. On roofs like this even 30lb felt on a hot summer day tears easily.  This is the main reason why roofers prefer synthetic.


Conclusion- Overall we all have to understand that both materials outweigh each other on different things.  There really isn’t a wrong choice. Our main point that we’d like you to take out of this blog is, synthetic is a new phenomenon and we need to first see what the after effects can bring us in the future.  We know what felt brings but there is still a verdict out to be reached for synthetic. Our house is where the bulk of most of our wealth is held and it’s a little scary to take on this risk.

Recent Posts