15lb vs 30lb vs Synthetic Felt Underlayment

GAF Feltbuster Synthetic Underlayment

When it comes to your roofing, you will have multiple decisions to make, including what type of underlayment or felt to use. Traditionally, you could choose between 15-pound and 30-pound felt. Now, however, you also have the option of synthetic underlayment, sometimes referred to as synthetic felt.


It is easiest to compare these types of felt with two separate sets of comparisons. You can compare synthetic and felt underlayment to start, then if you opt for felt, you can compare 15- and 30-pound options. If you were to complete a three-way comparison, there would be too many similarities between the two non-synthetic options.

Understanding Underlayment Options


Before we get into the comparisons, you need to have a clear understanding of the various options. The felt and synthetics are all types of roofing underlayment. Underlayment offers an additional layer of protection in case water penetrates the shingles on the roof. It also provides protection to the inner structure and the decking when the shingles are not in place, either during installation or repairs. It can also boost your home’s fire rating and provides roofers with slip protection as they install the shingles.


Roofing felt is similar to fleece made from polyester or fiberglass and then soaked in waterproofing agent. 15 pound and 30 pound refers to the thickness of the felt, with the numbers indicating the weight of 100 square feet of this paper back when they were originally developed. Now, however, they weigh less, although the names stuck.


Synthetic underlayment can come in many forms, with many spun or woven using polypropylene or polyethylene. Synthetic underlayment allows for a higher level of customization, since the manufacturing process can be adjusted to change properties like walkability and exposure time.

Felt vs Synthetic Underlayment


GAF Feltbuster Synthetic UnderlaymentTo start, take a look at asphalt felt as a general category and synthetic as the other.


Perm Rating


It is common, although not always the case, for synthetic underlayment to have a lower perm rating than felt. As such, moisture within the attic finds it significantly harder to escape via the roof. This means that while attic ventilation is always important, it is absolutely crucial when dealing with synthetic roofs and just very wise for felt ones.




The prices of your underlayment will vary based on the specific product you choose, but synthetic tends to be more expensive, with a few key factors to keep in mind.


Heavier synthetics will cost more, as they typically have higher-performance qualities, such as longer exposure times or better walkability. Thinner synthetics are usually comparable in price to high-quality 15-pound felt. Some of the lower-quality ones can easily be cheaper than 15-pound felt, and even some of moderate quality fall into this price range.




When you look at how the underlayment is packaged when you buy it, there are also some differences. Roofing felt comes in rolls that are three feet long and will typically cover around 400 square feet. By contrast, synthetic rolls usually cover about 1,000 square feet and are about four feet long.


The larger rolls of synthetic felt can make it quicker to cover the roof.




When the first synthetics were produced, felt was safer since the synthetics could become very slippery when wet. Now, however, synthetics frequently have better walkability than felt, although this does depend on the specific product.


Weather Conditions Affect Underlayment


Synthetic underlayment tends to stay better in place in windy conditions compared to felt, assuming that it is fastened properly.


Synthetic also capture less heat due to their color, which is gray compared to the black color of felt. This results in felt getting hotter, which can be unpleasant at times.


Synthetics even have some advantages in the cold. Roofing felt tends to get stiffer and keep a curl when the temperatures get lower. This is not a problem with synthetics.




Synthetic underlayment will always be lighter than felt underlayment. Ten square feet of felt has an average weight of about 130 pounds, while an equivalent amount of synthetic would be just 23 pounds. This obviously makes synthetic underlayment easier to get up to the roof.


15lb vs 30lb Felt Underlayment


Now, we can take a closer look at how the two weights of felt underlayment compare to each other to get a better feel for how all three compare as a whole.


The Main Difference


As mentioned, the main difference between the two is how much 100 square feet of the felt originally weighed. Historically, 15-pound felt underlayment weighed 15 pounds for this amount while 30-pound felt weighed 30 pounds. As such, the 30-pound felt weighed twice as much.


Over the years advancements in technology have made changes that reduced the weight, although the names have stuck. Not only is felt lighter than it used to be, but it also has better tensile strength.


Roof Slope


In the case of roofs with a steeper slope, you should always opt for 30-pound felt instead of 15-pound. This is essential due to its increased slip protection and resistance to tearing. If you opt for thinner felt on a steep roof, it is more likely to rip.


Tear Resistance


Generally speaking, 30-pound felt is less likely to experience tears than 15-pound felt. This is beneficial for installation, roof protection, and all of its other functions.


Additionally, if a shingle comes off in a storm, 30-pound felt is much less likely to tear along with the shingle, keeping your roof protected. By contrast, 15-pound felt may get torn off, leaving the underlying structures of the roof unprotected.




Because of the heavier nature of the 30-pound felt, it should come as no surprise that it is thicker than 15-pound felt.



The thickness of 30-pound felt also means that tends to have better wear over time.


Current Preferences


Most roofing contractors prefer to use synthetic felt due to its various advantages in terms of strength, weight, safety, and cost. However, not all types of roof are compatible with synthetic felt and in that case, most will suggest that you opt for 30-pound felt if you can afford it. 15-pound felt does provide enough protection in most cases, but the others do a better job.


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