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The Essential Guide to Roof Underlayment



Your One Stop Shop for Understanding Roofing Underlayment


On a classic gable roof, you have a variety of different top layers to choose from, ranging between asphalt shingles, clay tiles, metal sheets, and more. But what about the roof underlayment that sits underneath it?

 

There are actually a number of different types of roof underlayment that you could choose from when renovating a home, but before you make your choice you need to understand the pros and cons of each. We’ve put together this guide to explain what roof underlayment is, go into more detail on the different types used in the industry, and investigate some of the basics of the installation process in simple English.

 

What is Roof Underlayment?

 

Roof underlayment is the waterproof layer installed directly to the deck of your roof, underneath the typical roofing materials that act as the external barrier. Underlayment acts as a secondary line of defense against the elements, providing your home with more protection from wind, rain, sleet, and snow. While shingles and tiles offer protection, the fact that the underlayment is totally sealed-up makes it the perfect support system for the roof.

 

Along with protecting the structural elements of your roof from leaks and air damage, the underlayment also keeps the main decking from being visually exposed under the shingles or tiles. Underlayment is also known for providing some sound-dampening qualities against rain and hail. It’s also known for providing a degree of insulation for your roof, which is being taken more and more seriously as energy prices rise and environmental awareness grows.

 

Roof Underlayment Types

 

There are three primary types of roof underlayment used in American roofing, with your choices ranging between:

 

● Asphalt Felt Underlayment

● Rubberized Asphalt Underlayment

● Synthetic Plastic Underlayment

 

It’s important to understand that each of these choices comes with its own benefits, drawbacks, and price tag, with some being better suited for different environments than others. It’s also worth noting that the different underlayments will operate differently with different roof styles, so make sure you match your materials properly.

 

Asphalt Felt Roof Underlayment

 

This is the form of roof underlayment that has been in use for the longest, along with being the cheapest choice. It’s essentially a massive sheet of cellulose or fiberglass paper that’s been thoroughly coated in bitumen, which is a semi-solid form of petroleum. It can come in both 15-pound or 30-pound forms, with the latter being thicker and more durable.

 

Benefits of Using Asphalt Felt Roof Underlayment

 

Being the cheapest form of underlayment, this material is great if you’re looking to pull off a renovation or roof replacement on a budget. It also offers a reasonable degree of water resistance, which is great for areas with mild to medium rainfall, like New York State.

 

Drawbacks to Using Asphalt Felt Roof Underlayment

 

Despite being water-resistant, it’s not totally waterproof, which means it’s not the best underlayment for rainier states. Along with this, it’s not UV-resistant, meaning it needs to be thoroughly covered, especially in the sunnier states of the US. It’s also one of the weaker options, meaning it can easily be broken by pressure or even during installation, along with being the underlayment with the shortest lifespan.

 

Which Material Should I Match it With?

 

If choosing to use asphalt felt roof underlayment, you should match it with traditional asphalt shingles. 15-pound underlayment should provide a decent degree of protection, while still allowing ventilation to the roof, granted you’re in an area without overly harsh weather. If you live in an area with snow or high winds, 30-pound underlayment is a better call.

 

Rubberized Asphalt Underlayment

 

Rubberized asphalt underlayment is a catch-all term used to describe any asphalt underlayment material that’s been treated to mimic the properties of rubber. Generally, these materials are used in peel and stick underlayment, which features a self-adhesive backing that has to be revealed through peeling off a protective layer, then secured to the roof without fasteners.

 

Benefits of Using Rubberized Asphalt Underlayment

 

Rubberized asphalt basically does everything that classic felt underlayment does, but better. It’s more resistant to water, heat, and wind, while also being stronger and generally longer lasting. Along with this, there’s not much rush to install your primary roofing materials, as rubberized asphalt will remain sturdy when uncovered for between 90 and 180 days, perfect for larger projects.

 

Drawbacks to  Using Rubberized Asphalt Underlayment

 

The only real drawbacks to this material are that the best peel and stick roof underlayment comes with a hefty price tag, along with being slightly more complicated to install.

 

Which Material Should I Match it With?

 

Rubberized asphalt underlay is suitable for most primary roofing materials, being a great choice for options like shingles, clay tiles, and wooden shakes. As these are the more high-profile, long-lasting, and expensive choices for shingling/tiling, it only makes sense to match them with an underlayment that matches them in quality.

 

Synthetic Plastic Underlayment

 

This material differs from the last two in that it contains no asphalt whatsoever, rather being made from plastic compounds such as polymers, polypropylene, or polyethylene. This is the most modern form of roofing underlayment available.

 

Benefits of Using Synthetic Plastic Underlayment

 

This material has the highest tear and wind resistance of all three options, while also being resistant to degradation in ways that asphalt can’t compare to. It’s also very lightweight and durable, making it a safe choice for installation. Along with this, it’s highly resistant to UV radiation, making it a great choice for hotter, sunnier areas, especially when left uncovered.

 

Drawbacks to Using Synthetic Plastic Underlayment

 

Much like rubberized asphalt, the problems with synthetic roof underlayment are mostly associated with its high price tag. It’s not a great choice if you’re working within a slightly tighter budget.

 

Which Material Should I Match it With?

 

Being highly resistant to water and heat makes synthetic underlayment a great choice for most roofing materials, being especially well-suited to metal roofs that trap more heat. It’s a great choice for hotter or rainier climates. However, many would argue that heavy-duty felt is a better choice for snowier areas.

 

Roof Underlayment Installation

 

The installation process for each of these roofing underlayment materials is different from the others, with all of them having their own specific steps and techniques. While we’re not going to describe the processes in full detail, we’re going to illuminate those differences for you.

 

Remember that in all cases, you must prepare your roof decking by clearing away any debris and ensuring there isn’t an excessive amount of moisture. Of course, you also need to remove any existing roofing materials prior to installation to ensure the longest-lasting roof.

Traditional Asphalt Felt Installation

 

This process requires the use of extra materials, along with applying multiple layers of the underlayment on top of one another. The first layer needs to be secured to the decking with nails, then followed by another layer kept sturdy with fasteners, nails, and adhesives that you add yourself.

 

Peel & Stick Installation

 

In the case of peel and stick installation, there’s no need to use nails or adhesives to hold the underlayment in place, as it has its own adhesive on the back. In the case of the membrane sheets of rubberized asphalt, you simply align them on the roof, remove the backing, and press them onto the decking, smoothing them out with hand rollers.

 

Synthetic Underlayment Installation

 

Synthetic underlayment has to be installed using fasteners and nails, and also cannot be rolled out to a point where it’s tight and stretched. It needs to be rolled out levelly, but if it’s too tight it won’t allow for slight shifts in size brought on by environmental factors.

 

While each of these descriptions has been majorly abridged, we’d still advise hiring a reputable roofer to install any of the options. Roofing is a task that requires a certain degree of technical know-how and precision, which is hard to pull off as a DIY enthusiast.

 

Roof Underlayment: Final Thoughts

 

Picking the right form of roof underlayment for your home comes down to a few different factors.

 

First of all, you have to consider your budget. Ideally, you’d be able to afford rubberized asphalt or synthetic plastic, but if you’re working with a little less money, traditional asphalt with shingles can last up to 25 years if properly maintained.

 

Then you have to consider your climate. If you’re living in an area with more intense weather patterns then you need to go for a more heavy-duty option, while a milder climate will allow you to get away with a cheaper underlayment.

 

Finally, consider your primary roof material. We described some of the best matches in this blog, so you can know what to partner your underlayment with.

 

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