Do you need a new roof? Are you trying to understand what roof underlayment is and the role it plays in your roof replacement?
There are a variety of roofing components and materials that make up your roof system. Each performs a specific function to prevent water from entering your home.
While they all play a role, one of the most important for waterproofing is roof underlayment. But what is roof underlayment, and what does it do?
For over 35 years, the team at Bill Ragan Roofing has helped homeowners understand all parts of the roofing puzzle. So, let’s break down everything you need to know about roof underlayment, from what it is, types, cost, and more.
By the end of this article, you’ll have the answers to the following questions:
- What is roof underlayment?
- What are the types of roof underlayment?
- How much does roof underlayment cost?
- Which roof underlayment is best for Nashville, TN?
- Does underlayment get replaced during a roof replacement?
What is roof underlayment?
Roof underlayment is a water-resistant roofing component installed directly over the top of your roof decking. It lies between your decking and roofing material to provide an extra layer of protection from the elements.
Think of it as a safety net if water gets under your shingles that prevents it from getting to the wood underneath. Underlayment also acts as a barrier if shingles get blown off during storms with high winds.
Because Nashville gets high humidity, heavy spring storms, and occasional ice events, underlayment plays a big role in long-term roof health.
What are the types of roof underlayment?
The two types of roof underlayment are felt and synthetic. Felt was the preferred underlayment before the invention of synthetic, so synthetic underlayment is more common today.
You’ll still find roofers who use felt underlayment. However, synthetic underlayment is by far the better option.
Felt roof underlayment
The “old-school” underlayment that was used before the invention of synthetic underlayment. It’s made from a paper or fiberglass mat soaked in asphalt that comes in 15lb or 30lb.
Pros of felt roof decking
- Cheap
- Better than having no underlayment
Cons of felt roof decking
- Tears easily in high winds
- Absorbs water rather than repelling it
- Not ideal for humid climates like Nashville or Middle Tennessee.
Synthetic roof underlayment
Synthetic underlayment is made from woven/spun polyethylene or polypropylene polymers that resist tearing and moisture. This is the standard roof underlayment used on most homes in the United States and Middle Tennessee.
Pros of synthetic roof decking:
- Tear-resistant in wind
- Repels water
- Lightweight
- Safer for installers to walk on
- Performs well in heat and humidity
Cons of synthetic roof decking:
- Slightly more expensive than felt
Ice & Water Shield
While technically it’s own thing, ice and water shield is also a waterproof membrane installed on roof decking that’s fully waterproof. It’s more commonly used to reinforce vulnerable roof areas prone to leaks (valleys, against walls, etc.) instead of the entire roof decking.
However, high heat ice and water shield is used as underlayment on low slope roofs or premium roof systems, such as metal, synthetic, etc. So, while it’s technically not underlayment, ice and water shield goes hand in hand with it.
How much does roof underlayment cost?
The cost of roof underlayment is included in the overall cost of your roof replacement, so it’s impossible to nail down an exact price. However, each type of roof underlayment has its own general price range.
Typical roof underlayment cost ranges (material only):
- Felt underlayment: around $0.25 per square foot
- Synthetic underlayment: $0.25–$0.50+ per square foot
- Ice & water shield: $0.60–$1.25+ per square foot
Keep in mind that there are many factors that determine roofing costs, such as:
- Roof pitch
- Roof size
- Number of valleys, walls, penetrations, etc.
- Underlayment brand
- Local supplier costs and labor rates in your area or ours of Middle Tennessee
At the end of the day, roof underlayment is wrapped into the overall price of your roof replacement. However, it must be included on every roof, so it will always affect the cost.
Which roof underlayment is best for Nashville, TN?
Because we get heavy rain, high winds, humidity, and occasional ice, synthetic underlayment is the best option in Nashville and every area of the United States. However, it’s crucial that it’s combined with ice & water shield in vulnerable areas for maximum protection.
This combination provides the best balance of protection, durability, and cost-efficiency for Nashville and the Middle Tennessee climate.
Does underlayment get replaced during a roof replacement?
Yes, roof underlayment is a necessary roofing component that should be replaced every time a new roof is installed. Even more importantly, it must actually be installed.
Without underlayment, this water eventually rots your roof decking, leading to leaks, mold/mildew forming, and even structural damage. No matter what you read or hear, you should never trust a roofer who says your roof doesn’t need new underlayment.
If you do, you’re not getting the protection your roof and home need.
What other roofing components make up a roof system?
By the end of this article, you should know which roof underlayment is right for you. But your roof’s underlayment is just one crucial roofing component that makes up a complete roof system.
Knowing the rest of the components and materials helps you understand what you’ll pay for when getting a roof replacement. It also ensures you know what must be included in your roof estimate.
That’s why I wrote another article breaking down the rest of the roofing components that make up a roof system and their function.
Check out 9 Materials Included With Your Roof Replacement to learn what you’re paying for when it’s time for a new roof.
