What Is Roof Underlayment?

The Complete Guide To Roof Underlayment So You Can Make The Right Decision On Which UNderlayment Is Right For You!


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Roof Underlayment Basics

The roof is arguably the most important part of a house. It not only keeps the harsh weather at bay but also plays an undeniable role in the ventilation, insulation, and overall beauty of the home. That said, how much do you know about your roof?
Most people only envision the visible exterior when they think of roofing, not realizing that a lot more goes into making the roof sturdy than just slapping some shingles over your home. There is the skeleton, roof deck, and roof underlayment aspect to it.
roofing underlayment being put on a roof
The skeleton is what holds up the roof and is usually constructed from rafters or trusses. On top of it lies the roof deck, which consists of boards fastened onto the rafters or trusses to help stabilize the skeleton and give structure to the roof. The roof deck is usually made from plywood, OSB, or step sheathing.
Roof underlayment is the protective material that goes on top of the roof deck (and below the shingles/metal sheets) to shield the boards from water intrusion.

Why Is Roofing Underlayment Important?

Shingles and metal sheets face the bulk of the rain, snow, UV rays, and hailstones that nature throws at your house. However, there are small gaps and spaces in the places where the shingles/metal sheets overlap. These spaces provide an avenue for water to pass through and weaken your roof’s structure, that is, unless you have roofing underlayment installed.
Roofing underlayment is the material that stops the water and snow from coming in contact with your roof deck.

Certain sections of a roof- the valley, dormer, and hips- are likely to give you roof leaks during heavy rain. Since shingles and/or metal roofs provide inadequate coverage in such areas, roofing underlayment is important to protect your home from water leakage.

Types Of Roofing Underlayment

There are three types of roof underlayment: felt, synthetic, and self-adhered. Each is structured differently and carries its own advantages and disadvantages. Your choice of roof underlayment will depend on the budget, roof design, roofing material, and local climate. Fortunately, a good roofing contractor will suggest the best underlayment for your home, so you don’t have to go through the trouble of analyzing every factor.

Nonetheless, here is everything you need to know about the various types of roofing underlayment.

1. Felt Roofing Underlayment

Also known as tar paper or felt paper, felt roofing underlayment is the oldest underlayment technology in the market. It was the most popular underlayment for decades until synthetic products entered the market. Felt roof underlayment consists of an organic mat or paper made from a cellulose base and saturated with water to make it water-resistant. It can be applied in multiple layers for extra protection and is available in two standard weights;
  • No. 15 felt, which weighs 15 pounds per 100 square feet of roofing. This felt underlayment is typically used for lightweight projects. However, it tears easily and tends to absorb water and wrinkle if left exposed for too long.
  • No. 30 felt, which weighs 30 pounds for every 100 square feet of roofing. It’s heavier and, thus, more durable than No.15 felt.
  • Felt is the cheapest underlayment option.
  • Felt underlayment is quite heavy, making it harder for contractors to work with. Its weight also means that felt carries less material per roll. This leads to more trips up the ladder and more seams to cover 100 square feet of roof.
  • It has a slippery surface that makes it difficult and riskier to install
  • It’s prone to tearing under strain; during installation or when exposed to high winds
  • Felt absorbs water and develops wrinkles when exposed to moisture. These wrinkles make it hard to lay shingles flat on the roof.
  • Some roofing manufacturers require synthetic underlayment. Thus, installing felt underlayment may void your warranty.

2. Synthetic Roofing Underlayment

Synthetic underlayment is currently the most popular option in the market. Different manufacturers have their ways of doing it, but generally, it’s made from durable, moisture-resistant polymers. This type of underlayment costs more than felt but, if installed correctly, offers better protection than its asphalt counterpart.
  • Synthetic roof underlayment is tough and durable. It doesn’t tear easily, even under the boot traffic of roofers during installation. In addition, some brands of synthetic underlayment can withstand exposure to UV rays and moisture without sustaining damage.
  • It’s also considerably lighter than felt roofing, making it easier and more convenient to install.
  • Being plastic, it repels moisture instead of absorbing it and is resistant to mold and fungal growth.
  • Synthetic roof underlayment also carries more material per roll, reducing the number of trips up the ladder roofers have to make to cover 100 square feet of roof.
  • Many synthetic roof underlayment brands have slip-resistant surfaces, reducing the risk of slip and fall accidents during installation.
  • Synthetic roofing underlayment is relatively more expensive than felt.

3. Self-Adhered Underlayment

In the world of roofing, self-adhered underlayment is as premium as they come. These products contain high percentages of rubber and asphalt polymers to make them water-resistant. They also have sticky undersides that adhere to the roof deck to create a waterproof seal around nails and the sun will help the sealant waterproof all nail holes leaving an impenetrable roof covering.

Self-adhered underlayment is best for leak-prone areas like chimneys, vents, skylights, eaves, and valleys, but can also be applied to the entire roof deck for extra leak protection. In addition, the upper surface may contain polyesters or polyethylene materials to make it more weather-resistant. Thus, this type of underlayment is best suited for regions with severe winters and snowfall.
  • Self-adhered underlayment is very durable.
  • It’s the only underlayment with a 100% waterproof seal.
  • It’s a self-sealing underlayment, making it ideal for sealing around fasteners.
  • Self-adhered underlayment has slip-resistant surfaces that make it safer for roofers to work on.
  • Self-adhered underlayment is the most expensive of the three options available.

Find Your Ideal Roofing Underlayment

Whether you’re reroofing your home or building a new one, choosing the right roofing underlayment is just as important as picking the right roofing material. So when you are deciding on your next roof and thinking about things like is a metal roof or shingles better, what thickness of roofing underlayment should I get or what types of roofing materials should I have installed, remember to go for what makes the most sense for you and your property long term. 

If you want a quick and affordable solution, asphalt-saturated felt underlayment should work well enough. But if you’re after longevity, synthetic underlayment is your best bet at a more durable and modern solution.

Those that have no limit on their budget and/or live in areas with harsh winters can find the best quality in self-adhered roofing underlayment. If you want an experienced residential and commercial roofing contractor that will handle all your roofing needs, contact us today for a free consultation.