Deciding Which Type of Roof Underlayment is Best for You in 2024

You know your roof protects your home from the elements, but you might not know everything that goes into a solid roofing system. One critical yet hidden roofing component is the roof underlayment — also called a membrane or sheeting. The roof underlayment is the weather-proofing layer between the deck (or sheathing) and the shingles. To pick the right type of underlayment, you need to consider various factors, including the type of roofing involved.

In this guide, we help you match the ideal membrane with the appropriate roofing and take any guesswork you might have for which material is best for your home.

What Is Roof Underlayment?

Roof underlayment provides a robust layer that protects against leaks. The sheeting allows air and humidity to escape thanks to its semi-breathable membrane. This characteristic provides a protective coating for the roof assembly by forming a seal on points where flashing and shingles tack in.

Underlayment is available in three main types:

• Asphalt-saturated felt (felt paper)
• Non-bitumen synthetic (synthetic felt paper), meaning it doesn’t contain asphalt
• Rubberized asphalt

Most contractors use asphalt-saturated felt for residential roofing. The material works well with different types of roofs. However, homeowners can also opt for non-bitumen synthetic underlayment, which provides several benefits for residential construction work.

Contractors attach the materials using fasteners and, occasionally, adhesive-backed moisture barriers to enhance the protective layer. Proper installation prevents water intrusion even if structural damage compromises the integrity of the outer layer. The underlayment plays a complementary role in the roofing system.

Types of Roof Underlayment

Understanding the characteristics of various types of underlayment is key to making the right selection. Each variant comes with distinct properties capable of providing maximum protection in specific roofing applications.

Asphalt-saturated felt (felt paper)

Asphalt-saturated felt is ideal for steep, sloping roofs requiring substantial water resistance. Although not waterproof, this type of underlayment provides adequate protection in high-humidity environments.

Some felt underlayment materials have a cellulose base (natural plant fibers), while some felt underlayment is made of fiberglass substrate. The cellulose base underlayment is sometimes referred to as “organic” underlayment because of the plant-based material used to construct it.

Asphalt-saturated felt is available in 15- and 30-pound options, which refer to the thickness of the felt. Opting for a 30-pound felt is a practical choice when looking to protect the roof for longer. This weight is stiffer and thicker, so it better withstands the elements.

Non-bitumen synthetic

This type of underlayment leverages the protective qualities of synthetic polymers like polypropylene and polyethylene to improve the membrane’s performance. Asphalt and fiberglass enhance the stability and tear-resistance of the roof underlayment.

A synthetic sheet creates a reliable protective barrier across the whole plywood roof deck that covers the rafters. Its advantages include:

• High strength
• Lightweight material
• Wrinkle-free surfaces
• Resistance to fungal growth
• Superior resistance to moisture
• Immunity to the sun’s ultraviolet (UV) damage

Rubberized asphalt

Manufacturers use various rubber materials to produce dependable roof underlayment products. The membranes come with an adhesive that sticks to the surface of the deck. The rubberized asphalt underlayment has excellent sealing properties, as well.

However, this type of underlayment is pricier than other variants since it contains more levels of rubber polymers and asphalt. The benefit of choosing rubberized membranes is that they are waterproof. Contractors use this type of membrane to prevent water damage.

Some products incorporate a polyester or polyethylene layer on the upper surface. The materials enhance the weather-resistant and non-skid characteristics of the roof underlayment. Manufacturers may attach a polymer film to bolster moisture resistance on the upper surface.

It’s also possible to find a rubberized asphalt underlayment reinforced with fiberglass. Some products include a mineral coating on the upper surface or another coating specially designed for high-temperature conditions.

These enhancements enable membranes to provide sufficient protection in temperatures of up to 250 degrees Fahrenheit without risking deterioration of the adhesive.

Water-Resistant vs. Waterproof Roof Underlayment

When choosing the right roof membrane, it’s crucial to understand the difference between waterproof and water-resistant underlayment.

According to industry standards, waterproof materials prevent water intrusion under any circumstances, while a water-resistant membrane provides a moisture barrier to protect the roof deck. On the downside, a water-resistant membrane doesn’t provide a reliable barrier against water leaks. Water intrusion may still occur around fasteners.

A water-resistant underlayment doesn’t form a robust seal on nails, caps, or staples. Thus, a water-resistant underlayment can’t prevent water leaks when punctured by shingle fasteners. Leaks may still occur even if contractors seal the membrane with caps.

The peel-and-stick underlayment that top roof specialists use provides a truly waterproof barrier. The roof assembly benefits from the self-sealing properties associated with the rubberized asphalt roof underlayment. Unlike water-resistant membranes, a peel-and-stick underlayment seals installation punctures.

Granular vs. Smooth Roof Underlayment

With multiple underlayment selections available, homeowners need to identify a protective solution capable of serving specific purposes. A well-chosen membrane acts as a line of defense in the roof assembly. An additional factor to consider is the choice of a granular or smooth underlayment.

The surface of an underlayment plays an integral role in performance. Thus, it’s vital to understand the differences between smooth and granular surfaces.

Granular underlayment

This roof membrane comes with a rough surface, with the thickness of the membrane between 1.6 and 2.4 inches. An ever-increasing number of homeowners opt for a granular underlayment because it’s more affordable and has slip-resistant properties due to its sand-like surface.

Depending on their specific requirements, homeowners can let the roofing specialists install a granular membrane to protect the deck. The thickness of the material simplifies the installation process.

On the downside, a granular underlayment isn’t ideal for high-temperature environments. Granules can soften and become loose. Furthermore, the uneven surface presents challenges when trying to create watertight seals at overlaps. Contractors often use roofing cement to improve sealing at overlaps.

Smooth underlayment

A smooth, self-adhering underlayment has a finer film surface capable of enhancing the roof’s waterproofing. The membrane seals the roof deck and around fasteners more aggressively. For this reason, a smooth underlayment provides a watertight sealing bond.

The roof installers rely on the smooth membrane to adhere correctly to the surface at overlaps. Strong adherence creates watertight laps, eliminating the need for roofing sealants or cement. Products with a thick layer of aggressive adhesive provide superior protection. These products perform well under various conditions, including severe coastal and alpine environments.

Opting for a smooth underlayment with thinner adhesive yields a less aggressive sealing performance. Homeowners can expect ease of installation and cost benefits. For example, if you need to replace your roof in the future, contractors can apply a new layer of smooth membrane over the existing underlayment so that you don’t have to pay for it again.

Find the Best Roof Underlayment for Your Home

Underlayment plays a critical role in the roof assembly. Therefore, it’s vital to avoid overlooking its importance in protecting the roof deck from water damage. Working closely with a knowledgeable roof contractor enables you to make the right choice during roof installation or repair.

Discuss the types of underlayment types and select the one that is best for your roofing project.

Frequently Asked Questions about Roof Underlayment

Find answers to common questions about roofing underlayment here.

Is synthetic underlayment worth the extra cost?

Synthetic underlayment is often worth the slightly higher cost due to its numerous benefits. It is lightweight, durable, resistant to tearing and water damage, and provides better protection against leaks. It is also easier to install and offers better traction for roofers during the installation process.

Can roof underlayment be reused during roof replacements?

Typically, roof underlayment cannot be reused during roof replacements. During a roof replacement, the underlayment is often damaged or deteriorated, necessitating replacement. Reusing old underlayment may compromise the new roof's integrity and increase leak risks. It is recommended to install new underlayment during roof replacements for optimal protection.

How often should roof underlayment be replaced?

Generally, it is advised to replace roof underlayment every 20-30 years. Over time, underlayment can deteriorate due to exposure to sunlight, moisture, and other environmental factors. Regular inspections by a professional roofer can help determine the underlayment's condition and whether replacement is necessary.